The method by which the cycloid is generated shows that it consists of an infinite number of cusps placed along the fixed line and separated by a constant distance equal to the circumference of the rolling circle. The name cycloid is usually restricted to the portion between two consecutive cusps (fig. 1, curvea); the fixed line LM is termed the base, and the line PQ which divides the curve symmetrically is theaxis. The co-ordinates of any point R on the cycloid are expressible in the form x = a(θ + sin θ); y = a(1 − cos θ), where the co-ordinate axes are the tangent at the vertex O and the axis of the curve, a is the radius of the generating circle, and θ the angle R′CO, where RR′ is parallel to LM and C is the centre of the circle in its symmetric position. Eliminating θ between these two relations the equation is obtained in the form x = (2ay − y²)½ + a vers-¹ y/a. The clumsiness of the relation renders it practically useless, and the two separate relations in terms of a single parameter θ suffice for the deduction of most of the properties of the curve. The length of any arc may be determined by geometrical considerations or by the methods of the integral calculus. When measured from the vertex the results may be expressed in the forms s = 4a sin ½θ and s = √(8ay); the total length of the curve is 8a. The intrinsic equation is s = 4a sin ψ, and the equation to the evolute is s = 4a cos ψ, which proves the evolute to be a similar cycloid placed as in fig. 2, in which the curve QOP is the evolute and QPR the original cycloid. The radius of curvature at any point is readily deduced from the intrinsic equation and has the value ρ = 4 cos ½θ, and is equal to twice the normal which is 2a cos ½θ.Thetrochoidswere studied by Torricelli and F. van Schooten, and more completely by John Wallis, who showed that they possessed properties similar to those of the common cycloid. The cartesian equation in terms similar to those used above is x = aθ + b sin θ; y = a − b cos θ, where a is the radius of the generating circle and b the distance of the carried point from the centre of the circle. If the point is without the circle,i.e.if a < b, then the curve exhibits a succession of nodes or loops (fig. 1, curveb); if within the circle,i.e.if a > b, the curve has the form shown in fig. 1, curvec.Thecompanion to the cycloidis a curve so named on account of its similarity of construction, form and equation to the common cycloid. It is generated as follows: Let ABC be a circle having AB for a diameter. Draw any line DE perpendicular to AB and meeting the circle in E, and take a point P on DE such that the line DP = arc BE; then the locus of P is the companion to the cycloid. The curve is shown in fig. 3. The cartesian equation, referred to the fixed diameter and the tangent at B as axes may be expressed in the forms x = aθ, y = a(1 − cos θ) and y − a = a sin (x/a − ½π); the latter form shows that the locus is the harmonic curve.For epi- and hypo-cycloids and epi- and hypo-trochoids seeEpicycloid.References.—Geometrical constructions relating to the curves above described are to be found in T. H. Eagles,Constructive Geometry of Plane Curves. For the mechanical and analytical investigation, reference may be made to articlesMechanicsandInfinitesimal Calculus. A historical bibliography of these curves is given in Brocard,Notes de bibliographie des courbes géométriques(1897). See also Moritz Cantor,Geschichte der Mathematik(1894-1901).
The method by which the cycloid is generated shows that it consists of an infinite number of cusps placed along the fixed line and separated by a constant distance equal to the circumference of the rolling circle. The name cycloid is usually restricted to the portion between two consecutive cusps (fig. 1, curvea); the fixed line LM is termed the base, and the line PQ which divides the curve symmetrically is theaxis. The co-ordinates of any point R on the cycloid are expressible in the form x = a(θ + sin θ); y = a(1 − cos θ), where the co-ordinate axes are the tangent at the vertex O and the axis of the curve, a is the radius of the generating circle, and θ the angle R′CO, where RR′ is parallel to LM and C is the centre of the circle in its symmetric position. Eliminating θ between these two relations the equation is obtained in the form x = (2ay − y²)½ + a vers-¹ y/a. The clumsiness of the relation renders it practically useless, and the two separate relations in terms of a single parameter θ suffice for the deduction of most of the properties of the curve. The length of any arc may be determined by geometrical considerations or by the methods of the integral calculus. When measured from the vertex the results may be expressed in the forms s = 4a sin ½θ and s = √(8ay); the total length of the curve is 8a. The intrinsic equation is s = 4a sin ψ, and the equation to the evolute is s = 4a cos ψ, which proves the evolute to be a similar cycloid placed as in fig. 2, in which the curve QOP is the evolute and QPR the original cycloid. The radius of curvature at any point is readily deduced from the intrinsic equation and has the value ρ = 4 cos ½θ, and is equal to twice the normal which is 2a cos ½θ.
Thetrochoidswere studied by Torricelli and F. van Schooten, and more completely by John Wallis, who showed that they possessed properties similar to those of the common cycloid. The cartesian equation in terms similar to those used above is x = aθ + b sin θ; y = a − b cos θ, where a is the radius of the generating circle and b the distance of the carried point from the centre of the circle. If the point is without the circle,i.e.if a < b, then the curve exhibits a succession of nodes or loops (fig. 1, curveb); if within the circle,i.e.if a > b, the curve has the form shown in fig. 1, curvec.
Thecompanion to the cycloidis a curve so named on account of its similarity of construction, form and equation to the common cycloid. It is generated as follows: Let ABC be a circle having AB for a diameter. Draw any line DE perpendicular to AB and meeting the circle in E, and take a point P on DE such that the line DP = arc BE; then the locus of P is the companion to the cycloid. The curve is shown in fig. 3. The cartesian equation, referred to the fixed diameter and the tangent at B as axes may be expressed in the forms x = aθ, y = a(1 − cos θ) and y − a = a sin (x/a − ½π); the latter form shows that the locus is the harmonic curve.
For epi- and hypo-cycloids and epi- and hypo-trochoids seeEpicycloid.
References.—Geometrical constructions relating to the curves above described are to be found in T. H. Eagles,Constructive Geometry of Plane Curves. For the mechanical and analytical investigation, reference may be made to articlesMechanicsandInfinitesimal Calculus. A historical bibliography of these curves is given in Brocard,Notes de bibliographie des courbes géométriques(1897). See also Moritz Cantor,Geschichte der Mathematik(1894-1901).
CYCLOMETER(Gr.κύκλος, circle, andμέτρον, measure), an instrument used especially by cyclists to determine the distance they have traversed. In a common form a stud attached to one spoke of the wheel engages with a toothed pinion and moves it on one tooth at each revolution. The pinion is connected with a train of clockwork, the gearing of which bears such a ratio to the circumference of the wheel that the distance corresponding to the number of times it has revolved is shown on a dial in miles or other units.
CYCLONE(Gr.κυκλῶν, whirling, fromκύκλος, a circle), an atmospheric system where the pressure is lowest at the centre. The winds in consequence tend to blow towards the centre, but being diverted according to Ferrel’s law they rotate spirally inwards at the surface of the earth in a direction contrary to the movement of the hands of a watch in the northern hemisphere, and the reverse in the southern hemisphere. The whole system has a motion of translation, being usually carried forward with the great wind-drifts like eddies upon a swift stream. Thus their direction of movement over the British Islands is usually from S.W. to N.E., though they may remain stationary or move in other directions. The strength of the winds depends upon the atmospheric gradients. (SeeMeteorology.)
CYCLOPEAN MASONRY(from the Cyclopes, the supposed builders of the walls of Mycenae), a term in architecture, used, in conjunction with Pelasgic, to define the rude polygonal construction employed by the Greeks and the Etruscans in the walls of their cities. In the earliest examples they consist only of huge masses of rock, of irregular shape, piled one on the other and trusting to their great size and weight for cohesion; sometimes smaller pieces of rock filled up the interstices. The walls and gates of Tiryns and Mycenae were thus constructed. Later, these blocks were rudely shaped to fit one another. It is not always possible to decide the period by the type of construction, as this depended on the material; where stratified rocks could be obtained, horizontal coursing might be adopted; in fact, there are instances in Greece, where a later wall of cyclopean construction has been built over one with horizontal courses.
CYCLOPES(Κύκλωπες, the round-eyed, plural of Cyclops), a type of beings variously described in Greek mythology. In Homer they are gigantic cave-dwellers, cannibals having only one eye, living a pastoral life in the far west (Sicily), ignorant of law and order, fearing neither gods nor men. The most prominent among them was Polyphemus. In Hesiod (Theogony, 264) they are the three sons of Uranus and Gaea—Brontes, Steropes and Arges,—storm-gods belonging to the family of the Titans, who furnished Zeus with thunder and lightning out of gratitude for his having released them from Tartarus. They were slain by Apollo for having forged the thunderbolt with which Zeus slew Asclepius. Later legend transferred their abode to Mt Aetna, the Lipari islands or Lemnos, where they assisted Hephaestus at his forge. A third class of Cyclopes are the builders of the so-called “Cyclopean” walls of Mycenae and Tiryns, giants with arms in their belly, who were said to have been brought by Proetus from Lycia to Argos, his original home (Pausanias ii. 16. 5; 25. 8). Like the Curetes and Telchines they are mythical types of prehistoric workmen and architects, and as such the objects of worship.
The standard work on these and similar mythological characters is M. Mayer,Die Giganten und Titanen(1887); see also A. Boltz,Die Kyklopen(1885), who endeavours to show that they were an historical people; W. Mannhardt,Wald- und Feldkulte(1904); J. E. Harrison,Myths of the Odyssey(1882); and article in Roscher’sLexikon der Mythologie(bibliography).
The standard work on these and similar mythological characters is M. Mayer,Die Giganten und Titanen(1887); see also A. Boltz,Die Kyklopen(1885), who endeavours to show that they were an historical people; W. Mannhardt,Wald- und Feldkulte(1904); J. E. Harrison,Myths of the Odyssey(1882); and article in Roscher’sLexikon der Mythologie(bibliography).
CYCLOSTOMATA, orMarsipobranchii, a group of fishes including the ordinary lampreys and hagfish, and so called from the wide permanently gaping mouth which is without the hinged jaws characteristic of other vertebrates (Gnathostomata).The class Cyclostomata consists of two orders, the Myxinoids (or Hyperotreti) and the Petromyzontes (or Hyperoartii), which, while showing sufficient resemblance in structure to warrant their inclusion in the same class, are yet marked off by such deep-seated differences as to indicate that they commenced to diverge from one another far back in evolutionary time. The order Myxinoids includes the hagfish (Myxine), common off the eastern, and occurring also, though less commonly, off the western coasts of the north Atlantic, and the genusBdellostoma(also known asHomea,Eptatretus, in part—Polistotrema), including the “borers” of the western American coast, New Zealand and the Cape of Good Hope. The order Petromyzontes includes the widely distributed lampreys. The original genusPetromyzon(which it is now customary to subdivide into a number of genera) includes the large sea lamprey (P. marinus) of the north Atlantic coasts and the two fresh-water lampreys of European streams (P. fluviatilisandP. planeri, the latter of which is possibly only a small-sized variety of the former species). In North America nine or ten species of lampreys are known to occur, descriptions of which are given by Jordan and Evermann (1). In the southern hemisphere occur the two genera Mordacia (Chile, Tasmania) andGeotria(Chile, Australia, New Zealand) (2).
The Cyclostomes are remarkable among vertebrates in that they are semiparasitic in habit. The lampreys—except some of the small fresh-water forms—attach themselves to other fishes by their suctorial mouth and proceed to rasp off the flesh by means of the horny teeth carried by the highly-developed tongue. The Myxinoids have gone a step further and actually bore their way right into the body of their prey, devouring all the soft parts and leaving the skin behind as a mere shell, empty but for the bones. Where the hagfish or borers are abundant, as in certain localities off the east coast of Scotland and off the west coast of California, they may do great damage to fisheries from their habit of attacking fishes which are in difficulties through being caught by a hook or in a net; the fish when drawn up being frequently completely deprived of their flesh.
The Myxinoids retain the ancestral marine habitat, but the lampreys have sought refuge from the struggle for existence by taking to fresh water to a less or greater extent. Such a form asPetromyzon marinusorEntosphenus tridentatusof the west coast of America is what is known as anadromous in habit,i.e.it takes refuge in fresh water during the breeding season, ascending rivers like the salmon for the purpose of spawning. Certain species of lampreys, on the other hand, have completely deserted the sea and spend their whole lives in fresh-water streams or lakes. The lake lampreys show a reminiscence of their ancestral migratory habits in leaving lakes and ascending streams in order to deposit their spawn.
Anatomy.—In structural features, the Cyclostomes show a curious mixture of features which must be looked on as primitive with others which are indicative of high specialization for their peculiar mode of life. In general appearance they are “eel-like”: they are elongated in shape and adapted for swimming in eel fashion,i.e.the body is propelled forward by the backward passage along it of waves of lateral flexure. There are, however, certain conspicuous differences which at once serve to distinguish a Cyclostome from any other fishes of eel-like shape:—(1) the circular permanently open mouth, (2) the absence of all trace of paired limbs, (3) the absence of paired external nasal openings, and (4) the presence on the roof or at the tip of the head of a conspicuous median opening—the pituitary opening.
It will be convenient, in describing the structural features of the group, to take as a basis for the description the marine lamprey,Petromyzon marinus. A marine lamprey is an eel-like creature 70 to 75 cm. in length. At the anterior end and situated somewhat ventrally is the circular widely gaping mouth or buccal cavity, its lining studded with sharply pointed thorn-like “teeth” and its edge fringed with numerous sensory papillae. On the dorsal side of the head is the conspicuous circular pituitary opening with prominent lips, while on the sides are seen the eyes, and behind these a row of somewhat rounded branchial openings or gill-clefts. At about the beginning of the posterior fourth of the body, and in the midventral line, is the anal opening, and immediately behind it is the prominent papilla carrying the opening of the urogenital sinus. The hinder portion of the body, in accordance with its function in locomotion, is flattened from side to side, while its surface is increased by the development of a median fin fold, divided, except in early stages of development, into three portions, known as the first and second dorsal fins and the caudal fin. The last mentioned is of the primitive protocercal type. The whole surface of the body—which shows a conspicuous dark marbling, especially dorsally, on a light ground—is covered with highly glandular epidermis. An important feature is the complete absence of all trace of the calcified placoid plates which are so characteristic of the Elasmobranchii.The Myxinoids differ from the lampreys in regard to several of the above-mentioned characters. The edges of the mouth carry tentacle-like barbels. The pituitary opening is close to the anterior edge of the mouth opening instead of being right up on the dorsal side of the head. The eyes are invisible, being greatly reduced and sunk far below the surface, and inMyxine, though not inBdellostoma, the row of gill openings is represented by a single opening on each side nearly in the midventral line and situated at about the end of the first quarter of the body length. Ventrally the Myxinoid possesses on each side of the body a row of remarkable epidermal glands which can produce at will enormous quantities of glutinous slime. This secretion, which, no doubt, is of much value as a protection from attack, is composed of very fine threads, formed by the conversion of the protoplasm of certain cells of the epidermal glands (“thread cells”) into an extremely fine, tightly coiled filament, which becomes unwound when discharged to the exterior.From D. Starr Jordan,A Guide to the Study of Fishes, by permission of A. Constable & Co., Ltd.Fig. 1.—The Marine Lamprey (Petromyzon marinus, L.).Pituitary Tube.—A remarkable peculiarity of the Cyclostomes lies in the fact that the pituitary ingrowth of ectoderm does not, as in other forms, become involved in the inpushing of ectoderm which forms the buccal cavity. On the contrary, it lies outside the edge of the stomodaeum, and in the case of the lampreys active growth takes place in the tissue between the pituitary and stomodaeal ingrowths, so that the two openings come to be widely separated, the pituitary opening being pushed back on to the dorsal side of the head. The pituitary opening remains patent throughout life, as is the case with Crossopterygians alone amongst Gnathostomata. InMyxinea further remarkable peculiarity in regard to the hypophysis, probably adaptive in nature, occurs, inasmuch as the pituitary invagination develops an opening at its posterior end into the pharynx.Nervous System.—The anterior end of the nervous tube is enlarged and differentiated to form a brain as in other Vertebrates, but this brain in the lampreys at least shows remarkably primitive features. The enlargement as compared with the spinal cord is seen to be comparatively slight: the brain is much elongated, and its various regions lie in a straight line one behind the other: the roof of the brain retains to a great extent the primitive epithelial condition. On each side anteriorly there is present a comparatively large olfactory lobe, and this is continued posteriorly into a small cerebral hemisphere.The lampreys are amongst those vertebrates in which there is an eye-like apparatus (3) connected with the roof of the thalamencephalon. There grow out from the roof of the thalamencephalon two processes, a posterior (the pineal process), and an anterior (the parapineal process). The pineal process grows forwards so as to overlie the parapineal process. Each of these projections from the roof of the thalamencephalon dilates to form a vesicle, and each vesicle shows certain eye-like characteristics, its deep wall forming a “retina” and its superficial wall being clear and translucent (“pellucida”). The retinal cells are packed in the case of the pineal organ with opaque white pigment: similar pigment occurs in smaller quantity in the parapineal organ. Definite sensory cells are also present with rod-like structures projecting into the lumen of the vesicle. Nerve fibres have been traced—from the pineal organ into the posterior commissure and possibly into the right habenular ganglion. As regards other parts of the brain, the chief point to note is that the cerebellum is in a most rudimentary condition, forming merely a slight transverse thickening of the hind-brain roof at its anterior end. In Myxinoids the brain is much larger as compared with the spinal cord, and it differs from that of the lampreys by being relatively much shorter in an anteroposterior direction. A remarkable negative feature lies in the complete absence of the pineal and parapineal organs so conspicuous in the lampreys. The olfactory organ of Cyclostomes is remarkable for two special characteristics, firstly, that the two olfactory organs of other vertebrates are here represented by a single median structure, and secondly,that the olfactory organ becomes sunk down beneath the surface through becoming involved in the ectodermal ingrowth which forms the pituitary tube. As a further consequence in the case of the lampreys the olfactory organ becomes transported to the roof of the head along with the pituitary opening, which latter functions as an external nostril. That the unpaired olfactory organ of existing Cyclostomes has passed through, in their ancestors, a paired condition such as exists in other vertebrates, is indicated by the fact that it retains a pair of olfactory nerves.The eyes in adult lampreys are of moderate size, while in the Myxinoids they are greatly reduced—sunk beneath the skin (Bdellostoma) or even in amongst the muscles of the head (Myxine). The lens is completely absent, also the ocular muscles. The otocyst or auditory organ is unique amongst craniate vertebrates in regard to the semicircular canals. In the lampreys there are only two instead of the normal three, while the Myxinoids have only one.Alimentary Canal.—The widely gaping buccal funnel is morphologically an inpushing of the outer skin,i.e.it is stomodaeal in nature. The thorn-like teeth which stud its lining are formed simply by cornification of the epidermal cells (4) like the provisional horny teeth of a tadpole, and are not homologous with the true teeth of ordinary vertebrates. As to whether they represent the remnant of a once present system of epidermal scales, which may have preceded the coating of placoid elements in the evolution of the vertebrate, there is no evidence.Modified from T. J. Parker, Zootomy, fig. 4, by permission of Macmillan & Co., Ltd.Fig. 2.—Median longitudinal section through anterior end ofPetromyzon.a.v.o, Atrio-ventricular opening.br, Brain.br.o, Internal opening of gill sac.d.a, Dorsal aorta.d.c, Ductus cuvieri.h.v, Hepatic vein.i.j.v, Inferior jugular vein.N, Notochord.oes, Oesophagus.olf, Olfactory organ.pc, Pericardium.p.c.v, Left posterior cardinal vein.pit, Pituitary tube.V, Ventricle.v, Velum.The pharyngeal region, closely associated with the respiratory function, possesses, on each side, a series of gill-sacs (six inMyxine: seven inPetromyzon, besides an anterior one which is laid down in the embryo but disappears later: up to as many as fourteen inBdellostoma) opening on the one hand to the pharynx and on the other to the exterior. InBdellostomaand in the larva ofPetromyzonthe gill-sacs open directly from the pharynx to the exterior, but in the adult lamprey and inMyxinethe original relations are modified. InMyxine, the external openings of the gill-sacs have migrated backwards along the side of the body and become coincident at a point slightly posterior to the last sac. It follows from this that each sac is connected with the common aperture by a tube, longest in the case of the first sac, shortest in the case of the last. In the adult lamprey a different modification is found. Here the dorsal portion of the pharynx has become nipped off as a narrow tube which functions as an oesophagus from the larger ventral portion, which forms an elongated saccular structure ending blindly at its hinder end and having in its lateral wall the internal openings of the gill-sacs.Breathing.—The inspiratory current passes inwards by the mouth opening in the larval lamprey, by the pituitary tube inMyxine, while in the adult lamprey both expiration and inspiration takes place through the external gill-openings. In the case of the lampreys the elastic skeleton of the branchial region (see below) plays an important part in respiration. The branchial region shows rhythmic contraction through the agency of the transverse muscles—and expansion, through the elasticity of the branchial skeleton—in the adult lamprey. These rhythmic movements of the branchial region cause successive inflow and outflow through the branchial openings. In the larva, on the other hand, the respiratory current always passes in one direction—backwards. This is helped by the presence of a velar fold at the front end of the pharynx, which acts as a valve opening only backwards, and to the presence of membranous flaps projecting back from the anterior border of each gill-opening and acting as valves which open only outwards.Behind the pharynx comes the truly digestive part of the alimentary canal in the form of a straight tube showing little differentiation into special regions. The lining of the intestine is increased in area by an inwardly projecting fold, which is compared by some morphologists with the spiral valve of certain other groups. In the mature river lamprey the digestive tract becomes in great part degenerate.Coelomic Organs.—The chief point of interest about the splanchnocoele or perivisceral cavity is that in the Myxinoids the adult shows a persistent embryonic condition in that the pericardiac portion never becomes isolated from the main body cavity.The renal organs are of special interest in the Myxinoids from their very simple character. The kidney duct is seen running along the roof of the coelom on either side. Into the duct open short segmentally arranged tubes, each possessing at its closed rounded extremity a Malpighian body. Each of these short tubes is morphologically a nephric tubule, which, however, in correlation with its shortness, is without the turns and twists so characteristic of such tubules generally. A further consequence of the short simple character of the tubules is that they are quite separate from one another, instead of being massed together to form a compact gland such as the kidney is elsewhere. InPetromyzonthe kidney has the ordinary compact form, and here also the Malpighian bodies are shut off from the splanchnocoele.The ovary or testis is a large unpaired structure hanging from the dorsal wall of the splanchnocoele and shedding its products into it; from the coelomic space the genital products pass into the urogenital sinus—formed by the fusion of the kidney ducts at their hinder ends—through a small opening, one at each side. This opening, which leads directly from coelom into urogenital sinus, is known as the genital pore. Its morphological significance is doubtful.Skeleton.—The vertebral column of the lamprey is represented by a persistent notochord surrounded by a thick sheath, which shows no signs of invasion by cartilage cells or of segmentation. Resting on the sheath are paired dorsal arch elements, more numerous than the neuromuscular segments. In the tail region these are united into a continuous band of cartilage on each side: similar cartilaginous bands represent the ventral arch elements of the tail region. The skeleton of the head region consists of a cartilaginous cranium, into the formation of which enter typical parachordal and trabecular elements, together with olfactory and auditory capsules. In addition to these, there are a number of other cartilaginous pieces present in the head region, the homologies of which are doubtful.Branchial Basket.—One of the most characteristic features of the skeleton of the lamprey is the remarkable cartilaginous “branchial basket,” which supports the gill region. In an adult river lamprey the basketwork consists on each side of a series of eight vertical half-hoops of cartilage. The hoops of each side are connected together dorsally by a pair of longitudinal bars, lying ventral to the notochord, and ventrally by a similar pair of rods which are fused in the middle line. Slender cartilaginous projections arise from the anterior and posterior sides of the hoops, and certain of these meeting at their ends form additional longitudinal bars connecting together successive hoops. Connected with the basketwork posteriorly is a remarkable cup-shaped cartilage, which supports the hind wall of the pericardium. The series of cartilaginous half-hoops naturally suggest the half-hoops of cartilage which form the skeleton of the visceral arches in the Gnathostomata. They are, however, more superficial in position, and this has led many to doubt their actual homology with the cartilaginous visceral arches. Taking into account, however, our present knowledge of the development of the two sets of structures, it seems on the whole probable that a true homology exists and that the branchial basket of the lamprey represents merely a set of visceral arches modified in accordance with the peculiar breathing methods of the creature. In the Myxinoids the branchial basket is reduced to a few vestigial masses of cartilage.Vascular System.—The heart (5) of the lamprey consists of an atrium and a single ventricle, the atrium on the left, the ventricle on the right. Into the atrium, on its right side, and behind the atrio-ventricular opening, there opens a nearly vertical chamber usually termed the sinus venosus (see below), the opening guarded by a pair of vertically placed valves. The ventricle passes anteriorly into what is clearly the homologue of the conus arteriosus of other forms. In its interior are present a pair of laterally placed longitudinal ridges similar to the ridges which occur in other forms in the conus. The opening from ventricle into conus is guarded by a pair of laterally placed pocket valves situated just within the boundary of the ventricle.The arterial system is of the ordinary piscine type. From the heart there passes forwards a ventral aorta, split into two separate vessels in its anterior half, and giving off on each side a series of efferent vessels to the gill-sacs, one passing between each two gill-sacs and an additional one to the front wall of the front sac and to the posterior wall of the last. The blood is collected from the walls of the gill-sacs by a series of efferent vessels which open into the dorsal aorta. It is to be noted that the dorsal aorta retains the probably primitive unpaired condition, except for a very short extent at its anterior end, where it is split so as to form two short aortic roots.Venous System.—The main venous channels are like those in other fishes, though their connexion with the heart becomes modified in the adult. The two posterior cardinals—with their continuations forwards, the anterior cardinals—approach the median plane and undergo fusion in the region of their opening into the two ductus Cuvieri. The left ductus Cuvieri then atrophies so that all the blood from the cardinals reaches the heart by way of the originally rightductus Cuvieri. It is this right ductus Cuvieri which forms the dorsal part of what is usually termed the sinus venosus. The inferior jugular veins which return the blood from the ventral side of the head also become replaced in the adult by a median unpaired vein which opens posteriorly into the sinus venosus by what probably represents the hinder end of the original right inferior jugular. It is interesting to note that inPolypterus, one of the Crossopterygian ganoids, there is a somewhat similar asymmetrical condition of inferior jugulars and ductus Cuvieri.Oviposition of Lamprey(6).—The lamprey chooses as spawning ground a part of the stream with fairly rapid current and where the bottom is composed of sand with scattered stones. By means of the suctorial mouth, stones are removed from more or less circular area so as to form a shallow excavation. The male and female frequently work together at the task of preparing the nest. When oviposition is about to take place, the male may be seen to suddenly attach himself to the dorsal surface of the head of the female which holds on to one of the stones at the upper margin of the nest. The urogenital opening of the male, with its specially prominent papilla, is approximated to that of the female, and with a peculiar quivering movement the eggs and sperms are emitted synchronously amidst clouds of sand stirred up by the movements of the tail. The eggs fertilized thus at the moment of exit are very sticky from their coating of albumen, and become weighted down by adherent grains of sand.Development.—The development of the lamprey is of much morphological importance from the archaic nature of the creature and from the fact that the egg is comparatively small (about 1 mm. in diameter), so that development is not greatly modified by a large mass of yolk. It has been worked out so far only in the river lamprey (7). Segmentation is complete and unequal. It, as well as the process of gastrulation, agrees in its main features with the same phenomenon inAmia, Dipnoans and Urodele amphibians. The blastopore persists as the anal opening of the adult. The mesoderm arises in a manner closely comparable with that which occurs inAmphioxus, the chief difference being that the mesoderm segments are solid instead of hollow, except in the anterior head region, where they are true hollow enterocoelic pouches. The rudiment of the central nervous system has the form of a solid keel-like ingrowth of ectoderm along the mid-dorsal line, which only secondarily becomes hollowed out—just as happens in Teleostean fishes. The young lamprey, after completing its embryonic development, passes three or four years, in fact its whole life up to the time of sexual maturity, in a prolonged larval condition in which its structure shows important differences from that of the adult. This larval stage of the fresh-water lamprey of Europe was long supposed to be a separate genus of Cyclostomes and was calledAmmocoetes. TheAmmocoeteslives in the mud and breathes and feeds by means of a current of water produced by ciliary action, which carries Flagellates and other microscopic organisms in through the mouth opening. Correlated with this mode of feeding the buccal cavity is without the teeth so characteristic of the adult. A number of complicated branched sensory processes grow into and nearly occlude the cavity, forming a kind of sieve with only narrow chinks through which the ingoing current passes. The water passes out by the gill openings, which inAmmocoetesopen direct from pharynx to exterior. Certain arrangements of the pharyngeal wall ofAmmocoetesshow a remarkable resemblance to what is found inAmphioxus. The thyroid, which in the adult is a complicated ductless gland, has in the youngAmmocoetesthe form of a longitudinal groove of the ventral wall of the pharynx. This groove is lined by columnar cells, some carrying cilia, others being glandular and secreting sticky slime. These gland cells are arranged in four longitudinal bands. The thyroid is, in fact, in this stage in a condition corresponding exactly with the endostyle ofAmphioxus. The agreement extends to function the secretion, forming sticky threads which entangle food particles. Anteriorly a pair of peripharyngeal bands pass dorsalwards, one on each side, to bend back suprapharyngeal bands which are continued to the hinder end of the pharynx. Here again the resemblance to what occurs inAmphioxusis very close.TheAmmocoetespossesses a functional liver with bileduct, while in the adult river lamprey the alimentary canal is degenerate. It has no arch elements on its notochord. Its eyes are sunk beneath the surface and nonfunctional, and they retain to a great extent an embryonic character (8). There is a rapid process of metamorphosis from the larval to the adult condition, the details of which are by no means sufficiently known. After the metamorphosis the now mature lamprey accomplishes the act of reproduction and then apparently dies almost immediately. The development of the Myxinoids is much less well known than that of the lampreys. As regards the common hagfish (Myxine glutinosa), we are indeed still in complete ignorance in regard to its developmental history in spite of persistent efforts to obtain embryological material. It seems probable that during the breeding period the hagfishes retire into some particularly inaccessible habitat. Within the last few years, however, abundant material illustrating the developmental history ofBdellostoma(9) has been obtained on the Californian coast, and this when fully worked out will give us a good idea of the general lines of Myxinoid development. The egg differs greatly from that of the lampreys. It is—as is that ofMyxine—of large size, richly yolked and of a shortened-up sausage shape. It measures about 22 mm. by 8 mm. Surrounding the egg is a protective capsule of a yellow horny appearance. At one end a cap-like portion of this forms a detachable operculum, in the middle of which is a minute opening, the micropyle. Each end of the capsule is prolonged into a group of stiff processes with anchor-like expansions at their tips. Segmentation is, as in other richly yolked eggs, incomplete, confined to the germinal disk at the opercular pole. The central nervous system inBdellostomadevelops by the overarching of medullary folds, not out of a solid keel as is the case with the lampreys.History in Time.—The softness of the skeletal tissues and the absence of scales in Cyclostomata provide little opportunity for the preservation of fossil remains of this group, and no known fossils can be referred with certainty to the Cyclostomata. The DevonianPalaeospondylus gunnihas been regarded as a Cyclostome by some authors, but this relationship is at the least doubtful. Other authors have associated the Ostracoderms, the oldest known vertebrates, with this group.References.—1. D. S. Jordan and B. W. Evermann,Fishes of North and Middle America(Washington, 1896), part i. p. 8;2. L. Plate,SB. Ges. Naturf.(Berlin, Jg. 1897), p. 137;3. F. Studnicka in Oppel’sLehrbuch der vergleichenden mikroskopischen Anatomie der Wirbeltiere(Jena, 1905), Teil v. s. i.;4. E. Warren,Q. J. Micr. Sci.xlv. (1902) p. 631;5. L. Vialleton,Arch. d’anat. micr.T. vi. (1903) p. 283;6. H. A. Surface in D. S. Jordan’sFishes(1905), vol. i. p. 494;7. A. E. Shipley,Q. J. Micr. Sci.xxvii. (1887), W. B. Scott,Journ. Morphol.i. (1887), C. Kupffer,Arch. mikr. Anat.xxxv. (1890), A. Goette,Entwick. des Flussneunauges(Hamburg and Leipzig, 1890);8. C. Kohl, inBibliotheca zoologica, Heft 13 (Cassel, 1892);9. Bashford Dean in Kupffer’sFestschrift(Jena, 1899).
It will be convenient, in describing the structural features of the group, to take as a basis for the description the marine lamprey,Petromyzon marinus. A marine lamprey is an eel-like creature 70 to 75 cm. in length. At the anterior end and situated somewhat ventrally is the circular widely gaping mouth or buccal cavity, its lining studded with sharply pointed thorn-like “teeth” and its edge fringed with numerous sensory papillae. On the dorsal side of the head is the conspicuous circular pituitary opening with prominent lips, while on the sides are seen the eyes, and behind these a row of somewhat rounded branchial openings or gill-clefts. At about the beginning of the posterior fourth of the body, and in the midventral line, is the anal opening, and immediately behind it is the prominent papilla carrying the opening of the urogenital sinus. The hinder portion of the body, in accordance with its function in locomotion, is flattened from side to side, while its surface is increased by the development of a median fin fold, divided, except in early stages of development, into three portions, known as the first and second dorsal fins and the caudal fin. The last mentioned is of the primitive protocercal type. The whole surface of the body—which shows a conspicuous dark marbling, especially dorsally, on a light ground—is covered with highly glandular epidermis. An important feature is the complete absence of all trace of the calcified placoid plates which are so characteristic of the Elasmobranchii.
The Myxinoids differ from the lampreys in regard to several of the above-mentioned characters. The edges of the mouth carry tentacle-like barbels. The pituitary opening is close to the anterior edge of the mouth opening instead of being right up on the dorsal side of the head. The eyes are invisible, being greatly reduced and sunk far below the surface, and inMyxine, though not inBdellostoma, the row of gill openings is represented by a single opening on each side nearly in the midventral line and situated at about the end of the first quarter of the body length. Ventrally the Myxinoid possesses on each side of the body a row of remarkable epidermal glands which can produce at will enormous quantities of glutinous slime. This secretion, which, no doubt, is of much value as a protection from attack, is composed of very fine threads, formed by the conversion of the protoplasm of certain cells of the epidermal glands (“thread cells”) into an extremely fine, tightly coiled filament, which becomes unwound when discharged to the exterior.
Pituitary Tube.—A remarkable peculiarity of the Cyclostomes lies in the fact that the pituitary ingrowth of ectoderm does not, as in other forms, become involved in the inpushing of ectoderm which forms the buccal cavity. On the contrary, it lies outside the edge of the stomodaeum, and in the case of the lampreys active growth takes place in the tissue between the pituitary and stomodaeal ingrowths, so that the two openings come to be widely separated, the pituitary opening being pushed back on to the dorsal side of the head. The pituitary opening remains patent throughout life, as is the case with Crossopterygians alone amongst Gnathostomata. InMyxinea further remarkable peculiarity in regard to the hypophysis, probably adaptive in nature, occurs, inasmuch as the pituitary invagination develops an opening at its posterior end into the pharynx.
Nervous System.—The anterior end of the nervous tube is enlarged and differentiated to form a brain as in other Vertebrates, but this brain in the lampreys at least shows remarkably primitive features. The enlargement as compared with the spinal cord is seen to be comparatively slight: the brain is much elongated, and its various regions lie in a straight line one behind the other: the roof of the brain retains to a great extent the primitive epithelial condition. On each side anteriorly there is present a comparatively large olfactory lobe, and this is continued posteriorly into a small cerebral hemisphere.
The lampreys are amongst those vertebrates in which there is an eye-like apparatus (3) connected with the roof of the thalamencephalon. There grow out from the roof of the thalamencephalon two processes, a posterior (the pineal process), and an anterior (the parapineal process). The pineal process grows forwards so as to overlie the parapineal process. Each of these projections from the roof of the thalamencephalon dilates to form a vesicle, and each vesicle shows certain eye-like characteristics, its deep wall forming a “retina” and its superficial wall being clear and translucent (“pellucida”). The retinal cells are packed in the case of the pineal organ with opaque white pigment: similar pigment occurs in smaller quantity in the parapineal organ. Definite sensory cells are also present with rod-like structures projecting into the lumen of the vesicle. Nerve fibres have been traced—from the pineal organ into the posterior commissure and possibly into the right habenular ganglion. As regards other parts of the brain, the chief point to note is that the cerebellum is in a most rudimentary condition, forming merely a slight transverse thickening of the hind-brain roof at its anterior end. In Myxinoids the brain is much larger as compared with the spinal cord, and it differs from that of the lampreys by being relatively much shorter in an anteroposterior direction. A remarkable negative feature lies in the complete absence of the pineal and parapineal organs so conspicuous in the lampreys. The olfactory organ of Cyclostomes is remarkable for two special characteristics, firstly, that the two olfactory organs of other vertebrates are here represented by a single median structure, and secondly,that the olfactory organ becomes sunk down beneath the surface through becoming involved in the ectodermal ingrowth which forms the pituitary tube. As a further consequence in the case of the lampreys the olfactory organ becomes transported to the roof of the head along with the pituitary opening, which latter functions as an external nostril. That the unpaired olfactory organ of existing Cyclostomes has passed through, in their ancestors, a paired condition such as exists in other vertebrates, is indicated by the fact that it retains a pair of olfactory nerves.
The eyes in adult lampreys are of moderate size, while in the Myxinoids they are greatly reduced—sunk beneath the skin (Bdellostoma) or even in amongst the muscles of the head (Myxine). The lens is completely absent, also the ocular muscles. The otocyst or auditory organ is unique amongst craniate vertebrates in regard to the semicircular canals. In the lampreys there are only two instead of the normal three, while the Myxinoids have only one.
Alimentary Canal.—The widely gaping buccal funnel is morphologically an inpushing of the outer skin,i.e.it is stomodaeal in nature. The thorn-like teeth which stud its lining are formed simply by cornification of the epidermal cells (4) like the provisional horny teeth of a tadpole, and are not homologous with the true teeth of ordinary vertebrates. As to whether they represent the remnant of a once present system of epidermal scales, which may have preceded the coating of placoid elements in the evolution of the vertebrate, there is no evidence.
a.v.o, Atrio-ventricular opening.
br, Brain.
br.o, Internal opening of gill sac.
d.a, Dorsal aorta.
d.c, Ductus cuvieri.
h.v, Hepatic vein.
i.j.v, Inferior jugular vein.
N, Notochord.
oes, Oesophagus.
olf, Olfactory organ.
pc, Pericardium.
p.c.v, Left posterior cardinal vein.
pit, Pituitary tube.
V, Ventricle.
v, Velum.
The pharyngeal region, closely associated with the respiratory function, possesses, on each side, a series of gill-sacs (six inMyxine: seven inPetromyzon, besides an anterior one which is laid down in the embryo but disappears later: up to as many as fourteen inBdellostoma) opening on the one hand to the pharynx and on the other to the exterior. InBdellostomaand in the larva ofPetromyzonthe gill-sacs open directly from the pharynx to the exterior, but in the adult lamprey and inMyxinethe original relations are modified. InMyxine, the external openings of the gill-sacs have migrated backwards along the side of the body and become coincident at a point slightly posterior to the last sac. It follows from this that each sac is connected with the common aperture by a tube, longest in the case of the first sac, shortest in the case of the last. In the adult lamprey a different modification is found. Here the dorsal portion of the pharynx has become nipped off as a narrow tube which functions as an oesophagus from the larger ventral portion, which forms an elongated saccular structure ending blindly at its hinder end and having in its lateral wall the internal openings of the gill-sacs.
Breathing.—The inspiratory current passes inwards by the mouth opening in the larval lamprey, by the pituitary tube inMyxine, while in the adult lamprey both expiration and inspiration takes place through the external gill-openings. In the case of the lampreys the elastic skeleton of the branchial region (see below) plays an important part in respiration. The branchial region shows rhythmic contraction through the agency of the transverse muscles—and expansion, through the elasticity of the branchial skeleton—in the adult lamprey. These rhythmic movements of the branchial region cause successive inflow and outflow through the branchial openings. In the larva, on the other hand, the respiratory current always passes in one direction—backwards. This is helped by the presence of a velar fold at the front end of the pharynx, which acts as a valve opening only backwards, and to the presence of membranous flaps projecting back from the anterior border of each gill-opening and acting as valves which open only outwards.
Behind the pharynx comes the truly digestive part of the alimentary canal in the form of a straight tube showing little differentiation into special regions. The lining of the intestine is increased in area by an inwardly projecting fold, which is compared by some morphologists with the spiral valve of certain other groups. In the mature river lamprey the digestive tract becomes in great part degenerate.
Coelomic Organs.—The chief point of interest about the splanchnocoele or perivisceral cavity is that in the Myxinoids the adult shows a persistent embryonic condition in that the pericardiac portion never becomes isolated from the main body cavity.
The renal organs are of special interest in the Myxinoids from their very simple character. The kidney duct is seen running along the roof of the coelom on either side. Into the duct open short segmentally arranged tubes, each possessing at its closed rounded extremity a Malpighian body. Each of these short tubes is morphologically a nephric tubule, which, however, in correlation with its shortness, is without the turns and twists so characteristic of such tubules generally. A further consequence of the short simple character of the tubules is that they are quite separate from one another, instead of being massed together to form a compact gland such as the kidney is elsewhere. InPetromyzonthe kidney has the ordinary compact form, and here also the Malpighian bodies are shut off from the splanchnocoele.
The ovary or testis is a large unpaired structure hanging from the dorsal wall of the splanchnocoele and shedding its products into it; from the coelomic space the genital products pass into the urogenital sinus—formed by the fusion of the kidney ducts at their hinder ends—through a small opening, one at each side. This opening, which leads directly from coelom into urogenital sinus, is known as the genital pore. Its morphological significance is doubtful.
Skeleton.—The vertebral column of the lamprey is represented by a persistent notochord surrounded by a thick sheath, which shows no signs of invasion by cartilage cells or of segmentation. Resting on the sheath are paired dorsal arch elements, more numerous than the neuromuscular segments. In the tail region these are united into a continuous band of cartilage on each side: similar cartilaginous bands represent the ventral arch elements of the tail region. The skeleton of the head region consists of a cartilaginous cranium, into the formation of which enter typical parachordal and trabecular elements, together with olfactory and auditory capsules. In addition to these, there are a number of other cartilaginous pieces present in the head region, the homologies of which are doubtful.
Branchial Basket.—One of the most characteristic features of the skeleton of the lamprey is the remarkable cartilaginous “branchial basket,” which supports the gill region. In an adult river lamprey the basketwork consists on each side of a series of eight vertical half-hoops of cartilage. The hoops of each side are connected together dorsally by a pair of longitudinal bars, lying ventral to the notochord, and ventrally by a similar pair of rods which are fused in the middle line. Slender cartilaginous projections arise from the anterior and posterior sides of the hoops, and certain of these meeting at their ends form additional longitudinal bars connecting together successive hoops. Connected with the basketwork posteriorly is a remarkable cup-shaped cartilage, which supports the hind wall of the pericardium. The series of cartilaginous half-hoops naturally suggest the half-hoops of cartilage which form the skeleton of the visceral arches in the Gnathostomata. They are, however, more superficial in position, and this has led many to doubt their actual homology with the cartilaginous visceral arches. Taking into account, however, our present knowledge of the development of the two sets of structures, it seems on the whole probable that a true homology exists and that the branchial basket of the lamprey represents merely a set of visceral arches modified in accordance with the peculiar breathing methods of the creature. In the Myxinoids the branchial basket is reduced to a few vestigial masses of cartilage.
Vascular System.—The heart (5) of the lamprey consists of an atrium and a single ventricle, the atrium on the left, the ventricle on the right. Into the atrium, on its right side, and behind the atrio-ventricular opening, there opens a nearly vertical chamber usually termed the sinus venosus (see below), the opening guarded by a pair of vertically placed valves. The ventricle passes anteriorly into what is clearly the homologue of the conus arteriosus of other forms. In its interior are present a pair of laterally placed longitudinal ridges similar to the ridges which occur in other forms in the conus. The opening from ventricle into conus is guarded by a pair of laterally placed pocket valves situated just within the boundary of the ventricle.
The arterial system is of the ordinary piscine type. From the heart there passes forwards a ventral aorta, split into two separate vessels in its anterior half, and giving off on each side a series of efferent vessels to the gill-sacs, one passing between each two gill-sacs and an additional one to the front wall of the front sac and to the posterior wall of the last. The blood is collected from the walls of the gill-sacs by a series of efferent vessels which open into the dorsal aorta. It is to be noted that the dorsal aorta retains the probably primitive unpaired condition, except for a very short extent at its anterior end, where it is split so as to form two short aortic roots.
Venous System.—The main venous channels are like those in other fishes, though their connexion with the heart becomes modified in the adult. The two posterior cardinals—with their continuations forwards, the anterior cardinals—approach the median plane and undergo fusion in the region of their opening into the two ductus Cuvieri. The left ductus Cuvieri then atrophies so that all the blood from the cardinals reaches the heart by way of the originally rightductus Cuvieri. It is this right ductus Cuvieri which forms the dorsal part of what is usually termed the sinus venosus. The inferior jugular veins which return the blood from the ventral side of the head also become replaced in the adult by a median unpaired vein which opens posteriorly into the sinus venosus by what probably represents the hinder end of the original right inferior jugular. It is interesting to note that inPolypterus, one of the Crossopterygian ganoids, there is a somewhat similar asymmetrical condition of inferior jugulars and ductus Cuvieri.
Oviposition of Lamprey(6).—The lamprey chooses as spawning ground a part of the stream with fairly rapid current and where the bottom is composed of sand with scattered stones. By means of the suctorial mouth, stones are removed from more or less circular area so as to form a shallow excavation. The male and female frequently work together at the task of preparing the nest. When oviposition is about to take place, the male may be seen to suddenly attach himself to the dorsal surface of the head of the female which holds on to one of the stones at the upper margin of the nest. The urogenital opening of the male, with its specially prominent papilla, is approximated to that of the female, and with a peculiar quivering movement the eggs and sperms are emitted synchronously amidst clouds of sand stirred up by the movements of the tail. The eggs fertilized thus at the moment of exit are very sticky from their coating of albumen, and become weighted down by adherent grains of sand.
Development.—The development of the lamprey is of much morphological importance from the archaic nature of the creature and from the fact that the egg is comparatively small (about 1 mm. in diameter), so that development is not greatly modified by a large mass of yolk. It has been worked out so far only in the river lamprey (7). Segmentation is complete and unequal. It, as well as the process of gastrulation, agrees in its main features with the same phenomenon inAmia, Dipnoans and Urodele amphibians. The blastopore persists as the anal opening of the adult. The mesoderm arises in a manner closely comparable with that which occurs inAmphioxus, the chief difference being that the mesoderm segments are solid instead of hollow, except in the anterior head region, where they are true hollow enterocoelic pouches. The rudiment of the central nervous system has the form of a solid keel-like ingrowth of ectoderm along the mid-dorsal line, which only secondarily becomes hollowed out—just as happens in Teleostean fishes. The young lamprey, after completing its embryonic development, passes three or four years, in fact its whole life up to the time of sexual maturity, in a prolonged larval condition in which its structure shows important differences from that of the adult. This larval stage of the fresh-water lamprey of Europe was long supposed to be a separate genus of Cyclostomes and was calledAmmocoetes. TheAmmocoeteslives in the mud and breathes and feeds by means of a current of water produced by ciliary action, which carries Flagellates and other microscopic organisms in through the mouth opening. Correlated with this mode of feeding the buccal cavity is without the teeth so characteristic of the adult. A number of complicated branched sensory processes grow into and nearly occlude the cavity, forming a kind of sieve with only narrow chinks through which the ingoing current passes. The water passes out by the gill openings, which inAmmocoetesopen direct from pharynx to exterior. Certain arrangements of the pharyngeal wall ofAmmocoetesshow a remarkable resemblance to what is found inAmphioxus. The thyroid, which in the adult is a complicated ductless gland, has in the youngAmmocoetesthe form of a longitudinal groove of the ventral wall of the pharynx. This groove is lined by columnar cells, some carrying cilia, others being glandular and secreting sticky slime. These gland cells are arranged in four longitudinal bands. The thyroid is, in fact, in this stage in a condition corresponding exactly with the endostyle ofAmphioxus. The agreement extends to function the secretion, forming sticky threads which entangle food particles. Anteriorly a pair of peripharyngeal bands pass dorsalwards, one on each side, to bend back suprapharyngeal bands which are continued to the hinder end of the pharynx. Here again the resemblance to what occurs inAmphioxusis very close.
TheAmmocoetespossesses a functional liver with bileduct, while in the adult river lamprey the alimentary canal is degenerate. It has no arch elements on its notochord. Its eyes are sunk beneath the surface and nonfunctional, and they retain to a great extent an embryonic character (8). There is a rapid process of metamorphosis from the larval to the adult condition, the details of which are by no means sufficiently known. After the metamorphosis the now mature lamprey accomplishes the act of reproduction and then apparently dies almost immediately. The development of the Myxinoids is much less well known than that of the lampreys. As regards the common hagfish (Myxine glutinosa), we are indeed still in complete ignorance in regard to its developmental history in spite of persistent efforts to obtain embryological material. It seems probable that during the breeding period the hagfishes retire into some particularly inaccessible habitat. Within the last few years, however, abundant material illustrating the developmental history ofBdellostoma(9) has been obtained on the Californian coast, and this when fully worked out will give us a good idea of the general lines of Myxinoid development. The egg differs greatly from that of the lampreys. It is—as is that ofMyxine—of large size, richly yolked and of a shortened-up sausage shape. It measures about 22 mm. by 8 mm. Surrounding the egg is a protective capsule of a yellow horny appearance. At one end a cap-like portion of this forms a detachable operculum, in the middle of which is a minute opening, the micropyle. Each end of the capsule is prolonged into a group of stiff processes with anchor-like expansions at their tips. Segmentation is, as in other richly yolked eggs, incomplete, confined to the germinal disk at the opercular pole. The central nervous system inBdellostomadevelops by the overarching of medullary folds, not out of a solid keel as is the case with the lampreys.
History in Time.—The softness of the skeletal tissues and the absence of scales in Cyclostomata provide little opportunity for the preservation of fossil remains of this group, and no known fossils can be referred with certainty to the Cyclostomata. The DevonianPalaeospondylus gunnihas been regarded as a Cyclostome by some authors, but this relationship is at the least doubtful. Other authors have associated the Ostracoderms, the oldest known vertebrates, with this group.
References.—1. D. S. Jordan and B. W. Evermann,Fishes of North and Middle America(Washington, 1896), part i. p. 8;2. L. Plate,SB. Ges. Naturf.(Berlin, Jg. 1897), p. 137;3. F. Studnicka in Oppel’sLehrbuch der vergleichenden mikroskopischen Anatomie der Wirbeltiere(Jena, 1905), Teil v. s. i.;4. E. Warren,Q. J. Micr. Sci.xlv. (1902) p. 631;5. L. Vialleton,Arch. d’anat. micr.T. vi. (1903) p. 283;6. H. A. Surface in D. S. Jordan’sFishes(1905), vol. i. p. 494;7. A. E. Shipley,Q. J. Micr. Sci.xxvii. (1887), W. B. Scott,Journ. Morphol.i. (1887), C. Kupffer,Arch. mikr. Anat.xxxv. (1890), A. Goette,Entwick. des Flussneunauges(Hamburg and Leipzig, 1890);8. C. Kohl, inBibliotheca zoologica, Heft 13 (Cassel, 1892);9. Bashford Dean in Kupffer’sFestschrift(Jena, 1899).
(J. G. K.)
CYCLOSTYLE(Gr.κύκλος, a circle, andστῦλος, a column), a term used in architecture. A structure composed of a circular range of columns without a core is cyclostylar; with a core the range would be peristyle. This is the species of edifice called by Vitruvius monopteral.
CYGNUS(“The Swan”), in astronomy, a constellation of the northern hemisphere, mentioned by Eudoxus (4th centuryB.C.) and Aratus (3rd centuryB.C.), and fabled by the Greeks to be the swan in the form of which Zeus seduced Leda. Ptolemy catalogued 19 stars, Tycho Brahe 18, and Hevelius 47. In this constellation βCygniis a fine coloured double star, consisting of a yellow star, magnitude 3, and a blue star, magnitude 5½. The fine double star, μCygni, separated by Sir William Herschel in 1779, has magnitudes 4 and 5; it has a companion, of magnitude 7½, which, however, does not form part of the system. A double star,61 Cygni, of magnitudes 5.3 and 5.9, was the first star whose distance was determined; its parallax is 0″.39, and it is therefore the nearest star in the northern hemisphere with the exception of σCentauri. A regular variable, χCygni, has extreme magnitudes of 5 to 13.5, and its period is 406 days.Nova Cygniis a “new” star discovered by Johann Schmidt in 1876. There is also an extended nebula in the constellation.
CYLINDER(Gr.κύλινδρος, fromκυλίνδειν, to roll). A cylindrical surface, or briefly a cylinder, is the surface traced out by a line, named the generatrix, which moves parallel to itself and always passes through the circumference of a curve, named the directrix; the name cylinder is also given to the solid contained between such a surface and two parallel planes which intersect a generatrix. A “right cylinder” is the solid traced out by a rectangle which revolves about one of its sides, or the curved surface of this solid; the surface may also be defined as the locus of a line which passes through the circumference of a circle, and is always perpendicular to the plane of the circle. If the moving line be not perpendicular to the plane of the circle, but moves parallel to itself, and always passes through the circumference, it traces an “oblique cylinder.” The “axis” of a circular cylinder is the line joining the centres of two circular sections; it is the line through the centre of the directrix parallel to the generators. The characteristic property of all cylindrical surfaces is that the tangent planes are parallel to the axis. They are “developable” surfaces,i.e.they can be applied to a plane surface without crinkling or tearing (seeSurface).
Any section of a cylinder which contains the axis is termed a “principal section”; in the case of the solids this section is a rectangle; in the case of the surfaces, two parallel straight lines. A section of the right cylinder parallel to the base is obviously a circle; any other section, excepting those limited by twogenerators, is an ellipse. This last proposition may be stated in the form:—“The orthogonal projection of a circle is an ellipse”; and it permits the ready deduction of many properties of the ellipse from the circle. The section of an oblique cylinder by a plane perpendicular to the principal section, and inclined to the axis at the same angle as the base, is named the “subcontrary section,” and is always a circle; any other section is an ellipse.
The mensuration of the cylinder was worked out by Archimedes, who showed that the volume of any cylinder was equal to the product of the area of the base into the height of the solid, and that the area of the curved surface was equal to that of a rectangle having its sides equal to the circumference of the base, and to the height of the solid. If the base be a circle of radius r, and the height h, the volume is πr²h and the area of the curved surface 2πrh. Archimedes also deduced relations between the sphere (q.v.) and cone (q.v.) and the circumscribing cylinder.
The name “cylindroid” has been given to two different surfaces. Thus it is a cylinder having equal and parallel elliptical bases;i.e.the surface traced out by an ellipse moving parallel to itself so that every point passes along a straight line, or by a line moving parallel to itself and always passing through the circumference of a fixed ellipse. The name was also given by Arthur Cayley to the conoidal cubic surface which has for its equation z(x² + y²) = 2mxy; every point on this surface lies on the line given by the intersection of the planes y = x tan θ, z = m sin 2θ, for by eliminating θ we obtain the equation to the surface.
CYLLENE(mod.Ziria), a mountain in Greece, in the N.E. of Arcadia (7789 ft.). It was specially sacred to Hermes, who was born in a cave on the mountain, and had a temple and an ancient statue on its summit. The name Cyllene belongs also to an ancient port town in Elis, and, owing to doubtful identification with this, to a modern port at Glarentza, and also to some mineral baths a little to the south of it.
CYMA(Gr.κῦμα, wave), in architecture, a moulding of double curvature, concave at one end, convex at the other. When the concave part is uppermost, it is called acyma recta; but if the convex portion is at the top, it is called acyma reversa. When the crowning moulding of an entablature is of the cyma form, it is called a “cymatium.”
CYMBALS(Fr.cymbales; Ger.Becken; Ital.piattiorcinelli), a modern instrument of percussion of indefinite musical pitch, whereas the small ancient cup-shaped cymbals sounded a definite note. Cymbals consist of two thin round plates of an alloy containing 8 parts of copper to two of tin, each having a handle-strap set in the little knob surmounting the centre of the plate. The sound is obtained not by clashing them against each other, but by rubbing their edges together by a sliding movement. Sometimes a weird effect is obtained by suspending one of the cymbals by the strap and letting a drummer execute a roll upon it as it swings; or by holding a cymbal in the left hand and striking it with the soft stick of the bass drum, which produces a sound akin to that of the tam-tam. All gradations ofpianoandfortecan be obtained on the cymbals. The composer indicates his intention of letting the cymbals vibrate by “Let them vibrate,” and the contrary effect by “Damp the sound.” To stop the vibrations the performer presses the cymbals against his chest, as soon as he has played a note. The duration of the vibration is indicated by thevalueof the note placed upon the staff; the name signifies nothing, since the pitch of the cymbals is indefinite. The instrument is played from the same part of the score as the bass drum, unless otherwise indicated bysenza piatti, orpiatti soliif the bass drum is to remain silent. Although cymbals are not often required they form part of every orchestra; their chief use is for marking the rhythm and for producing weird, fantastic effects or adding military colour, and their shrill notes hold their own against a full orchestra playingfortissimo. Cymbals are specially suited for suggesting frenzy, fury or bacchanalian revels, as in the Venus music in Wagner’sTannhäuserand Grieg’sPeer Gyntsuite. Damping gives a suggestion of impending evil or tragedy. Thetimbreof the ancient cymbals is entirely different, more like that of small hand-bells or of the notes of the keyed harmonica. They are not struck full against each other, but by one of their edges, and the note given out by them is higher in proportion as they are thicker and smaller. Berlioz inRomeo and Julietscored for two pairs of cymbals, modelled on some ancient Pompeian instruments no larger than the hand (some are no larger than a crown piece), and tuned toand
The origin of the cymbals must be referred to prehistoric times. The ancient Egyptian cymbals closely resembled our own. The British Museum possesses two pairs, 51⁄3in. in diameter, one of which was found in the coffin of the mummy of Ankhhapē, a sacred musician; they are shown in the same case as the mummy, and have been reproduced by Carl Engel.1Those used by the Assyrians were both plate- and cup-shaped. The Greek cymbals were cup- or bell-shaped, and are to be seen in the hands of fauns and satyrs innumerable in sculptures and on painted vases. The word cymbal is derived fromκύμβη(Lat.cymba), a hollow vessel, andκύμβαλα= small cymbals. During the middle ages the word cymbal was applied to theGlockenspiel, or peal of small bells, and later to the dulcimer, perhaps on account of the clear bell-like tone produced by the hammers striking the wire strings. After the introduction or invention of the keyed dulcimer or clavichord, and of the spinet, the word clavicymbal was used in the Romance languages to denote the varieties of spinet and harpsichord. Ancient cymbals are among the instruments played by King David and his musicians in the 9th-century illuminated MS. known as the Bible of Charles the Bald in the Bibliothèque Nationale, Paris.
The origin of the cymbals must be referred to prehistoric times. The ancient Egyptian cymbals closely resembled our own. The British Museum possesses two pairs, 51⁄3in. in diameter, one of which was found in the coffin of the mummy of Ankhhapē, a sacred musician; they are shown in the same case as the mummy, and have been reproduced by Carl Engel.1Those used by the Assyrians were both plate- and cup-shaped. The Greek cymbals were cup- or bell-shaped, and are to be seen in the hands of fauns and satyrs innumerable in sculptures and on painted vases. The word cymbal is derived fromκύμβη(Lat.cymba), a hollow vessel, andκύμβαλα= small cymbals. During the middle ages the word cymbal was applied to theGlockenspiel, or peal of small bells, and later to the dulcimer, perhaps on account of the clear bell-like tone produced by the hammers striking the wire strings. After the introduction or invention of the keyed dulcimer or clavichord, and of the spinet, the word clavicymbal was used in the Romance languages to denote the varieties of spinet and harpsichord. Ancient cymbals are among the instruments played by King David and his musicians in the 9th-century illuminated MS. known as the Bible of Charles the Bald in the Bibliothèque Nationale, Paris.
(K. S.)