Fig. 367.—Hexabranchus.
Fig. 367.—Hexabranchus.
I must pass by many groups and orders to more aberrant types, represented by the naked-gilled orders, Opisthobranchiata and Nudibranchiata. These gastropods constitute a large sub-order of extremely beautiful molluscs, remarkable in shape, and often brilliant in colour. The distinguishing character of these typical forms consists in the peculiar nature and situation of their breathing organs, which are exposed on the back of the animal or around the anterior part, and are not protected by the mantle. But the situation is varied, and the gills are sometimes placed on each side of the body, respiration being effected by the ciliated surface of the whole.For these and other reasons they have been placed in four groups. Nudibranchs are found in all parts of the world, and are most abundant in depths where the choicest seaweeds and corallines abound. Their fecundity is very great, as many as sixty thousand eggs being deposited by a single female at one time. They are eaten as a luxury where they most abound.
Fig. 368.—Longitudinal section ofPleurobranchus aurantiacus, showing circulation and gills or branchiæ.—(Warne.)
Fig. 368.—Longitudinal section ofPleurobranchus aurantiacus, showing circulation and gills or branchiæ.—(Warne.)
In the Opisthobranchs the branched veins as well as the auricle are placed behind the ventricle of the heart. They differ from Nudibranchs inasmuch as they are usually furnished with a pair of tentacles and labial palpi, or an expansion of the skin like the veil of the larval form. To clearly understand the character of the internal organisation of these curious animals, the longitudinal section given inFig. 368must be consulted:pis the foot;athe mouth, covered above with the veil-like expansion, over which are the tentacles,c; the branchial veins,v, carry the blood to the gills, from which it flows into the heart ath. This disposition is the opposite of that which characterises the Prosobranchus. Another anatomical peculiarity, which may here be referred to, is the direct communication of the system of blood vessels with the surrounding medium; a characteristic common to most other molluscs, and on which depends the changeable external appearance of the animal. In the illustration of Pleurobranchus here given,gindicates the opening of the duct which conveys water direct to the blood, and through which the blood vessels permeate the back and foot. Like the holes in the sponges, it can be filled or emptied at the will of the animal.
Although this, in the main, is the principle of the circulation in most of this order, one branch possesses no special breathing organs, respiration being carried on throughout the naked skin of the body.
With regard to the Nudibranchiata, the group having the most symmetrical form is the extensive family Dorididæ, characterised by differences in the branchiæ, the relative proportion of the mantle to the foot, and variations in the radula and jaws. The general aspect of the genus Doris, although drawn on a small scale, is represented inPlate XVII., Fig.b. The whole sub-order of Nudibranchs has become more generally known and admired since the publication of Alder and Hancock’s monograph with its many attractive coloured illustrations.
These gastropods can be kept alive for some time in a small aquarium if the precaution is observed of often changing the water and adding a little fresh seaweed. Numerous curious microscopic forms of life may be found adhering to them.
Fig. 369.—Aplysia dipilans.
Fig. 369.—Aplysia dipilans.
Tunicata.—The most remarkable group of animals belonging to this sub-order are the Ascidians. They derive their name from the test or tunic, a membranous consistence, in which they dwell, and which often includes calcareous spicules. The test has two orifices, within which is the mantle. Few microscopic spectacles are more interesting than the circulation along this network of muslin-like fabric, and that of the ciliary movement by which the fluid is kept moving. In the transparent species, as Clavelina and Perophora, the ciliary movement is seen to greater advantage. The animals are found adhering to the broad fronds of fuci near low water-mark. They thrive in tanks, and multiply both by fission and budding. Two species are figured inPlate XVII., Figs.iandk, the zooids of which were found arranged in clusters, as represented.
Aplysiidæ(sea-hares), so called on account of a slight resemblance to a crouching hare. The body form is elongated with a partially developed neck and head, oral and dorsal tentacles, and furnished beneath the mantle with a shelly plate to protect the branchiæ. The mouth is provided with horny jaws, and the gizzard is armed with spines, to prepare the food for digestion. The side lobesare thin and large, and are either folded over the back or used in swimming.Fig. 369is a reduced drawing ofA. dipilans.
The Pectinibranchs are known as violet sea-snails, Ianthinidæ and Scalariidæ. The radula consists of numerous rows of pointed teeth arranged in cross series, forming an angle in the middle. There is no central or rachidian tooth, and they have thin trochiform shells adapted for a pelagic life. They are mostly of a violet colour, from which they derive their name, the colour being more vivid on the underside, which is turned up towards the light when the animal is swimming near the surface of the sea (Fig. 370).
Fig. 370.—Ianthinia, Violet Sea-snail.—(Warne.)The bubbleb, drawn somewhat too large, is about to be joined to the anterior end of the float;c.Shell;l.Float;p.Foot;t.Head.
Fig. 370.—Ianthinia, Violet Sea-snail.—(Warne.)
The bubbleb, drawn somewhat too large, is about to be joined to the anterior end of the float;c.Shell;l.Float;p.Foot;t.Head.
The most interesting feature in connection with these oceanic snails is the curious float which they construct to support their egg-capsules. It is a gelatinous raft, in fact, enclosing air-bubbles, which is attached to the foot, the egg capsules being suspended from its under-surface.They are unable to sink so long as they are in connection with their floats, and are therefore often cast on shore during storms, and furnish an endless series of microscopic specimens. The violet snails feed on various kinds of jelly-fish, and occur in shoals.
Pond Snails.—The three families, Limnœidæ, Physidæ, and Chilinidæ, form a special group of the pulminate, sessile-eyed fresh-water snails. The larger family of these belongs to the genus Limnœa, having a compressed and triangular head with two tentacles and eyes placed at their inner base. They are prolific and gregarious, and their ova are enclosed in transparent gelatinous capsules, deposited in continuous series, and firmly glued to submerged stems and leaves of aquatic plants.L. stagnalisis common in all ponds, marshes and slow-running rivers of Great Britain.
Fig. 371.—Ova and young ofLimnæus stagnalis.
Fig. 371.—Ova and young ofLimnæus stagnalis.
One of the species,L. trancatula, is the host of the liver-fluke so fatal to sheep. The fluke parasite passes one stage of its existence in the intestine of the pond snail.
Each ova-sac of Limnœa contains from fifty to sixty ova (represented inFig. 371, ata). If examined with a low power soon after the eggs are deposited, they appear to consist simply of a pellucid protoplasmic substance. In about twenty-four hours a very minute yellowish spot, the nucleus, is discovered near the cell-wall. In another twenty-four hours the nucleus referred to is seen to have assumed a somewhat deeper colour and to contain within it a minute spot—a nucleolus.
On the fourth day the nucleus has changed its position, and isenlarged to double the size; a slightly magnified view is seen atb. On a closer examination a tranverse fissure is seen; this on the eighth day divides the small mass as atc, and the outer wall is thickened. The embryo becomes detached from the side of the cell, and moves with a rotatory motion around the interior; the direction of this motion is from the right to the left, and is always increased when sunlight falls upon it. The increase is gradual up to the eighteenth day, when the changes are more distinctly visible, and the ova crowd down to the mouth of the ova-sac, as atd. By employing a higher magnifying power a minute black spec, the future eye (e) and tentacles of the snail, is quite visible. Upon closely observing it, a fringe of cilia is noticed in motion near the edge of the shell. It is now apparent that the rotatory motion first observed must have been in a great measure due to this; and the current kept up in the fluid contents of the cell by the ciliary fringes. For days after the young animal has escaped from the egg, this ciliary motion is carried on, not alone by the fringe surrounding the mouth, but by cilia entirely surrounding the tentacles themselves, which whips up a supply of nourishment, and at the same time aeration of the blood is effected. From the twenty-sixth to the twenty-eighth day it appears actively engaged near the side of the egg, using force to break through the cell-wall, which at length it succeeds in accomplishing; leaving its shell in the ova-sac, and immediately attaching itself to the side of the glass its ciliary action recommences, and it appears to have advanced a stage, as atf. It is still some months before the embryo grows to the perfect form,Fig. 372; the animal is here shown with its sucker-like foot adhering closely to the glass of the aquarium. A single snail will deposit from two to three of these ova-sacs a week, producing, in the course of six weeks or two months, from 900 to 1,000 young.
Fig. 372.—Limnæus stagnalis(natural size).
Fig. 372.—Limnæus stagnalis(natural size).
The shell itself is deposited in minute cells, which take up a circular position around the axis; on its under-surface a hyaline membrane is secreted. The integument expands, and at various points an internal colouring-matter or pigment is deposited. Theincrease of the animal goes on until the expanded foot is formed, the outer edge of which is rounded off and turned over by condensed tissue in the form of a twisted wire; this encloses a network of small vessels filled with a fluid in constant and rapid motion. The course of the blood or fluid, as it passes from the heart, may be traced through the larger branches to the respiratory organs, consisting of branchial-fringes placed near the mouth; the blood may also be seen returning through other vessels. The heart, a strong muscular apparatus, is pear-shaped, and enclosed within a pericardium or extremely thin and pellucid enveloping membrane. The heart is seen to be furnished with muscular bands of considerable strength, the action of which appears like the alternate to-and-fro motion occasioned by drawing out a band of indiarubber, and which, although so minute, are clearly analogous to the muscular fibres of the mammal heart; it beats or contracts at the rate of about sixty times a minute, and is placed rather far back in the body, towards the axis of the shell. The nervous system is made up of ganglia, or nervous centres, and distributed throughout the various portions of the body.
The singular arrangement of the eye cannot be omitted; it appears at an early stage of life to be within the tentacle, and consequently capable of being retracted into it. In the adult animal the eye is situated at the base of the tentacle; and although it can be protruded at pleasure for a short distance, it seems to depend much upon the tentacle for protection as a coverlid—it invariably draws down the tentacle over the eye when that organ needs protection. The eye itself is pyriform, somewhat resembling the round figure of the human eye-ball, with its optic-nerve attached. In colour it is very dark, having a central pupillary-opening for the admission of light. The tentacle, which is cylindrical in the young animal, becomes flat and triangular in shape in the adult. The tentacles serve in some respect to distinguish species. In Limnœa they are, as I have said, compressed and triangular, with the eyes at their inner base. In Physa they are cylindrical and slender and without lateral mantle lobes. The development of the lingual membrane is delayed; consequently, the young animal does not early take to a vegetable sustenance: in place of teeth it has two rows of cilia, as before stated, which drop off when the teeth are fully formed. The lingual bandbearing the teeth, or the “tongue,” as it is termed, consists of several rows of cutting spines, pointed with silica.
It is a fact of some interest, physiologically, to know that if the young animal is kept in fresh water alone, without vegetable matter of any kind, it retains its cilia, and arrest of development follows, and it more slowly acquires gastric teeth, and attains to perfection in form or size. If, at the same time, it is confined within a narrow cell or space, it grows only to such a size as will enable it to move about freely; thus it is made to adapt itself to the necessities of a restricted state of existence. Some young animals in a narrow glass-cell, at the end of six months, were alive and well; the cilia were seen to be retained around the tentacles in constant activity, whilst other animals of the same brood and age, placed in a situation favourable to growth, attained their full size, and produced young, which grew in three weeks to the size of their elder relations.71
My experimental investigations were further extended to the development of the lingual membrane, or teeth, of Gastropoda, as well as the jaw and radula. In Limnœa, the teeth when fully developed resemble those of Helix; that is to say, in the fully grown animal are found several rows or bands of similar teeth, with simple obtuse cusps and a much suppressed central tooth. In the young snail a high power of the microscope is required to make them out. The dental band, however, in most Mollusca is disposed in longitudinal series, but varies a good deal in this respect, as will be seen on reference to my several papers, with illustrations of upwards of a hundred different species, published in “Linnæan Transactions” of 1866, and in the “Microscopical Society’s Transactions” of 1868. By way of example I may say, in the Pulmonata the lingual band usually consists of a single median row, the laterals on each side being broad and similar. But in many other groups the teeth are arranged in three, five, or seven dissimilar series. Taking Nerita as a type, thebroad teeth on each side of the median are termedlaterals; and the numerous small teeth on the outside of the band, known as thepleuræ, are termeduncini.
Since the investigations of Lovén into the lingual dentition of the Mollusca, various observers have studied the subject, with great advantage to our knowledge of the affinities of these animals. That these investigations have proved of value is shown by the light which has been shed on the true position of many species. When once we have ascertained the homology of a genus, whose relations were otherwise somewhat doubtful, it is surprising how other characteristics, even of the shell, probably misunderstood before, concur to bear out the affinities indicated by the lingual band. These tooth-bearing membranes, armed with sharp cutting points, admirably adapted for the division of the food on which they feed, are most of them beautiful objects for the microscope.
Fig. 373.1. Palate ofBuccinum undatum, common Whelk, seen under polarised light; 2. Palate ofDoris tuberculata, Sea-slug.
Fig. 373.
1. Palate ofBuccinum undatum, common Whelk, seen under polarised light; 2. Palate ofDoris tuberculata, Sea-slug.
The two ends of each longitudinal row of teeth are connected with muscles attached to the upper and lower surfaces of cartilaginous cushions; the alternate contractions and extensions of the muscles cause the bands of teeth to work backwards and forwards, after the fashion of a chain-saw, or rather of a rasp, upon any substance to which it is applied, and the resulting wear and tear of the anterior teeth are made good by a development of new teeth in the secreting sac in which the hinder end of the band is lodged. Besides the chain-saw-like motion of the band the lingual membrane has a kind of licking or scraping action as a whole. With the constant growth of the band new teeth are developed, when the teeth on theextreme portion of the band differ much in size and form from those in the median line.
As I have shown in the papers already referred to, that as each row is a repetition of the first, the arrangement of teeth admits of easy representation by a numerical formula, in which, when the uncini are very numerous, they are indicated by the sign ∞ (infinity), and the others by the proper figure. Thus, ∞ · 5 · 1 · 5 · ∞, which, in the genus Trochus, signifies that each row consists of one median, flanked on both sides by five lateral teeth, and these again by a large number of uncini. When only three areas are found, the outer ones must be considered the pleuræ, inasmuch as there is frequently a manifest division in the membrane between them and the lateral areas.
Most of the Cephalopod molluscs are provided with well-developed teeth, and they are, as we know, carnivorous. The teeth of the cuttle-fish,Sepia officinalis(Plate V., No. 111), resemble those of the Pteropoda, and have the same formula, 3 · 1 · 3. Sepia are also furnished with a retractile proboscis, and a prehensile spiny collar, apparently for the purpose of seizing and holding prey while the teeth are tearing it to pieces. In the squid Loligo (Plate V., No. 113) the median teeth are broad at the base, approach the tricuspid form with a prolonged acute central cusp, while the uncini are much prolonged and slightly curved. The lingual band increases in breadth towards the base, sometimes to twice that of the anterior portion. This band, mounted dry, forms an attractive object for black-ground illumination.
In another family, that of the rock-limpet,Patella radiata, the lingual band (Plate V., No. 116) well serves to distinguish it from the better-known common limpet. It is furnished with a remarkable long ribbon, studded by numerous rows of strong dark-brown tricuspid teeth. The lingual membrane when not in use lies folded up in the abdominal cavity. The teeth of Acmæa are somewhat differently arranged (Plate V., No. 117); their formula is 3 · 1 · 3.
Testacella maugei, belonging to Pulmonifera, is slug-like in appearance, and subterranean in its habits, chiefly feeding on earth-worms. During winter and in dry weather it forms a kind of cocoon, and thus completely encloses itself in an opaque white mantle; in this way it protects itself from frost and cold. Its lingual membraneis large, and covered with about fifty rows of divergent teeth, gradually diminishing in size towards the median row; each tooth is barbed and pointed, broader towards the base, and with an articulating nipple set in the basement membrane. A few rows are represented slightly magnified (Plate V., No. 121). Their formula is 0 0 · 1 · 0 0.
Tongues, etc., of Gasteropods.Tuffen West, del.W. F. Maples, ad. nat. del.Edmund Evans.Plate V.
Tongues, etc., of Gasteropods.
Tuffen West, del.W. F. Maples, ad. nat. del.Edmund Evans.
Plate V.
The boat-shell,Cymba olla, belonging to the Velutinidæ, formula 0 · 1 · 0, or 1 · 1 · 1. The lingual band (Plate V., No. 118) is narrow and ribbon-like in its appearance, with numerous trident-shaped teeth set on a strong muscular membrane. The end of the band and its connection with the muscles at the extremity of the cartilaginous cushion is shown in the drawing. The blueish appearance is produced by a selenite film and polarised light. InScapander ligniariusthe band (Plate V., No. 119) is also narrow, but the teeth are bold and of extraordinary size; their formula is 1 · 0 · 1. This mollusc is said to be eyeless.Pleurobranchus plumulabelongs to the same family; its teeth are simple, recurved, and convex, and arranged in numerous divergent rows, the medians of which are largest. The mandible (Plate V., No. 122) presents an exceedingly pretty tesselated appearance, and the numerous divergent rows of teeth are tricuspid.
The velvety-shell,Velutina lævigata, formula 3 · 1 · 3. The teeth (Plate V., No. 108) are small and fine; medians recurved, with a series of delicate denticulations on either side of the central cusp, which is much prolonged: 1st laterals, denticulate, with outer cusp prolonged; 2nd and 3rd laterals, simple curved or hooked-shaped. The mandible (No. 109), divided in the centre, forms two plates of divergent denticulations.
The ear-shell,Haliotis tuberculatus, is a well-known beautiful shell, much used for ornamental purposes. The lingual band (Plate V., No. 114), is well developed. The medians are flattened-out, recurved obtuse teeth; 1st laterals, trapezoidal or beam-like; uncini numerous, about sixty, denticulate, the few first pairs prolonged into strong pointed cusps.
The top-shell,Turbo marmoratus. After the outer layer of shell is removed, it presents a delicate pearly appearance. Its lingual band (No. 123) closely resembles Trochus; it is long and narrow, the median teeth are broadest, with five recurved laterals, and numerousrows of uncini, slender and hooked. A single row only is represented in the plate.
Cyclotus translucidus, a family of operculate land-shells, belongs to the Cyclostomatidæ. The teeth shown in No. 110, formula 3 · 1 · 3, are arranged in slightly divergent rows on a narrow band; they are more or less subquadrate, recurved, with their central cusps prolonged.Cistula catenata, one of the family Cyclophoridæ; its band (No. 115) formula, 2 · 1 · 2. Its teeth resemble those of Littorina. The lingual band of Cyclostomatidæ points out a near alliance to the Trochidæ; but this question can only be determined by an examination of several species, when it may, perhaps, be decided to give them rank as a sub-order. They are numerous enough; the West Indian islands alone furnish 200 species.
The length of the lingual band, and number of rows of teeth borne on it, vary greatly in different species. But it is among the Pulmonifera we meet with the most astonishing instances of large numbers of teeth.Limax maximuspossesses 26,800, distributed through 180 rows of 160 each, the individual teeth measuring only one 10,000th of an inch.Helix pomatiahas 21,000, and its comparatively dwarfed congener,H. absoluta, no less than 15,000.
Structure of the Shell of Mollusca.—In my opening sketch of the sub-order Mollusca an idea may have been gathered of the general character of the shell covering of these animals. The simplest form of shell occurs in the rudimentary oval plate of the common slug,Limax rufus. It is embedded in the shield situated at the back, near the head of the animal. In the Chitons, a small but singular group of molluscs allied to the univalve limpets, we have an ovoid shell, made up of eight segments, or movable plates, which give them a resemblance to enormous woodlice. These have been regarded as forming a transition series—a link between one division and the other. The shell in by far the greater portion of all the molluscs is developed from cells that in process of growth have become hardened by the deposition of calcareous matter in the interior. This earthy matter consists principally of calcium carbonate deposited in a crystalline state; and in certain shells, as in that of the oyster (Plate XVIII.,Fig. 8), from the animal cell not having sufficientlycontrolled the mode of deposition of the earth particles, they have assumed the form of perfect rhomboidal crystals.72
PLATE XVIII.SECTIONS OF SHELL-STRUCTURE.
PLATE XVIII.
SECTIONS OF SHELL-STRUCTURE.
The shell of the wing-shells,Pinna ingens(Plate XVIII., No. 7), is composed of hexagonal cells, filled with partially translucent calcareous matter, the outer layer of which can be split up into prism-like columns. Figs. 3 and 6 are horizontal sections of theHaliotis splendens, with stellate pigment in a portion of the section, and wavy lines, as in the dentine of the human tooth, and ofTerebratulata rubicuna, showing radiating perforations. Nos. 4 and 5, sections of the shell of a crab, show pigment granules beneath the articular layer and the general hexagonal structure of the next layer.
Some difference of opinion has been expressed with regard to the formation of pearls, but it is now generally understood to be a diseased condition. Pearls are matured on a nucleus, consisting of the same matter as that from which the new layers of shell proceed at the edge of the mussel or oyster. The finest kinds are formed in the body of the animal, or originate in the pearly-looking part of the shell. It is from the size, roundness, and brilliancy of pearls that their value is estimated.
The microscope discloses a difference in the structure of pearls: those having a prismatic cellular structure have a brown horny nucleus, surrounded by small imperfectly-formed prismatic cells;there is also a ring of horny matter, followed by other prisms, and so on, as represented inFig. 374; and all transverse sections of pearls from oysters show the same successive rings of growth or deposit.
Fig. 374.1. A transverse section of a Pearl from Oyster, showing its prismatic structure 2. A transverse section of another Pearl, showing its central cellular structure, with outside rings of true pearly matter. (Magnified 50 diameters.)
Fig. 374.
1. A transverse section of a Pearl from Oyster, showing its prismatic structure 2. A transverse section of another Pearl, showing its central cellular structure, with outside rings of true pearly matter. (Magnified 50 diameters.)
In a segment of a transverse section of a small purple pearl from a species of Mytilus (Fig. 375), all trace of prismatic structure has disappeared, and only a series of fine curved or radiating lines is seen. This pearl consists of a beautiful purple-coloured series of regular laminæ, many of which have a series of concentric zones, and are of a yellow tint. The most beautiful sections for microscopic examination are obtained from Scotch pearls.
Preparation of the Teeth and Shell of Mollusca for Microscopical Examination.—The method of preparing lingual membranes of Mollusca is as follows: Under a dissecting microscope, and with a large bull’s eye lens, cut open and expose to view the floor of themouth; pin back the cut edges throughout its length, and work out the dental band with knife and forceps. The band being detached, place it in a watch-glass, and boil in caustic potash solution for a few minutes. Having by this process freed the tongue from its integuments, remove it, wash it well, and place it for a short time in a dilute acid solution, either acetic or hydrochloric. Wash it well and float it upon a slide; with a fine sable brush open it out flat, and remove whatever dirt or fibre may be adhering to it. Lastly, place it in weak spirit and water, and there let it remain for a few days before mounting in formalin. Canada balsam renders them rather too pellucid, and the finer teeth are thereby lost.
Fig. 375.1. Transverse section of a small Pearl from a Mytilus; 2. Horizontal section magnified 240 diameters to show prismatic structure and transverse striæ.
Fig. 375.
1. Transverse section of a small Pearl from a Mytilus; 2. Horizontal section magnified 240 diameters to show prismatic structure and transverse striæ.
The preparation of shell structure must be proceeded with with some amount of care and caution, or the delicate reticulated network membrane will be destroyed. If any acid solvent be used to remove the calcareous structure it should be much diluted, so that the action may proceed slowly rather than hastily. In the young hermit-crab, for example, where the calcareous and membranous portions of the shell are continuous, and the calcium carbonate ina relatively small proportion, a strong acid solution would entirely destroy the specimen. In the case of nacreous shells the process of cutting and grinding must also be proceeded with with some amount of caution. The operation should be examined as the process proceeds, and under polarised light. Sections of shell structure are usually mounted in Canada balsam. Under the headingTechniquemuch useful information on this and kindred subjects will be found in the “Journal of the Royal Microscopical Society.”
The Annulosa of Huxley embraces the lowest grade of articulated animals, most of which are now grouped with Metazoa, while some writers place them in a sub-kingdom Vermes. It appears to me then only possible to describe this heterogeneous group of worm-like animals among those which resemble each other in certain negative features, but not possessing any of the distinctive characters of those previously described. There are numerous species among Entozoa, every one of which is of the highest interest to mankind in general, and to animal life as a whole. To these I shall devote some attention, from the wide-spread importance attached to them. They are characterised by having a soft absorbent body with little or no colour, in consequence of being excluded from light, living within the bodies of animals and absorbing their vital juices, thereby inflicting a large amount of injury and death upon the whole vertebrate kingdom. They bear in this respect a close analogy to parasitic Fungi in the nature of their destructive action upon plant life, which I have fully discussed in a previous chapter.
The relations which obtain between parasites and their hosts are in all respects conditioned by their natural history; and without a detailed knowledge of the organisation, the development, and the mode of life of the different species, it is impossible to determine the nature and extent of the pathological conditions to which they give rise, and at the same time find means of protection against guests in every way so unwelcome.
The nutritive system of the entozoa must be regarded as in the lowest state of development, yet there are some among them of a higher grade, as will be seen as we proceed. All are remarkablealike for their vast productiveness and for their peculiar metamorphoses. For example, the greater number of the Tænia begin their lives as sexless, encysted larvæ, and on entering their final abode, segments are successively added, until the worm has finally reached the adult stage. Again, the tapeworm of the cat has its origin in the encysted larvæ found in the livers of the mouse and rat. Another species of entozoa inhabit the stomach of the stickle-back, and only attain their perfect form in the stomachs of aquatic birds that feed exclusively on fish. Another infests the mantle of pond-snails, and through their agency, the embryos pass into the stomach of sheep.
An almost endless number of similar transformations take place in other genera. The simplest form among internal parasites is the Gregarinæ, formerly grouped among Protozoa. They consist of a simple limiting membrane, with a mass of granular matter enclosed and surrounding a nucleus (Plate III., No. 53). These parasites pass through a crystoid stage in the body of one of the lower animals, usually the earthworm,Lumbricus agricola. In the more mature organism an envelope, differentiated from the protoplasm within, can be made out (No. 54); this affords an indication of greater differentiation in the subjacent layer of protoplasm. An anterior portion is in many cases separated by a constriction from the cylindrical or band-like body (No. 56). Gregarinæ multiply when encysted, and divide into a multitude of minutepseudo-navicula, so named from their resemblance in shape to a well-known form of Diatomaceæ. When a young pseudo-navicule escapes it behaves somewhat like an amœba, and if perchance it is swallowed by an appropriate host, it develops at once into the higher stage. The various forms are represented inPlate III., Nos. 53—61. Miescher, in 1843, described suchlike bodies, taken from the muscles of a mouse. A good account of specimens obtained from the muscles of a pig was published by the late Mr. Rainey in the “Philosophical Transactions,” 1857. He regarded them as cestoid entozoa. They have been described under a variety of names, as worm-nodules, egg-sacs, eggs of the fluke, young measles, &c. M. Lieberkühn carefully traced the pseudo-naviculæ after leaving the perivisceral cavity of the earth-worm; he found large numbers of small corpuscles, exhibiting amœba-likemovements, as well as pseudo-naviculæ, containing granules, formed in an encysted Gregarinæ. He imagines that these latter bodies burst, and that their contained granules develop into the amœbiform bodies which subsequently become Gregarinæ.
Professor Ray Lankester made a careful examination of more than a hundred worms for the purpose of studying these questions, but he succeeded in arriving at no other conclusion than that certain forms may be the by-products of encysted Gregarinæ. TheG. lumbricusis one of those forms which are unilocular. The vesicle is not always very distinct, and is sometimes altogether absent; occasionally it contains no granules, sometimes several, one of which is generally nucleated. In other of these cysts a number of nucleated cells may be seen developing from the enclosed Gregarina, which gradually become fused together and broken up, until the entire mass is converted into nucleated bodies, often seen in different stages of development, assuming the form of a double cone, as that presented by certain species of Diatomaceæ. At length the cyst contains nothing but pseudo-naviculæ, sometimes enclosing granules; these gradually disappear, and finally the cyst bursts. Encystation seems to take place much more rarely among the bilocular forms of Gregarinæ than in the unilocular species found in the earthworm and other Annelids.73
Dr. J. Leidy published in the “Transactions of the Philadelphia Society,” 1853, the results of his examinations of several new species of Gregarinæ. He described a double membrane “within the parietal tunic of the posterior sac, this being transparent, colourless, and marked by a most beautiful set of exceedingly regular parallel longitudinal lines.”
Professor R. Leuckart is the latest writer on the parasites of animals, and to him we are indebted for a more systematic account of the whole group, and their life-history, than to any previous investigator. I can only attempt to give a mere outline of the developmental stages of a few typical forms of parasites, commencing with the cystic tapeworm, Tænia. These worms are ribbon-like in appearance, and are divided throughout the greater part of their length into segments, and their usual habitation is the intestinalcavity of vertebrate animals. The anterior extremity of a tænoid worm is usually called the head, and bears the organ by which the animal attaches itself to the mucous membrane of the creature which it infests. These organs are either suckers, or hooks, or both conjoined. In Tænia, four suckers are combined with a circlet of hooks, disposed around a median terminal prominence. The embryo passes through certain stages of development—viz., four forms or changes: but the embryo itself is very peculiar, consisting of an oval non-ciliated mass, provided with six hooks, three upon each side of the middle line. Tænia are found enclosed in various situations besides that of the alimentary canal: the eye, the brain, the muscular tissues, the liver, &c., of animals. The following cystic worms are usually included in this genera,Cysticercus Anthocephalus,Cœnurus, andT. Echinococcus.Plate IV., No. 100, shows an adult specimen of the latter with rostellum suckers, and three successive segments, the last of which is the ova sac. The water-vascular system is represented coloured by carmine. This parasite infests the human body as frequently as many other species. My accurately-drawn figure is copied from Cobbold’s “Introduction to the Study of Entozoa.”
Cysticercus fasciolarisis developed within the liver of white mice;Cysticercus cellulosæin the muscles of the pig; hence we have the diseased state of pork familiarly known as “measly pork.” Should a lamb become infested with Tænia the final transformation will be different; within a fortnight symptoms of a disease known as “staggers” manifest themselves, and in the course of a few weeks theCœnurus cerebraliswill be developed within the brain. Von Siebold pointed out the bearing of this fact upon the important practical problem of the prevention of “staggers.” Others belonging to the same class of parasites are quite as remarkable in their preference for the alimentary canal of fishes. The Echinorhynchus is developed in the intestinal canal of the flounder,Triænophorus nodulusin the liver of the salmon. Thus, by careful and repeated observation with the microscope, a close connection is found to exist between the cystic and cestoid entozoa.
The Echinococcus (Plate IV., No. 101) infests the human liver. These parasites are always found in cysts, and in closed cavities in the interior of the body. They are united in fours by a very shortstalk or pedicle, common to the whole. By an increase of magnification the contents of a cyst present the several structures represented inFig. 376.
Echinorhynchus, or spiny-headed threadworms, constitute a group of entozoa which undergo a metamorphosis hardly, perhaps, less remarkable than that known to take place in other Nematode worms. Leuckart instituted, in 1861, a series of experiments with the ova ofEchinorhynchus proteusfound parasitic upon theGammarus pulex. The ova ofE. proteusresemble in form and structure those of allied species. They are of a fusiform shape, surrounded with two membranes, anexternalof a more albuminous nature, and aninternalchitinous one. When the eggs reach the intestine the outer of these membranes is absent, being in fact digested, while the inner remains intact until ruptured by the embryo.
Fig. 376.—Cystic Disease of Liver (Human).a.Cyst with Echinococcus enclosed;b.detached hooklets from the head of Echinococcus, magnified 250 diameters;c.crystals found in cyst, chiefly cholesterine;d.cylindrical epithelium, some enclosed in structureless vesicles;e.Puro-muculent granules, fat and blood corpuscles.
Fig. 376.—Cystic Disease of Liver (Human).
a.Cyst with Echinococcus enclosed;b.detached hooklets from the head of Echinococcus, magnified 250 diameters;c.crystals found in cyst, chiefly cholesterine;d.cylindrical epithelium, some enclosed in structureless vesicles;e.Puro-muculent granules, fat and blood corpuscles.
The typical Threadworm belonging to the order Nematoidea infest the intestines of children, and are a source of much suffering. The egg is elliptical, and contains a mass of granular protoplasm, the external wall of which soon becomes marked out into a layer of cells. The mouth of the worm appears as a depression at the end of the blunt head. When the muscular system and alimentary canal are developed the embryo hatches out, some few of which are free living forms; most of them lead a parasitic life. Their reproduction is enormous, representing thousands of eggs and embryos.
Of the non-parasitic species of thread-worm, the common vinegar eel, Anguillula,74affords an example. This is found in polluted water, bog-moss, and moist earth, as well as in vinegar; also in the alimentary canal of the pond-snail, the frog, fish, &c. Another species is met with in the ears of wheat affected with a blight termed the “cockle”; another, theA. glutinis, in sour paste. If grains of the affected wheat are soaked in water for an hour or two before they are cut open, the so called “eels” will be found. The paste-eel makes its appearance spontaneously just as the pasty mass is turning sour; the means of securing a supply for microscopical examination consists in allowing a portion of the paste in which they show themselves to dry up, and laying it by for stock; if at any time a portion of this is introduced into a little fresh-made paste, and the whole kept warm and moist for a few hours, it will be found to swarm with these wriggling little worms. A small portion of paste spread over the face of a Coddington lens is a ready way of viewing them.
Trichina spiralis.—One of the smallest and most dangerous of all human internal parasites isT. spiralis, since it finds its way into the muscles throughout the human body. The young animal presents the form of a spirally-coiled worm in the interior of a minute oval-shaped cyst (Plate IV., No. 104), a mere speck scarcely visible to the naked eye. In the muscular structure it resembles a small millet seed, somewhat calcareous in composition. The history of the development of Trichina in the human muscle is briefly that in a few hours after the ingestion of infected pork, Trichina, disengaged from the muscle, will be found in the stomach: hence they pass into the small intestine, where they are further developed. Continuing their migrations, they penetrate far into the interior of the primitive muscular fasciculi, where they will be found, in about three days after ingestion, in considerable numbers, and so far developed that the young entozoa have almost attained a size equal to that of the full-grown Trichina (Plate IV., No. 105). They quickly advance into the interior of the muscular fasciculi, where they liveand multiply in continuous series, while the surrounding structures as well as the muscular tissue undergo a process of histolysis. The destructive nature of the parasite is very great.
The number of progeny produced by one female may amount to several thousands, and as soon as they leave the egg they either penetrate through the blood-vessels, or are carried on by the circulation, and ultimately become lodged in the muscles situated in the most distant parts of the body. Here, as already explained, they become encysted.
Fig. 377.—Monads in Rat’s Blood, stained with methyl violet, showing membrane under different aspects; blood-corpuscles, some crenated and others with stained discs (× 1,200).—(Crookshank.)
Fig. 377.—Monads in Rat’s Blood, stained with methyl violet, showing membrane under different aspects; blood-corpuscles, some crenated and others with stained discs (× 1,200).—(Crookshank.)
Professor Virchow draws the following conclusions:—“1. The ingestion of pig’s flesh, fresh or badly dressed, containing Trichinæ, is attended with the greatest danger, and may prove the proximate cause of death. 2. The Trichinæ maintain their living properties in decomposed flesh; they resist immersion in water for weeks together, and when encysted may, without injury to their vitality, be plunged in a sufficiently dilute solution of chromic acid for at least ten days. 3. On the contrary, they perish and are deprived of all noxious influence in ham which has been well smoked, kept a sufficient length of time, and then well boiled before it is consumed.”
A more minute Filarian worm has been detected in the human blood-vessels, known asFilaria sanguinis hominis. This worm carries on its work of destruction throughout the night; during the day it remains perfectly passive. It increases rapidly, and produces swellings of the glandular structures of the body, somewhat after the nature of those characteristic of the Bombay plague, with aslight difference, that after death the swellings are seen to be due to the vast accumulations of theFilaria sanguinisblocking the blood-vessels. The accompanyingFig. 377shows a similar infiltration of monads in the blood of rats dying of plague in Bombay.
Trematode Worms.—In the order Trematoda, to which the fluke belongs, the body is unsegmented, and to the naked eye smooth throughout, with a blood circulatory system, and two suctorial discs at the hinder end. There is a distinct digestive canal, usually forked, furnished with only one aperture, the mouth. The excretory organs open out as in tape worms, and the male and female organs co-exist in the same individual.
The Fluke (shown inPlate IV., No. 103) is cone-shaped, and is theAmphistome conicumof Rudolphi. This parasite is common in oxen, sheep, and deer, and it has also been found in the Dorcas antelope. It invariably takes up its abode in the first stomach, or rumen, attaching itself to the papillated folds of the mucous membrane. In the full-grown, adult stage, it rarely exceeds half an inch in length. It is certainly one of the most remarkable in form and organisation of any of the internal parasites.
The larger fluke (Fasciola hepatica) often attains to an inch or more in size. It is not only of frequent occurrence in all varieties of grazing cattle, but has likewise been found in the horse, the ass, and also in the hare and rabbit and other animals. Its occurrence in man has been recorded by more than one observer. The oral sucker forming the mouth leads to a short œsophagus, which very soon divides into two primary stomachal or intestinal trunks, the latter in their turn sending off branches; the whole together forming that attractive dendritic system of vessels so often compared to plant-venation. This remarkably-formed digestive apparatus is represented inPlate IV., Nos. 106 and 107,Fasciola giganteaof Cobbold, and should be contrasted with the somewhat similarly racemose character of the water-vascular system. Let it be expressly noted, however, that in the digestive system the majority of the tubes branch out in a direction obliquely downwards, whereas those of the vascular system slope obliquely upwards. A further comparison of the disposition of these two systems of structure, with the same systems figured and described as characteristic of the Amphistoma, will at once serve to demonstrate the important differenceswhich subsist between the several members of the two genera, if we turn to the consideration of the habits ofFasciola hepatica, which, in so far as they relate to excitation of the liver disease in sheep, acquire the highest practical importance. Intelligent cattle-breeders, agriculturists, and veterinarians have all along observed that therot, as this disease is commonly called, is particularly prevalent after long-continued wet weather, and more especially so if there have been a succession of wet seasons; and from this circumstance they have very naturally inferred that the humidity of the atmosphere, coupled with a moist condition of the soil, forms the sole cause of the malady. Co-ordinating with these facts, it has likewise been noticed that the flocks grazing in low pastures and marshy districts are much more liable to the invasion of this endemic disease than are those pasturing on higher and drier grounds; a noteworthy exception occurring in the case of those flocks feeding in the salt-water marshes on our eastern shores.Plate IV., No. 106,Fasciola gigantea: the anterior surface is exposed to display oral and ventral suckers, and the dendriform digestive apparatus injected with ultra-marine; No. 107 shows the dorsal aspect of the specimen and the multiramose character of the water-vascular system, the vessels being injected with vermilion.
In their larval condition the Amphistoma live in or upon the body of the pond-snail. This we infer from the circumstance that the larvæ, or cercariæ, of a closely-allied species, theAmphistoma subclavatum, are known to infest the alimentary canal of frogs and newts, and have also been found on the body of the Planorbis by myself. The cercariæ larvæ are taken, it is believed, by the sheep and the cattle while drinking. The earliest embryotic stage in which I have found the embryo fluke is represented atFig. 378, No. 1. In the year 1854, whilst observing the habits of Limnœa and other water-snails, I brought home specimens from the ornamental water in the Botanic Gardens; upon these were discovered thousands of minute thread-like worms, subsequently met with on other embryos, and at first taken to be simple infusorial animals, but upon placing them in a glass vessel these minute bodies were observed to detach themselves and commence a free-swimming existence. A fringe of cilia was seen to surround the flask-shaped body (No. 1).