In Front of the conus vestige of the Teleost there is present a thick walledbulbus aortaediffering from the conus in not being rhythmically contractile, its walls being on the contrary richly provided with elastic tissue.
In Front of the conus vestige of the Teleost there is present a thick walledbulbus aortaediffering from the conus in not being rhythmically contractile, its walls being on the contrary richly provided with elastic tissue.
The Dipnoans37show an important advance on the conus as in atrium and ventricle. The conus has a characteristic spiral twist. Within it inNeoceratodusare a number oflongitudinalrows of pocket valves. One of these rows is marked out by the very large size of its valves and by the fact that they are not distinct from one another but even in the adult form a continuous, spirally-running, longitudinal fold. This ridge projecting into the lumen of the conus divides it incompletely into two channels, the one beginning (i.e.at its hinder end) on theleftside and ending in frontventrally, the other beginning on therightand endingdorsally. InProtopterusa similar condition occurs, only in the front end of the conus a second spiral fold is present opposite the first and, meeting this, completes the division of the conus cavity into two separate parts. The rows of pocket valves which do not enter into the formation of the spiral folds are here greatly reduced.
These arrangements in the conus of the Dipnoans are of the highest morphological interest, pointing in an unmistakable way towards the condition found in the higher lung-breathing vertebrates. Of the two cavities into which the conus is partially divided in the Dipneusti the one which begins posteriorly on the right receives the (venous) blood from the right side of the heart, and ending up anteriorly dorsal to the other cavity communicates only with aortic arches V. and VI. In the higher vertebrates this cavity has become completely split off to form the root of the pulmonary arteries, and a result of aortic arch V. receiving its blood along with the functionally much more important VI. (the pulmonary arch) from this special part of the conus has been the almost complete disappearance of this arch (V.) in all the higher vertebrates.
a.c.v, Anterior cardinal vein.
d.C, Ductus Cuvieri
h.v, Hepatic vein.
i.j.v, Inferior jugular vein.
ir.v, Inter-renal vein.
l.v, Lateral cutaneous vein.
p.c.v, Posterior cardinal vein.
p.n, Pronephros.
p.v, Pulmonary vein.
s, Subclavian vein.
s.v, Sinus venosus.
th, Thyroid.
v, Vein from pharyngeal wall.
* Anterior portion of left posterior cardinal vein.
Arterial System.—There are normally six aortic arches laid down corresponding with the visceral arches, the first (mandibular) and second (hyoidean) undergoing atrophy to a less or greater extent in post-embryonic life. Where an external gill is present the aortic arch loops out into this, a kind of short-circuiting of the blood-stream taking place as the external gill atrophies. As the walls of the clefts assume their respiratory function the aortic arch becomes broken into a network of capillaries in its respiratory portion, and there is now distinguished a ventral afferent and a dorsal efferent portion of each arch. Complicated developmental changes, into which it is unnecessary to enter,38may lead to each efferent vessel draining the two sides of a single cleft instead of the adjacent walls of two clefts as it does primitively. In the Crossopterygians and Dipnoans as in the higher vertebrates the sixth aortic arch gives off the pulmonary artery to the lung. Among the Actinopterygians this, probably primitive, blood-supply to the lung (swimbladder) persists only inAmia.
Venous System.—The most interesting variations from the general plan outlined have to do with the arrangements of the posterior cardinals. In the Selachians these are in their anterior portion wide and sinuslike, while in the region of the kidney they become broken into a sinusoidal network supplied by the postrenal portion now known as the renal portal vein. In the Teleostomes the chief noteworthy feature is the tendency to asymmetry, the right posterior cardinal being frequently considerably larger than the left and connected with it by transverse anastomotic vessels, the result being that most of the blood from the two kidneys passes forwards by the right posterior cardinal. The Dipnoans (fig. 27) show a similar asymmetry, but here the anterior end of the right posterior cardinal disappears, being replaced functionally by a new vessel which conveys the blood from the right posterior cardinal direct to the sinus venosus instead of to the outer end of the ductus Cuvieri. This new vessel is the posterior vena cava which thus in the series of vertebrates appears for the first time in the Dipneusti.
Pulmonary Veins.—InPolypterus(fig. 28) the blood is drained from the lungs by a pulmonary vein on each side which unites in front with its fellow and opens into the great hepatic vein behind the heart. In the Dipnoans the conjoined pulmonary veins open directly into the left section of the atrium as in higher forms. In the Actinopterygians with their specialized air-bladder the blood passes to the heart via posterior cardinals, or hepatic portal, or—a probably more primitive condition—directly into the left ductus Cuvieri (Amia).
Lymphatics.—More or less irregular lymphatic spaces occur in the fishes as elsewhere and, as in the Amphibia, localized muscular developments are present forming lymph hearts.
Central Nervous System.—The neural tube shows in very early stages an anterior dilated portion which forms the rudiment of the brain in contradistinction to the hinder, narrower part which forms the spinal cord. This enlargement of the brain is correlated with the increasing predominance of the nervecentres at the anterior end of the body which tend to assume more and more complete control over those lying behind.
Spinal Cord.—A remarkable peculiarity occurs in the sun fishes (Molidae), where the body is greatly shortened and where the spinal cord undergoes a corresponding abbreviation so as to be actually shorter than the brain.
Brain.—It is customary to divide the brain into three main regions, fore-, mid-, and hind-brain, as in the most familiar vertebrates there is frequently seen in the embryo a division of the primitive brain dilatation into three vesicles lying one behind the other. A consideration of the development of the brain in the various main groups of vertebrates shows that these divisions are not of equal importance. In those archaic groups where the egg is not encumbered by the presence of a large mass of yolk it is usual for the brain to show in its early stages a division into two main regions which we may term the primitive fore-brain or cerebrum and the primitive hind-brain or rhombencephalon. Only later does the hinder part of the primitive fore-brain become marked off as mid-brain. In the fully developed brain it is customary to recognize the series of regions indicated below, though the boundaries between these regions are not mathematical lines or surfaces any more than are any other biological boundaries:—
The myelencephalon or medulla oblongata calls for no special remark, except that in the case ofTorpedothere is a special upward bulging of its floor on each side of the middle line forming the electric lobe and containing the nucleus of origin of the nerves to the electric organ.
cer, Cerebellum.
c.h, Cerebral hemisphere.
th, Thalamencephalon.
f.b, Primitive fore-brain (in B the line points to the thickened wall of the fore-brain, the so-called “basal ganglia”).
G.p, Pineal body.
m.b, Roof of mid-brain, optic lobes,tectum opticum.
o.l, Olfactory lobe.
IV.v, Fourth ventricle.
The cerebellum occurs in its simplest form in lampreys and Dipnoans (fig. 29, C), where it forms a simple band-like thickening of the anterior end of the roof of the hind-brain. In Selachians it is very large and bulges upwards, forming a conspicuous organ in a dorsal view of the brain (fig. 29, A). In Teleosts (fig. 29, B) the cerebellum is also large. It projects back as a great tongue-like structure over the roof of the fourth ventricle, while in front it dips downwards and projects under the roof of the mid-brain forming a highly characteristicvalvula cerebelli. Avalvula cerebellioccurs also in ganoids, while in the Crossopterygians a similar extension of the cerebellum projects backwards into the IV. ventricle or cavity of the hind-brain (fig. 30).
The mesencephalon is a conspicuous structure in the fishes from its greatly developed roof (tectum opticum) which receives the end pencils of the optic nerve. Normally it projects upwards as a pair of large optic lobes, but in the Dipnoans (fig. 29, C) the lateral thickening is not sufficiently great to cause obvious lateral swellings in external view.
a.c, Anterior commissure.
cer, Cerebellum.
d.s, Dorsal sac.
g.h, Habenular ganglion.
h.c, Habenular commissure.
i.g, Infundibular gland.
l.p, Lateral plexus.
o.c, Optic chiasma.
pall, Pallium.
par, Paraphysis.
pin, Pineal body.
p.c, Posterior commissure.
s.v, Saccus vasculosus.
t.o, Tectum opticum.
v.III, Third ventricle.
v.IV, Fourth ventricle.
vel, Velum transversum.
The thalamencephalon is one of the most interesting parts of the brain from its remarkable uniformity throughout the Vertebrata. Even inAmphioxusthe appearance of a sagittal section strongly suggests vestiges of a once present thalamencephalon.39The roof—like that of the myelencephalon—remains to a great extent membranous, forming with the closely appliedpia matera vascular roof to the III. ventricle. Frequently a transverse fold of the roof dips down into the III. ventricle forming thevelum transversum(fig. 30).
The side walls of the thalamencephalon are greatly thickened forming thethalamus(epithalamus and hypothalamus), while a ganglionic thickening of the roof posteriorly on each side forms theganglia habenulaewhich receive olfactory fibres from the base of the hemisphere. The habenular ganglia are unusually large in the lampreys and are here strongly asymmetrical, the right being the larger.
The floor of the thalamencephalon projects downwards and backwards as the infundibulum. The side walls of this are thickened to form characteristiclobi inferiores, while the blind end develops glandular outgrowths (infundibular gland, fig. 30) overlaid by a rich development of blood sinuses and forming with them thesaccus vasculosus. The optic chiasma, where present, is involved in the floor of the thalamencephalon and forms a large, upwardly-projecting ridge. Farther forwards on the floor or anterior wall is the anterior commissure (see below).
Passing forwards from the mid-brain (cf. fig. 30) a series of interesting structures are found connected with the roof of the primitive fore-brain, viz.—posterior commissure (intercalary region), pineal organ, habenular commissure with anterior parietal organ, dorsal sac (= pineal cushion),velum transversum, paraphysis. The posterior commissure is situated in the boundary between thalamencephalon and mid-brain. It is formed offibres connecting up the right and left sides of the tectum opticum (?). The habenular or superior commissure situated farther forwards connects the two ganglia habenulae. In the immediate neighbourhood of these ganglia there project upwards two diverticula of the brain-roof known as the pineal organ and the parapineal (or anterior parietal) organ. The special interest of these organs40lies in the fact that in certain vertebrates one (parapineal inSphenodonand in lizards) or both (Petromyzon) exhibit histological features which show that they must be looked on as visual organs or eyes. In gnathostomatous fishes they do not show any definite eye-like structure, but in certain cases (Polyodon,Callichthys, &c.) the bony plates of the skull-roof are discontinuous over the pineal organ forming a definite parietal foramen such as exists in lizards where the eye-like structure is distinct. It is also usual to find in the epithelial wall of the pineal organ columnar cells which show club-shaped ends projecting into the lumen (exactly as in the young visual cells of the retina41) and are prolonged into a root-like process at the other end. Definite nerve fibres pass down from these parietal organs to the brain. It is stated that the fibres from the pineal organ pass into the posterior commissure, those of the parapineal organ into the habenular commissure.
The facts mentioned render it difficult to avoid the conclusion that these organs either have been sensory or are sensory. Possibly they represent the degenerate and altered vestiges of eye-like organs present in archaic vertebrates, or it may be that they represent the remains of organs not eye-like in function but which for some other reason lay close under the surface of the body. It would seem natural that a diverticulum of brain-tissue exposed to the influence of light-rays should exhibit the same reaction as is shown frequently elsewhere in the animal kingdom and tend to assume secondarily the characters of a visual organ. The presence of the rod-like features in the epithelial cells is perhaps in favour of the latter view. In evolution we should expect these to appear before the camera-like structure of a highly developed eye, while in the process of degeneration we should expect these fine histological characters to go first.
Selachians.—No parapineal organ is present. The pineal body (except inTorpedowhere it is absent) is in the form of a long slender tube ending in front in a dilated bulb lying near the front end of the brain in close contact with, or enclosed in, a definite foramen in the cranial roof.Holocephali and Crossopterygii.—Here also the pineal body is long and tubular: at its origin it passes dorsalwards or slightly backwards behind the large dorsal sac.Actinopterygian Ganoids resemble Selachians on the whole. InAmiaa parapineal organ is present, and it is said to lie towards the left side and to be connected by a thick nerve with thelefthabenular ganglion (cf.Petromyzon, articleCyclostomata). This is adduced to support the view that the pineal and parapineal bodies represent originally paired structures.Teleostei.—A parapineal rudiment appears in the embryo of some forms, but in the adult only the pineal organ is known to exist. This is usually short and club-shaped, its terminal part with much folded wall and glandular in character. In a few cases a parietal foramen occurs (Callichthys,Loricaria, &c.).Dipneusti.—The pineal organ is short and simple. No parapineal organ is developed.
Selachians.—No parapineal organ is present. The pineal body (except inTorpedowhere it is absent) is in the form of a long slender tube ending in front in a dilated bulb lying near the front end of the brain in close contact with, or enclosed in, a definite foramen in the cranial roof.
Holocephali and Crossopterygii.—Here also the pineal body is long and tubular: at its origin it passes dorsalwards or slightly backwards behind the large dorsal sac.
Actinopterygian Ganoids resemble Selachians on the whole. InAmiaa parapineal organ is present, and it is said to lie towards the left side and to be connected by a thick nerve with thelefthabenular ganglion (cf.Petromyzon, articleCyclostomata). This is adduced to support the view that the pineal and parapineal bodies represent originally paired structures.
Teleostei.—A parapineal rudiment appears in the embryo of some forms, but in the adult only the pineal organ is known to exist. This is usually short and club-shaped, its terminal part with much folded wall and glandular in character. In a few cases a parietal foramen occurs (Callichthys,Loricaria, &c.).
Dipneusti.—The pineal organ is short and simple. No parapineal organ is developed.
The dorsal sac is formed by that part of the roof of the thalamencephalon lying between the habenular commissure and the region of the velum. In some cases a longitudinal groove is present in which the pineal organ lies (Dipneusti). In the Crossopterygians the dorsal sac is particularly large and was formerly mistaken for the pineal organ.
Thevelum transversumis a transverse, inwardly-projecting fold of the roof of the primitive fore-brain in front of the dorsal sac. To those morphologists who regard the hemisphere region or telencephalon as a primitively unpaired structure the velum is an important landmark indicating the posterior limit of the telencephalon. Those who hold the view taken in this article that the hemispheres are to be regarded as paired outpushings of the side wall of the primitive fore-brain attribute less morphological importance to the velum. Physiologically the velum is frequently important from the plexus of blood-vessels which passes with it into the III. ventricle.
InPetromyzonandChimaerathe velum is not developed. In Dipnoans there are present in its placepairedtransverse folds which are probably merely extensions backwards of the lateral plexuses.
The Paraphysis is a projection from the roof of the primitive fore-brain near its anterior end. It is well seen in Dipnoans42(LepidosirenandProtopterus) where in the larva (exactly as in the urodele larva) it forms a blindly ending tube sloping upwards and forwards between the two hemispheres. In the adult it becomes mixed with the two lateral plexuses and is liable to be confused with them. In the other groups—except the Teleosts where it is small (Anguilla) or absent (most Teleosts)—the paraphysis is by no means such a definite structure, but generally there is present a more or less branched and divided diverticulum of the brain wall, frequently glandular, which is homologized with the paraphysis. The morphological significance of the paraphysis is uncertain. It may represent the remains of an ancient sense organ, or it may simply represent the last connexion between the brain and the external ectoderm from which it was derived.
An important derivative of the primitive fore-brain is seen in the pair of cerebral hemispheres which in the higher vertebrates become of such relatively gigantic dimensions. The hemispheres appear to be primitively associated with the special sense of smell, and they are prolonged anteriorly into a pair of olfactory lobes which come into close relation with the olfactory organ. From a consideration of their adult relations and of their development—particularly in those groups where there is no disturbing factor in the shape of a large yolk sac—it seems probable that the hemispheres are primitively paired outpushings of the lateral wall of the primitive fore-brain43—in order to give increased space for the increased mass of nervous matter associated with the olfactory sense. They are most highly developed in the Dipneusti amongst fishes. They are there (cf. fig. 29, C) of relatively enormous size with thick nervous floor (corpus striatum) and side walls and roof (pallium) surrounding a central cavity (lateral ventricle) which opens into the third ventricle. At the posterior end of the hemisphere a small area of its wall remains thin and membranous, and this becomes pushed into the lateral ventricle by an ingrowth of blood-vessel to form the huge lateral plexus (=plexus hemisphaerium). In this great size of the hemispheres44and also in the presence of a rudimentary cortex in the Dipnoi we see, as in many other features in these fishes, a distinct foreshadowing of conditions occurring in the higher groups of vertebrates. The Cyclostomes possess a distinct though small pair of hemispheres. In the Selachians the relatively archaicNotidanidae45possess a pair of thick-walled hemispheres, but in the majority of the members of the group the paired condition is obscured (fig. 29, A).
In the Teleostomes the mass of nervous matter which in other groups forms the hemispheres does not undergo any pushing outwards except as regards the small olfactory lobes. On the contrary, it remains as a great thickening of the lateral wall of the thalamencephalon (the so-called basal ganglia), additional space for which, however, may be obtained by a considerable increase in length of the fore-brain region (cf. fig. 30, A) or by actual involution into the third ventricle (Polypterus).46The great nervous thickenings of the thalamencephalic wall bulge into its cavity and are covered over by the thin epithelial roof of the thalamencephalon which is as a consequence liable to be confused with the pallium or roof of the hemispheres with which it has nothing to do: the homologue of the palliumas of other parts of the hemisphere is contained within the lateral thickening of the thelamencephalic wall, not in its membranous roof.47
Associated with the parts of the fore-brain devoted to the sense of smell (especially the corpora striata) is the important system of bridging fibres forming the anterior commissure which lies near the anterior end of the floor, or in the front wall, of the primitive fore-brain. It is of great interest to note the appearance in theDipnoans(LepidosirenandProtopterus) of a corpus callosum (cf. fig. 30 B) lying dorsal to the anterior commissure and composed of fibres connected with the pallial region of the two hemispheres.
Sense Organs.—The olfactory organs are of special interest in the Selachians, where each remains through life as a widely-open, saccular involution of the ectoderm which may be prolonged backwards to the margin of the buccal cavity by an open oronasal groove, thus retaining a condition familiar in the embryo of the higher vertebrates. In Dipnoans the olfactory organ communicates with the roof of the buccal cavity by definite posterior nares as in the higher forms—the communicating passage being doubtless the morphological equivalent of the oronasal groove, although there is no direct embryological evidence for this. In the Teleostomes the olfactory organ varies from a condition of great complexity in the Crossopterygians down to a condition of almost complete atrophy in certain Teleosts (Plectognathi).48
Theeyesare usually of large size. The lens is large and spherical and in the case of most Teleostomes accommodation for distant vision is effected by the lens being pulled bodily nearer the retina. This movement is brought about by the contraction of smooth muscle fibres contained in theprocessus falciformis, a projection from the choroid which terminates in contact with the lens in a swelling, thecampanula Halleri. InAmiaand in Teleosts a network of capillaries forming the so-called choroid gland surrounds the optic nerve just outside the retina. As a rule the eyes of fishes have a silvery, shining appearance due to the deposition of shining flakes of guanin in the outer layer of the choroid (Argentea) or, in the case of Selachians, in the inner layers (tapetum). Fishes which inhabit dark recesses,e.g.of caves or of the deep sea, show an enlargement, or, more frequently, a reduction, of the eyes. Certain deep-sea Teleosts possess remarkable telescopic eyes with a curious asymmetrical development of the retina.49
The otocyst or auditory organ agrees in its main features with that of other vertebrates. In Selachians the otocyst remains in the adult open to the exterior by theductus endolymphaticus. InSquatina50this is unusually wide and correlated; with this the calcareous otoconia are replaced by sand-grains from the exterior. In Dipnoans (LepidosirenandProtopterus) curious outgrowths arise from the ductus endolymphaticus and come to overlie the roof of the fourth ventricle, recalling the somewhat similar condition met with in certain Amphibians.
In various Teleosts the swimbladder enters into intimate relations with the otocyst. In the simplest condition these relations consist in the prolongation forwards of the swimbladder as a blindly ending tube on either side, the blind end coming into direct contact either with the wall of the otocyst itself or with the fluid surrounding it (perilymph) through a gap in the rigid periotic capsule. A wave of compression causing a slight inward movement of the swimbladder wall will bring about a greatly magnified movement of that part of the wall which is not in relation with the external medium, viz. the part in relation with the interior of the auditory capsule. In this way the perception of delicate sound waves may be rendered much more perfect. In the Ostariophysi (Sagemehl), including theCyprinidae, theSiluridae, theCharacinidaeand theGymnotidae, a physiologically similar connexion between swimbladder and otocyst is brought about by the intervention of a chain of auditory ossicles (Weberian ossicles) formed by modification of the anterior vertebrae.51
In various Teleosts the swimbladder enters into intimate relations with the otocyst. In the simplest condition these relations consist in the prolongation forwards of the swimbladder as a blindly ending tube on either side, the blind end coming into direct contact either with the wall of the otocyst itself or with the fluid surrounding it (perilymph) through a gap in the rigid periotic capsule. A wave of compression causing a slight inward movement of the swimbladder wall will bring about a greatly magnified movement of that part of the wall which is not in relation with the external medium, viz. the part in relation with the interior of the auditory capsule. In this way the perception of delicate sound waves may be rendered much more perfect. In the Ostariophysi (Sagemehl), including theCyprinidae, theSiluridae, theCharacinidaeand theGymnotidae, a physiologically similar connexion between swimbladder and otocyst is brought about by the intervention of a chain of auditory ossicles (Weberian ossicles) formed by modification of the anterior vertebrae.51
Lateral Line Organs.52—Epidermal sense buds are scattered about in the ectoderm of fishes. A special arrangement of these in lines along the sides of the body and on the head region form the highly characteristic sense organs of the lateral line system. InLepidosirenthese organs retain their superficial position; in other fishes they become sunk beneath the surface into a groove, which may remain open (some Selachians), but as a rule becomes closed into a tubular channel with openings at intervals. It has been suggested that the function of this system of sense organs is connected with the perception of vibratory disturbances of comparatively large wave length in the surrounding medium.
Peripheral Nerves.—In the Cyclostomes the dorsal afferent and ventral efferent nerves are still, as inAmphioxus, independent, but in the gnathostomatous fishes they are, as in the higher vertebrates, combined together into typical spinal nerves.
As regards the cranial nerves the chief peculiarities of fishes relate to (1) the persistence of the branchial clefts and (2) the presence of an elaborate system of cutaneous sense organs supplied by a group of nerves (lateralis) connected with a centre in the brain which develops in continuity with that which receives the auditory nerve. These points may be exemplified by the arrangements in Selachians (see fig. 31). I., II., III., IV. and VI. call for no special remark.
bucc, Buccal.
c, Commissure between pre- and postauditory parts of lateralis system.
d.r, Dorsal roots of spinal nerves.
g.g, Gasserian ganglion.
gn.g, (Geniculate) ganglion of VII.
hy, Hyomandibular.
l.n.X, Lateralis vagi.
m, Motor branches ofhy.
md, Mandibular.
md.ex, External mandibular.
mk.c, Meckel’s cartilage.
mx, Maxillary.
oc, Occipitospinal.
ol.o, Olfactory organ.
op.p, Ophthalmicus profundus.
op.s, Ophthalmicus superficialis.
pn, Palatine.
pq., Palatopterygo-quadrate cartilage.
s, Spiracle.
st, Supra-temporal branch of lateralis system.
t.a, Lateralis centre in brain.
v.n, Visceral nerve.
v.r, Ventral roots.
Trigeminus(V.).—Theophthalmicus profundusbranch (op.p.)—which probably is morphologically a distinct cranial nerve—passes forwards along the roof of the orbit to the skin of the snout. As it passes through the orbit it gives off the long ciliary nerves to the eyeball, and is connected with the small ciliary ganglion (also connected with III.) which in turn gives off the short ciliary nerves to the eyeball. Theophthalmicus superficialis(cut short in the figure) branch passes from the root ganglion of V. (Gasserian ganglion), and passes also over the orbit to the skin of the snout. It lies close to, or completely fused with, the corresponding branch of the lateralis system.
The main trunk of V. branches over the edge of the mouth into themaxillary(mx.) andmandibular(md.) divisions, the former, like the two branches already mentioned, purely sensory, the latter mixed—supplying the muscles of mastication as well as the teeth of the lower jaw and the lining of the buccal floor.
The main trunk of theFacialis(VII.) bifurcates over thespiracle into a pre-spiracular portion—the main portion of which passes to the mucous membrane of the palate as the palatine (pnVII.)—and a postspiracular portion, the hyomandibular (hy.) trunk which supplies the muscles of the hyoid arch and also sends a few sensory fibres to the lining of the spiracle, the floor of mouth and pharynx and the skin of the lower jaw. Combined with the main trunk of the facial are branches belonging to thelateralissystem.
Lateralis Group of Nerves.—Thelateralisgroup of nerves are charged with the innervation of the system of cutaneous sense organs and are all connected with the same central region in the medulla. A special sensory area of the ectoderm becomes involuted below the surface to form the otocyst, and the nerve fibres belonging to this form the auditory nerve (VIII.). Other portions of thelateralisgroup become mixed up with various other cranial nerves as follows:
(a) Facial portion.
(1)Ophthalmicus superficialis(op.s.VII.): passes to lining of nose or to the lateral line organs of the dorsal part of snout.
(2)Buccal(bucc.VII): lies close to maxillary division of V. and passes to the sensory canals of the lower side of the snout.
(3)External mandibular(md.ex.): lies in close association with the mandibular division of V., supplies the sensory canals of the lower jaw and hyoid region.
Lateralis vagi(l.n.X.) becomes closely associated with the vagus. It supplies the lateral line organs of the trunk.
In the lamprey and in Dipnoans thelateralis vagiloses its superficial position in the adult and comes into close relation with the notochord.
In Actinopterygians and at least some Selachians alateralisset of fibres is associated with IX., and in the former fishes a conspicuous trunk oflateralisfibres passes to some or all (Gadus) of the fins. This has been called thelateralis accessoriusand is apparently connected with V., VII., IX., X. and certain spinal nerves.53
Vagus Group(IX., X., XI.).—Theglossopharyngeus(IX.) forks over the first branchial cleft (pretrematic and post-trematic branches) and also gives off a palatine branch (pn.IX.). In some cases (various Selachians, Ganoids and Teleosts) it would seem that IX. includes a few fibres of thelateralisgroup.
Vagus (X.) is shown by its multiple roots arising from the medulla and also by the character of its peripheral distribution to be a compound structure formed by the fusion of a number of originally distinct nerves. It consists of (1) a number of branchial branches (X.¹ X.² &c.), one of which forks over each gill cleft behind the hyobranchial and which may (Selachians) arise by separate roots from the medulla; (2) an intestinal branch (v.n.X.) arising behind the last branchial and innervating the wall of the oesophagus and stomach and it may be even the intestine throughout the greater part of its length (Myxine).
Theaccessorius(XI.) is not in fishes separated as a distinct nerve from the vagus.
With increased development of the brain its hinder portion, giving rise to the vagus system, has apparently come to encroach on the anterior portion of the spinal cord, with the result that a number of spinal nerves have become reduced to a less or more vestigial condition. The dorsal roots of these nerves disappear entirely in the adult, but the ventral roots persist and are to be seen arising ventrally to the vagus roots. They supply certain muscles of the pectoral fins and of the visceral arches and are known as spino-occipital nerves.54
These nerves are divisible into an anterior more ancient set—the occipital nerves—and a posterior set of more recent origin—(occipito-spinal nerves). In Selachians 1-5 pairs of occipital nerves alone are recognizable: in Dipnoans 2-3 pairs of occipital and 2-3 pairs of occipito-spinal: in Ganoids 1-2 pairs occipital and 1-5 pairs occipito-spinal; in Teleosts finally the occipital nerves have entirely disappeared while there are 2 pairs of occipito-spinal. In Cyclostomes no special spino-occipital nerves have been described.
These nerves are divisible into an anterior more ancient set—the occipital nerves—and a posterior set of more recent origin—(occipito-spinal nerves). In Selachians 1-5 pairs of occipital nerves alone are recognizable: in Dipnoans 2-3 pairs of occipital and 2-3 pairs of occipito-spinal: in Ganoids 1-2 pairs occipital and 1-5 pairs occipito-spinal; in Teleosts finally the occipital nerves have entirely disappeared while there are 2 pairs of occipito-spinal. In Cyclostomes no special spino-occipital nerves have been described.
The fibres corresponding with those of theHypoglossus(XII.) of higher vertebrates spring from the anterior spinal nerves, which are here, as indeed in Amphibia, still free from the cranium.
Sympathetic.—The sympathetic portion of the nervous system does not in fishes attain the same degree of differentiation as in the higher groups. In Cyclostomes it is apparently represented by a fine plexus with small ganglia found in the neighbourhood of the dorsal aorta and on the surface of the heart and receiving branches from the spinal nerves. In Selachians also a plexus occurs in the neighbourhood of the cardinal veins and extends over the viscera: it receives visceral branches from the anterior spinal nerves. In Teleosts the plexus has become condensed to form a definite sympathetic trunk on each side, extending forwards into the head and communicating with the ganglia of certain of the cranial nerves.
(J. G. K.)
V. Distribution in Time and Space
The origin of Vertebrates, and how far back in time they extend, is unknown. The earliest fishes were in all probability devoid of hard parts and traces of their existence can scarcely be expected to be found. The hypothesis that they may be derived from the early Crustaceans, or Arachnids, is chiefly based on the somewhat striking resemblance which the mailed fishes of the Silurian period (Ostracodermi) bear to the Arthropods of that remote time, a resemblance, however, very superficial and regarded by most morphologists as an interesting example of mimetic resemblance—whatever this term may be taken to mean. The minute denticles known as conodonts, which first appear in the Ordovician, were once looked upon as teeth of Cyclostomes, but their histological structure does not afford any support to the identification and they are now generally dismissed altogether from the Vertebrates. As a compensation the Lower Silurian of Russia has yielded small teeth or spines which seem to have really belonged to fishes, although their exact affinities are not known (PalaeodusandArchodusof J. V. Rohon).
It is not until we reach the Upper Silurian that satisfactory remains of unquestionable fishes are found, and here they suddenly appear in a considerable variety of forms, very unlike modern fishes in every respect, but so highly developed as to convince us that we have to search in much earlier formations for their ancestors. These Upper Silurian fishes are theCoelolepidae, theAteleaspidae, theBirkeniidae, thePteraspidae, theTremataspidaeand theCephalaspidae, all referred to the Ostracophori. The three last types persist in the Devonian, in the middle of which period the Osteolepid Crossopterygii, the Dipneusti and the Arthrodira suddenly appear. The most primitive Selachian (Cladoselache), the Acanthodian Selachians (Diplacanthidae), the Chimaerids (Ptyctodus), and the Palaeoniscid ganoids (Chirolepis) appear in the Upper Devonian, along with the problematicPalaeospondylus.
In the Carboniferous period, the Ostracophori and Arthrodira have disappeared, the Crossopterygii and Dipneusti are still abundant, and the Selachians (Pleuracanthus, Acanthodians, truesharks) and Chondrostean ganoids (PalaeoniscidaeandPlatysomidae) are predominant. In the Upper Permian the Holostean ganoids (Acanthophorus) make their appearance, and the group becomes dominant in the Jurassic and the Lower Cretaceous. In the Trias, the Crossopterygii and Dipneusti dwindle in variety and theCeratodontidaeappear; the Chondrostean and Holostean ganoids are about equally represented, and are supplemented in the Jurassic by the first, annectant representatives of the Teleostei (Pholidophoridae,Leptolepidae). In the latter period, the Holostean ganoids are predominant, and with them we find numerous Cestraciont sharks, some primitive skates (SquatinidaeandRhinobatidae), Chimaerids and numerous Coelacanthid crossopterygians.
The fish-fauna of the Lower Cretaceous is similar to that of the Jurassic, whilst that of the Chalk and other Upper Cretaceous formations is quite modern in aspect, with only a slight admixture of Coelacanthid crossopterygians and Holostean ganoids, the Teleosteans being abundantly represented byElopidae,Albulidae,Halosauridae,ScopelidaeandBerycidae,many being close allies of the present inhabitants of the deep sea. At this period the spiny-rayed Teleosteans, dominant in the seas of the present day, made their first appearance.
With the Eocene, the fish-fauna has assumed the essential character which it now bears. A few Pycnodonts survive as the last representatives of typically Mesozoic ganoids, whilst in the marine deposits of Monte Bolca (Upper Eocene) the principal families of living marine fishes are represented by genera identical with or more or less closely allied to those still existing; it is highly remarkable that forms so highly specialized as the sucking-fish or remoras, the flat-fish (Pleuronectidae), the Pediculati, the Plectognaths, &c., were in existence, whilst in the freshwater deposits of North AmericaOsteoglossidaeandCichlidaewere already represented. Very little is known of the freshwater fishes of the early Tertiaries. What has been preserved of them from the Oligocene and Miocene shows that they differed very slightly from their modern representatives. We may conclude that from early Tertiary times fishes were practically as they are at present. The great hiatus in our knowledge lies in the period between the Cretaceous and the Eocene.
At the present day the Teleosteans are in immense preponderance, Selachians are still well represented, the Chondrostean ganoids are confined to the rivers and lakes of the temperate zone of the northern hemisphere (Acipenseridae,Polyodontidae), the Holostean ganoids are reduced to a few species (Lepidosteus,Amia) dwelling in the fresh waters of North America, Mexico and Cuba, the Crossopterygians are represented by the isolated groupPolypteridae, widely different from any of the known fossil forms, with about ten species inhabiting the rivers and lakes of Africa, whilst the Dipneusti linger in Australia (Neoceratodus), in South America (Lepidosiren), and in tropical Africa (Protopterus). The imperfections of the geological record preclude any attempt to deal with the distribution in space as regards extinct forms, but several types, at present very restricted in their habitat, once had a very wide distribution. TheCeratodontidae, for instance, of which only one species is now living, confined to the rivers of Queensland, has left remains in Triassic, Rhaetic, Jurassic and Cretaceous rocks of Europe, North America, Patagonia, North and South Africa, India and Australia; theAmiidaeandLepidosteidaewere abundant in Europe in Eocene and Miocene times; theOsteoglossidae, now living in Africa, S.E. Asia and South America, occurred in North America and Europe in the Eocene.
In treating of the geographical distribution of modern fishes, it is necessary to distinguish between fresh-water and marine forms. It is, however, not easy to draw a line between these categories, as a large number of forms are able to accommodate themselves to either fresh or salt water, whilst some periodically migrate from the one into the other. On the whole, fishes may be roughly divided into the following categories:—
I. Marine fishes. A. shore-fishes; B. pelagic fishes; C. deep-sea fishes.II. Brackish-water fishes.III. Fresh-water fishes.IV. Migratory fishes. A. anadromous (ascending fresh waters to spawn); B. catadromous (descending to the sea to spawn).
I. Marine fishes. A. shore-fishes; B. pelagic fishes; C. deep-sea fishes.
II. Brackish-water fishes.
III. Fresh-water fishes.
IV. Migratory fishes. A. anadromous (ascending fresh waters to spawn); B. catadromous (descending to the sea to spawn).
About two-thirds of the known recent fishes are marine. Such are nearly all the Selachians, and, among the Teleosteans, all theHeteromi,Pediculatiand the great majority ofApodes,Thoracostei,Percesoces,Anacanthini,AcanthopterygiiandPlectognathi. All theCrossopterygii,Dipneusti,Opisthomi,Symbranchii, and nearly all theGanoideiandOstariophysiare confined to fresh-water.
The three categories of marine fishes have thus been defined by Günther:—
“1.Shore Fishes—that is, fishes which chiefly inhabit parts of the sea in the immediate neighbourhood of land either actually raised above, or at least but little submerged below, the surface of the water. They do not descend to any great depth,—very few to 300 fathoms, and the majority live close to the surface. The distribution of these fishes is determined, not only by the temperature of the surface water, but also by the nature of the adjacent land and its animal and vegetable products,—some being confined to flat coasts with soft or sandy bottoms, others to rocky and fissured coasts, others to living coral formations. If it were not for the frequent mechanical and involuntary removals to which these fishes are exposed, their distribution within certain limits, as it no doubt originally existed, would resemble still more that of freshwater fishes than we find it actually does at the present period.2.Pelagic Fishes—that is, fishes which inhabit the surface and uppermost strata of the open ocean, and approach the shores only accidentally or occasionally (in search of prey), or periodically (for the purpose of spawning). The majority spawn in the open sea, their ova and young being always found at a great distance from the shore. With regard to their distribution, they are still subject to the influences of light and the temperature of the surface water; but they are independent of the variable local conditions which tie the shore fish to its original home, and therefore roam freely over a space which would take a freshwater or shore fish thousands of years to cover in its gradual dispersal. Such as are devoid of rapidity of motion are dispersed over similarly large areas by the oceanic currents, more slowly than the strong swimmers, but not less surely. An accurate definition, therefore, of their distribution within certain areas equivalent to the terrestrial regions is much less feasible than in the case of shore fishes.3.Deep-Sea Fishes—that is, fishes which inhabit such depths of the ocean that they are but little or not at all influenced by light or the surface temperature, and which, by their organization, are prevented from reaching the surface stratum in a healthy condition. Living almost under identical tellurian conditions, the same type, the same species, may inhabit an abyssal depth under the equator as well as one near the arctic or antarctic circle; and all that we know of these fishes points to the conclusion that no separate horizontal regions can be distinguished in the abyssal fauna, and that no division into bathymetrical strata can be attempted on the base of generic much less of family characters.”
“1.Shore Fishes—that is, fishes which chiefly inhabit parts of the sea in the immediate neighbourhood of land either actually raised above, or at least but little submerged below, the surface of the water. They do not descend to any great depth,—very few to 300 fathoms, and the majority live close to the surface. The distribution of these fishes is determined, not only by the temperature of the surface water, but also by the nature of the adjacent land and its animal and vegetable products,—some being confined to flat coasts with soft or sandy bottoms, others to rocky and fissured coasts, others to living coral formations. If it were not for the frequent mechanical and involuntary removals to which these fishes are exposed, their distribution within certain limits, as it no doubt originally existed, would resemble still more that of freshwater fishes than we find it actually does at the present period.
2.Pelagic Fishes—that is, fishes which inhabit the surface and uppermost strata of the open ocean, and approach the shores only accidentally or occasionally (in search of prey), or periodically (for the purpose of spawning). The majority spawn in the open sea, their ova and young being always found at a great distance from the shore. With regard to their distribution, they are still subject to the influences of light and the temperature of the surface water; but they are independent of the variable local conditions which tie the shore fish to its original home, and therefore roam freely over a space which would take a freshwater or shore fish thousands of years to cover in its gradual dispersal. Such as are devoid of rapidity of motion are dispersed over similarly large areas by the oceanic currents, more slowly than the strong swimmers, but not less surely. An accurate definition, therefore, of their distribution within certain areas equivalent to the terrestrial regions is much less feasible than in the case of shore fishes.
3.Deep-Sea Fishes—that is, fishes which inhabit such depths of the ocean that they are but little or not at all influenced by light or the surface temperature, and which, by their organization, are prevented from reaching the surface stratum in a healthy condition. Living almost under identical tellurian conditions, the same type, the same species, may inhabit an abyssal depth under the equator as well as one near the arctic or antarctic circle; and all that we know of these fishes points to the conclusion that no separate horizontal regions can be distinguished in the abyssal fauna, and that no division into bathymetrical strata can be attempted on the base of generic much less of family characters.”
A division of the world into regions according to the distribution of the shore-fishes is a much more difficult task than that of tracing continental areas. It is possible perhaps to distinguish four great divisions: the Arctic region, the Atlantic region, the Indo-Pacific region and the Antarctic region. The second and third may be again subdivided into three zones: Northern, Tropical and Southern. This appears to be a more satisfactory arrangement than that which has been proposed into three zones primarily, each again subdivided according to the different oceans. Perhaps a better division is that adopted by D. S. Jordan, who arranges the littoral fishes according to coast lines; we then have an East Atlantic area, a West Atlantic, an East Pacific and a West Pacific, the latter including the coasts of the Indian Ocean. The tropical zone, whatever be the ocean, is that in which fishes flourish in greatest abundance and where, especially about coral-reefs, they show the greatest variety of bizarre forms and the most gorgeous coloration. The fish-fauna of the Indo-Pacific is much richer than that of the Atlantic, both as regards genera and species.
As regards the Arctic and Antarctic regions, the continuity or circumpolar distribution of the shore fishes is well established. The former is chiefly characterized by its Cottids, Cyclopterids, Zoarcids and Gadids, the latter by its Nototheniids. The theory of bipolarity receives no support from the study of the fishes.
Pelagic fishes, among which we find the largest Selachians and Teleosteans, are far less limited in their distribution, which, for many species, is nearly world-wide. Some are dependent upon currents, but the great majority being rapid swimmers able to continue their course for weeks, apparently without the necessity of rest (many sharks, scombrids, sword-fishes), pass from one ocean into the other. Most numerous between the tropics, many of these fishes occasionally wander far north and south of their habitual range, and there are few genera that are at all limited in their distribution.
Deep-sea fishes, of which between seven hundred and eight hundred species are known, belong to the most diverse groups and quite a number of families are exclusively bathybial (Chlamydoselachidae,Stomiatidae,Alepocephalidae,Nemichthyidae,Synaphobranchidae,Saccopharyngidae,Cetomimidae,Halosauridae,Lipogenyidae,Notacanthidae,Chiasmodontidae,Icosteidae,Muraenolepididae,Macruridae,Anomalopidae,Podatelidae,Trachypteridae,Lophotidae,Ceratiidae,Gigantactinidae). But they are all comparatively slight modifications of the forms living on the surface of the sea or in the shallow parts, fromwhich they may be regarded as derived. In no instance do these types show a structure which may be termed archaic when compared with their surface allies. That these fishes are localized in their vertical distribution, between the 100-fathoms line, often taken as the arbitrary limit of the bathybial fauna, and the depth of 2750 fathoms, the lowest point whence fishes have been procured, there is little doubt. But our knowledge is still too fragmentary to allow of any general conclusions, and the same applies to the horizontal distribution. Yet the same species may occur at most distant points; as these fishes dwell beyond the influence of the sun’s rays, they are not affected by temperature, and living in the Arctic zone or under the equator makes little difference to them. A great deal of evidence has been accumulated to show the gradual transition of the surface into the bathybial forms; a large number of surface fishes have been met with in deep water (from 100 to 500 fathoms), and these animals afford no support to Alexander Agassiz’s supposition of the existence of an azoic zone between the 200-fathoms line and the bottom.
Brackish-water fishes occur also in salt and fresh water, in some localities at least, and belong to various groups of Teleosteans. Sticklebacks, gobies, grey mullets, blennies are among the best-known examples. The facility with which they accommodate themselves to changes in the medium in which they live has enabled them to spread readily over very large areas. The three-spined stickleback, for instance, occurs over nearly the whole of the cold and temperate parts of the northern hemisphere, whilst a grey mullet (Mugil capito) ranges without any appreciable difference in form from Scandinavia and the United States along all the Atlantic coasts to the Cape of Good Hope and Brazil. It would be hardly possible to base zoo-geographical divisions on the distribution of such forms.
The fresh-water fishes, however, invite to such attempts. How greatly their distribution differs from that of terrestrial animals has long ago been emphasized. The key to their mode of dispersal is, with few exceptions, to be found in the hydrography of the continents, latitude and climate, excepting of course very great altitudes, being inconsiderable factors, the fish-fauna of a country deriving its character from the headwaters of the river-system which flows through it. The lower Nile, for instance, is inhabited by fishes bearing a close resemblance to, or even specifically identical with, those of tropical Africa, thus strikingly contrasting with the land-fauna of its banks. The knowledge of the river-systems is, however, not sufficient for tracing areas of distribution, for we must bear in mind the movements which have taken place on the surface of the earth, owing to which present conditions may not have existed within comparatively recent times, geologically speaking; and this is where the systematic study of the aquatic animals affords scope for conclusions having a direct bearing on the physical geography of the near past. It is not possible here to enter into the discussion of the many problems which the distribution of fresh-water fishes involves; we limit ourselves to an indication of the principal regions into which the world may be divided from this point of view. The main divisions proposed by Günther in the 9th edition of theEncyclopædia Britannicastill appear the most satisfactory. They are as follows:—