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266
[ANATOMY
ICHTHYOLOGY


as of other parts of the hemisphere is contained within the lateral thickening of the thelamencephalic wall, not in its membranous roof.

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 lioor, or in the front wall, of the primitive fore-brain. It is of great interest to note the appearance in the Dipnoans (Lepidoslren and Protopterus) 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).2

The eyes are 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 hbres contained in the process us falczformis, a projection from the choroid which terminates in contact with the lens in a swelling, the campauula H alleri. In Amia and 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 (Argerttea) 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?

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 the ductus endolymphaticus. In .Squatina 4 this is unusually wide and correlated; with this the calcareous otoconia are replaced by sand-grains from the exterior. In Dipnoans (Lepidosiren and Protopterus) 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 swim-bladder enters into intimate relations with the otocyst. In the sim lest condition these relations consist in the prolongation forwards ofpthe swim-bladder 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 liuid surrounding it (perilymph)~through a gap in the rigid periotic capsule. A wave of compression causing a slight inward movement of the swim-bladder wall will bring about a greatly rnagnihcd 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 the Cypriuidae, the Siluridae, the Characirtidae and the Gyrnnotialae, a physiologically similar connexion between swim-bladder and otocyst is brought about by the intervention of a chain of auditory ossicles (Weberian ossicles) formed by modification of the anterior vertebrae.5 1

F. K. Studnicka, S.B. bohm. Gesell. (1901); j. Graham Kerr, Quart. Journ. Micr. Sci. xlvi., and The Budgett Memorial Volume.

(ZR) Wiedersheim, K6lliker's Festschrift: cf. also Ariat. Anz. 1887 .

A. Brauer, Verhandl. deutsch. zool. Gesell. (1902). C. Stewart, Journ. Linn. Soc. Zool. (1906), 439. T. W. Bridge and A. C. Haddon, Phil. Trans. 184 (1893). Lateral Line Organsf-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. In Lepidosiren these 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 in Amphioxus, 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 bronchial clefts and (2) the presence of an elaborate system' of cutaneous sense organs supplied by a group of nerves (lateral is) 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 lig. 31). I., II., III., IV. and VI. call for no special remark.

Trigeminus (V.) -The ophthalmic us profuudus branch (op.p.)which probably is morphologically a distinct cranial nerve-9""9 Z-

" ~~ -

Q- fix , a ' ~»~'~~ .»»;»;', Mi 'W ¢|lnv»;w|' |$X¢ f l' "~~11. rv

1 .hy ' .bc:I -V .

pn - ., f, |; pn .1 1/n x

Mig ~ j H wt' V

= ' ” 'W

-'F' .m 9 /1

- , xml

Wide/r

From Bridge, Cambridge Natural History, vol. vii. “Fishes " (by permission of Macmillan & Co., Ltd.). After Wiedersheixu, Grundrirs der vergleichenden Analomie (by permission of Gustav Fischer).

FIG. 31.—Diagram of Cranial nerves of a Fish. Cranial nerves a.nd bronchial clefts are numbered with Roman figures. Trigeminus black; Facialis dotted; Lateralis oblique shading; Glossopharyngeal cross-hatched; Vagus white.

bucc, Buccal.

c, Commissure between preand

post auditory parts of

lateral is system.

d.r, Dorsal roots of spinal nerves.

mx,

oc, Occipitospinal.

Olfactory organ.

Maxillary.

ol.o,

op.p, Ophthalmicus profundus.

op.s, Ophthalmicus superficial is.

g.g, Gasserian ganglion. pn, Palatine.

gn.g, (Geniculate) ganglion of pq., Palatopterygo-quadrate VII. cartilage.

hy, Hyomandibular. s, Spiracle.

l.n.X, Lateralis vagi. st,

Supra-temporal branch of

rn, Motor branches of hy.

lateral is system.

md, Mandibular. t.a, Lateralis centre in brain. . md.ex, External mandibular. 11.11, Visceral nerve. mk.c, Meckel's cartilage. ~v.r, Ventral roots. 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. The ophthalmic us superjieialis (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 lateral is system. The main trunk of V. branches over the edge of the mouth into the maxillary (mx.) and mandibular (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 iioor. The main trunk of the Facialis (VII.) bifurcates over the° For literature of lateral line organs see Cole, Trans. Linn. Soo.

vii. (1898).