XIII 



PHYLUM CHORDATA 163 



a narrow central canal, and givers origin L(j a largo number of 

 ]»aire(l spinal nerves, e;u'h nrising from it by two roots. 



Organs of Special Sense. — The olfactory organs arc rounded 

 chambers enclosed bv t lie cartilage of the olflictory capsules (jf the 

 skull, and opening on the exterior by the external nares on the 

 ventral surface of the head. The interior has its lining meuil)rano 

 rai.sed up into a luunber of close-set ridges rumiing out from 

 a median septum. The fibres of the olfactory nerves terminate in 

 cells of the epithelium covering the surface of these ridges. 



The eye has the general structure already described as char- 

 acterising the Craniata in general (p. 10!)). The sclerotic is 

 cartilaginous, the choroid has a shining metallic internal layer or 

 tapetum, and the; lens is spherical. There are the usual eye- 

 muscles, the two obliques situated anteriorly, the four recti 

 posteriorly, not embracing the optic nerve. There are no eyelids. 



The ear consists only of the membranous labyrinth (Fig. 788), 

 equivalent to the » intern al e;xr, of higher Craniata^iJie-m il d l e -mv^^ 



o uter ear being absentj~T'he membranous labyrinth consists of the 

 vestibule and tlireesem/ici rcular canals. The former, which is divided 

 into two parts by a constriction, communicates by a narrow passage 

 — the cndolyinphatic duct or aqueductus vestibuli — with the exterior, 

 in the position already mentioned. Of the three semicircular canals, 

 the anterior and posterior are vertical and the external horizontal, 

 as in Craniata in general. Each has an ampulla, that of the 

 anterior and external canals situated at their anterior ends, and 

 that of the posterior canal, which is the largest of the three 

 and forms an almost complete circle, at its posterior end. In 

 the fluid {endolymph) in the interior of the vestibule are suspended, 

 in a mass of gelatinous connective-tissue, numerous minute 

 calcareous particles or otoliths, giving it a milky character. 



The sensory canals of the integument running along the lateral 

 line and over the head contain special nerve-endings (neuromasts), 

 and doubtless function as organs of some special sense (see p. 108). 

 The same probably holds good of a number of unbranched canals 

 arranged in groups situated on the anterior portion of the trunk 

 and on the head, and being particularly numerous in the neigh- 

 bourhood of the snout. These are dilated internally into vesicles, 

 the ampulkr, provided with special nerve-endings. 



Urinogenital Organs.— In the female there is a single 

 ovary (Fig. 817, ov.), an elongated, soft, lobulated body, lying 

 a little to the right of the middle line of the abdominal cavity, 

 attached by a fold of peritoneum, the mesoarium. On its 

 surface are rounded elevations of various sizes, the Graafian 

 follicles, each containing an ovum of a bright yellow colour. There 

 are two oviducts (MUllerian ducts) entirely unconnected with the 

 ovaries. Each oviduct (Figs. 817 and 824, ovd.) is a greatly 

 elongated tube extending throughout the entire length of the 



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