XIII PHYLUM CHORDATA 163 



a narrow central canal, and gives origin to a large number of 

 paired spinal nerves, each arising from it by two roots. 



Organs of Special Sense. — The olfactory organs are rounded 

 chambers enclosed by the cartilage of the olfactory capsules of the 

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

 ventral surface of the head. The interior has its lining membrane 

 raised up into a number of close-set ridges running out from 

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

 cells of the epithelium covering the surface of these ridges. 



The eTje has the general structure already described as char- 

 acterising the Craniata in general (p. 109). 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 situate'd anteriorly, the four recti 

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



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

 equivalent to the internal ear of higher Craniata, the middle and 

 outer ear being absent. The membranous labyrinth consists of the 

 vestibide and three semicircular canals. The former, which is divided 

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

 — the endolymphatic duct or aqueductiis vestibuli — with the exterior, 

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

 the anterior and posterior are vertical and the external horizonfel, 

 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 ampullce, 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 (MuUerian 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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