xiii PHYLUM CHORDATA 151 



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. 103). The sclerotic is 

 cartilaginous, the choroid has a shining metallic internal layer or 

 tapetum, and the lens is spherical. The eyeball is attached to the 

 inner wall of the orbit by a cartilaginous stalk. There are the 

 usual eye-muscles, the two obliques situated anteriorly, the four 

 recti posteriorly. There are no eyelids. 



The ear consists only of the membranous labyrinth equivalent to 

 the internal ear of higher Craniata, the middle ear and the outer ear 

 being absent. The membranous labyrinth consists of the vestibule 

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

 two parts by a constriction, communicates by a narrow passage 

 the aqucductus 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 mucous canals of the integument contain special nerve- 

 endings, and doubtless function as organs of some special sense. 

 The same probably holds good of a number of minute canals 

 situated on the anterior portion of the trunk, and on the head, 

 being particularly numerous in the neighbourhood of the snout. 

 These are dilated internally into vesicles, the ampullce, provided 

 with special nerve-endings: 



Urino-genital Organs. In the female there is a single 

 ovary (Fig. 773, 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 (Mlillerian ducts) entirely unconnected with the 

 ovaries. Each oviduct (Fig. 773, ovd. ; Fig. 778) is a greatly 

 elongated tube extending throughout the entire length of the 

 abdominal cavity. In front the two unite behind the pericardium 

 to open into the abdominal cavity by a wide median aperture 

 (abd. ap.). At about the point of junction of the middle and 

 anterior thirds is a slight swelling marking the position 

 of the shell-gland (sh. gld.). The posterior part dilates to form 

 a wide chamber, and in Scyllium the two unite to open into 

 the cloaca by a common aperture situated just behind the 



