160 ZOOLOGY SECT. 



Their inner surface is raised up into a number of ridges on which 

 the fibres of the olfactory nerves are distributed. The eye has a 

 cartilaginous sclerotic and is in most cases attached to the inner 

 wall of the orbit by means of a cartilaginous stalk. A fold of the 

 conjunctiva corresponding to the nictitating membrane, or third 

 eyelid of higher Vertebrates, occurs in some Sharks. The ear 

 consists of the three semicircular canals with their ampullae; of 

 the membranous vestibule, which is partly divided into two ; and 

 of a canal the aqueductus vestibuli or endolymphatic duct which 

 opens on the exterior on the dorsal surface of the head. In the 

 Rays the semicircular canals form almost complete circles and 

 open separately into the vestibule by narrow ducts. 



Urine-genital Organs. The kidneys, as already noticed in 

 the account given of the Dog-fish, differ somewhat in their relations 

 in the two sexes. In the male the anterior portion persists as the 

 epididymis, and its duct becomes the spermiduct, while the 

 posterior portion, which is the functional kidney, has a duct, the 

 ureter, of its own. In the female there is no direct connection 

 between the reproductive and renal organs ; the anterior portion 

 of the kidney may be functional, and its duct persists, opening 

 along with those of the posterior portion. In the male the 

 ureters open into a median chamber the urino-genital sinus a 

 special compartment of the cloaca, which receives also the spermi- 

 ducts : this communicates with the general cavity of the cloaca 

 by a median opening situated on a papilla the urino-genital 

 papilla. In the female there is a median urinary sinus, into 

 which the ureters open, or the latter open separately into the 

 cloaca. 



Save in certain exceptional cases (e.g. the Dog-fish), there are 

 two ovaries, varying considerably in form, but always characterised 

 towards the breeding season by the great size of the Graafian 

 follicles enclosing the mature ova. The oviducts (Mlillerian 

 ducts) are quite separate from the ovaries. The right and left 

 -ctyiducts come into close relationship anteriorly, being united in 

 the middle on the ventral surface of the oesophagus, where each 

 open& by a wide orifice into the abdominal cavity, or both open 

 by a s!ngle median aperture. The following part of the oviduct 

 is very narrow ; at one point it exhibits a thickening, due to the 

 presence in its walls of the follicles of the shell-gland. Behind 

 this is a dilated portion which acts as a uterus ; and this communi- 

 cates with the cloaca through a wide vagina. A considerable 

 number of the Elasmobranchii are viviparous, and in these the 

 inner surface of the uterus is beset with numerous vascular villi, 

 while the shell-gland is small or vestigial. 



The testes are oval or elongate: the convoluted epididymis 

 is connected with the anterior end by efferent ducts, and passes 

 posteriorly into the vas deferens. The latter is dilated near its 



