180 ZOOLOGY 



SECT. 



surface of the head. In the Rays the semicircular canals form 

 almost complete circles and open separately into the vestibule by 

 narrow ducts. 



Urinogenital 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 in the 

 epididymis, and its duct becomes the spermiduct, while the 

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

 the ureter or ureters 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 urinogenital sinus 

 which extends into the cloaca, and receives also the spermi- 

 ducts : it communicates with the general cavity of the cloaca 

 by a median opening situated on a papilla the urinogenital 

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

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

 the cloaca. 



Save in certain exceptional cases (e.g. Scyllium), there are 

 two ovaries, varying considerably in form, but always characterised 

 towards the breeding season by the great size of the follicles 

 enclosing the mature ova. The oviducts (Miillerian ducts) are 

 quite separate from the ovaries. The right and left oviducts 

 come into close relationship anteriorly, being united in the 

 middle on the ventral surface of the oesophagus, where each 

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

 by a single 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 from 

 it arises the vas deferens. The latter is dilated near its opening 

 into the urinogenital sinus to form an ovoid sac the vesicula 

 seminalis. A sperm-sac is sometimes present, opening close to the 

 aperture of the vas deferens. The Miillerian ducts are vestigial 

 in the male. 



Impregnation is internal in all the Elasmobranchs with the 

 possible exception of Lasmargus (the Greenland Shark), the 

 claspers acting as intromittent organs by whose agency the semen 

 is transmitted into the interior of the oviducts. 



In all the Elasmobranchs the ova are very large, consisting of a 



