GENITAL ORGANS 465 



the ureters either separately, or by a common aperture (Fig. 349). 

 Internal fertilisation takes place by means of the claspers of the 

 male l (p. 486). 



In Mustelus aritarcticus the uterus becomes divided into several 

 compartments, each containing an embryo surrounded by a mem- 

 brane apparently representing the horny egg-shell of other forms ; 

 in others (e.g. Acanthias vulgaris, Trygon pastinaca, Trygorhiiui 

 fasciata, Rhinobates vincentianus), a common horny investment 

 encloses several eggs or embryos. 



The testis of Elasmobranchs is paired and symmetrical : it is 

 oval in Chimseroids and more elongated and relatively larger in 

 Plagiostomes, in which the two organs may become partially fused. 

 As already mentioned (p. 454), vasa efferentia, opening into a 

 longitudinal canal, connect each testis with the anterior end of the 

 corresponding mesonephros (epididymis) in Plagiostomes. the 

 coiled Wolffian duct serving mainly as a vas deferens and giving 

 rise to a dilated portion (vcsicula scminalis\ which communicates 

 with the urinogenital sinus ; a more or less elongated caeoal 

 sperm-sac also opens into the urinogenital sinus, which commu- 

 nicates with the cloaca at the apex of a papilla (Fig. 349). In 

 the Holocephali, the vas deferens forms a large, coiled mass 

 anteriorly, and is connected with the testis by mesenteric folds, 

 but in the absence of a mesonephric part of the epididymis it is 

 not clear how the sperms pass into it : they become aggregated 

 into spermatophores before entering the elongated and septate 

 seminal vesicles. Vestiges of the anterior end or even the 

 whole (Holocephali) of the Miilleriau ducts can be recognised 

 in adult Elasmobranchs. 



The ovaries and testes of most Teleosts, which usually 

 produce an enormous number of generative cells, closely corres- 

 pond with one another as regards position and the arrangement 

 of their ducts. Dorsal and ventral folds of the peritoneum are 

 developed in connection with the elongated ovary, and these 

 in most cases meet along its outer side, so as to enclose a portion 

 of the coelome and thus convert the ovary into a hollow sac 

 (" cystoarian " condition), which is blind anteriorly and on the 

 inner, folded walls of which the ova arise. The peritoneal folds 

 are eventually continued backwards to form the oviduct (Fig. 350), 

 which is generally short, and as a rule fuses with its fellow to 

 form an unpaired canal : this opens either by a genital pore (p. 390) 

 between the rectum and the urinary aperture on a level with 

 the integument, or on a papilla, which may become elongated to 

 form a tube or ''ovipositor"; or the ducts may communicate 

 with a urinogenital sinus. The testis is elongated, often tabulated 



1 The shell-gland secretes the horny material for the egg-case or "purse," 

 which in Plagiostomes is usually produced at its four angles into longer or shorter 

 tendril-like threads ; in Cestracion it has a spiral ridge, and in Callorhynchus it 

 is very large and expanded and covered on one side by hair-like processes. In 

 viviparous forms the egg-shell becomes more or less reduced. 



H H 



