132 ELASMOBRANCHII 



occurs in the Notidani, Heterodonti, and Squaliformes ; Type B 

 in the Scillioidei ; Type C in the Rajiformes. The copulatory 

 appendage of the Pleuracanthodii seems to have been of very 

 similar structure ; but that of the Holocephali acquires a trifid 

 instead of a tubular main cartilage (p. 174). 



The primitive optic chiasma, conns arteriosus with valves, and 

 spiral valve are all retained in living Elasmobranchs. Except in 

 Chlamydoselachus, winch retains a single efferent vessel as in the 

 embryo (Ayers [24]), the gill-bearing branchial arches are provided 

 with two efferent vessels. The posterior of one arch joins the 

 anterior of the next, above the slit, to form the epibranchial artery 

 (Fig. 71). 



The mesoriephric kidneys are differentiated into an anterior 

 'genital' region and a posterior excretory region (Fig. 90). The 

 latter, which is sometimes called the metanephros, is large and 

 normally developed; but its collecting ducts become to a great 

 extent separated off from that of the front tubules, and in the 

 male may join to a single duct or ureter on each side. They open 

 into the base of the mesonephric ducts, which join to a median 

 sinus in both sexes (Fig. 90). The excretory tubules of the 

 anterior region of the kidney are relatively unimportant. In 

 the male the mesonephric duct swells to a seminal vesicle behind, 

 and forms a coiled epididymis in front, receiving the vasa 

 efferentia from the testis. The urinogenital sinus receives the 

 seminal vesicles and the ureters in the male, and in the Selachii 

 is continued forwards in paired blind * sperm-sacs.' The urino- 

 genital papiHa opens into the cloaca behind the anus. 



The large oviducts of the female open independently into the 

 cloaca between the anus and the urinary papilla (Fig. 90). Their 

 anterior coelomic funnels join to a common ostium on the ventral 

 surface of the oesophagus and in front of the liver. 



The Miillerian ducj (oviduct) is developed in both sexes, re- 

 maining as a vestige in the adult male. It is derived from the 

 pronephros (Balfour [27], Eabl [337]). The rudimentary pronephric 

 funnels combine to one opening, shifting backwards to a position 1 

 behind the pericardial septum (p. 90). The duct is split off from 

 the archinephric duct, which thus becomes divided into an oviduct 

 or Miillerian duct, and a mesonephric or Wolffian duct. As a rule, 

 the egg is large, heavily laden with yolk ; fertilisation is internal, 

 and cleavage meroblastic. A large narrow-stalked yolk-sac remains 

 for a considerable time protruding from the ventral surface of the 

 embryo, often even after birth. A lobe of the yolk-sac may also 

 lie in the abdominal coelom. The vitello-intestinal duct arises from 

 the anterior, end of the intestine. 



A characteristic horny case, secreted by a special glandular 

 region of the oviduct (Fig. 91), and containing m albuminous 



