234 COMPARATIVE ANATOMY 



conspicuous tubes of varying diameter running forwards from 

 the cloaca, one on either side of the middle line. They are 

 attached to the dorsal \vall of the peritoneal cavity by a fold 

 of the peritoneum known as the mesometrium. Anteriorly the 

 two ducts curve inwards to the ventral side of the oesophagus 

 and their ends unite and open by a single large slit-like opening 

 in the same position as the rudimentary organ in the male. 

 At no great distance from their anterior ends the oviducts are 

 swollen and their walls thickened to form the oviducal glands. 

 Behind the oviducal glands the oviducts run straight back- 

 wards as rather thick tubes. Their hinder ends are somewhat 

 dilated, and they unite together dorsal to the rectum and open 

 into the cloaca by a large median aperture immediately above 

 the rectal opening. 



The secretion of the oviducal gland is poured round each 

 ovum as it passes down the oviduct and sets into a firm horny 

 semi-transparent egg-case or shell, which has the same relation 

 to the enclosed yolk that the egg-shell of a bird has to the yolk 

 within. The egg-cases of Scyllium are oblong capsules, about 

 two inches in length and rather less than an inch in width, 

 and their angles are produced into long tapering threads or 

 tendrils. The eggs are extruded by the female and the 

 tendrils are twined round a seaweed or sea-fan or other 

 branching object in shallow water, and there undergo their 

 development. The empty egg-cases of Scyllium and the larger 

 and blacker but similar cases of the skate are common objects 

 on the seashore, and are familiarly known as " sea-purses." 

 Complete eggs may be easily obtained by dredging in suitable 

 localities, and occasionally they are cast up on the shore. In 

 advanced eggs the embryo is seen attached to a voluminous 

 yolk-sac by a narrow umbilical stalk. Scylljum is thus 

 oviparous, but in Acanthias and many other dogfishes and 

 sharks the eggs are retained in the oviducts of the mother and 

 undergo their development there. 



The central nervous system of the dogfish presents the same 

 features as that of the frog, but the relative proportions of the 

 parts are different. The spinal cord has the typical vertebrate 

 structure, with a ventral and a dorsal fissure, and a central 

 canal, and the spinal nerves are given off in pairs in each 

 segment of the body, each member of a pair arising from a 

 dorsal and a ventral root which unite outside the neural canal 



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