xii PHYLUM CHORDATA 381 



ureters unite to open by a median aperture into the cloaca ; 

 in the male they open into a small chamber, the urino- 

 genital sinus (u. g. s.), which opens into the cloaca (cl.). 



In the male Dog-fish the testes are a pair of large soft 

 organs situated in the body-cavity, and united with one 

 another posteriorly. From the anterior end. of each arise 

 numerous delicate efferent ducts^ which enter a long con- 

 voluted spermiduct or vas deferens (v. def.), leading posteriorly 

 to the urinogenital sinus. In the female Scyllium there is a 

 single ovary suspended to the dorsal body-wall by a fold 

 of peritoneum. In the adult it is studded all over with 

 rounded projections, the ova. There are two oviducts, a 

 right and a left, which extend along the whole length of the 

 dorsal wall of the ccelome, below the kidneys. Anteriorly 

 they unite with one another below the gullet just in front of 

 the liver, and at the point of junction is a single aperture of 

 considerable size, by which both tubes communicate with the 

 ccelome ; posteriorly they open into the cloaca. About the 

 anterior third of each oviduct is narrow ; its posterior two- 

 thirds is wide and distensible, and at the junction of these 

 parts is a yellowish glandular mass, the shell-gland. 



Internal impregnation takes place, the spermatic fluid of 

 the male being passed, by means of the claspers, into the ovi- 

 ducts of the female. The ova, when ripe, break loose from 

 the surface of the ovary into the ccelome, and thence pass, 

 through the common aperture, into one or other of the 

 oviducts, where fertilisation occurs. As it passes into 

 the dilated portion of the oviduct, the oosperm of Scyllium 

 becomes surrounded by a horn-like egg-shell (Fig. 213)^ 

 secreted by the shell-gland, and having the form of a 

 pillow-case produced at each of its four corners into a long 

 tendril-like process. The eggs are laid among sea-weed, to 

 which they become attached by their tendrils. In some 



