396 THE DOGFISH less. 



general character of nephridia, opening on the one hand 

 by nephrostomes into the ccelome, and on the other into a 

 longitudinal duct which discharges into the cloaca. Thus 

 the primitive structure of the kidney furnishes another 

 instance of metamerism in the dogfish. 



In the male there is a single pair of spermaries (Fig. 103, 

 ts\ in the form of large soft organs, united with one another 

 posteriorly. They are suspended by a fold of peritoneum 

 to the dorsal body-wall. From the anterior end of each 

 arise numerous delicate efferent ducts, which enter a long, 

 convoluted spermiduct or vas deferens {^. def). This passes 

 along the ventral aspect of the kidneys and dilates into a 

 conical pouch, the vesicula seminalis {vs. sem), and the two 

 vesiculae open, along with the ureters and a pair of reser- 

 voirs called sperm-sacs {sp. s), into the urinogenital sinus. 



The female has a single ovary (Fig. 99, ovy) suspended to 

 the dorsal body-wall by a fold of peritoneum. In the adult it 

 is studded all over with rounded projections, the ova, varying 

 in diameter from 1 2-14 mm. downwards. The oviducts {ovd) 

 are paired and extend along the whole length of the dorsal 

 wall of the coelome, below the kidneys. Anteriorly they 

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

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

 considerable size {ovd), by which both tubes communicate with 

 the coelome : 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 the 

 two 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 

 oviducts of the female. The eggs, 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 



