335 



REPRODUCTION 



There is a single ovary, which is probably that of the right side. 

 It hangs into the body cavity and varies in size and appearance 

 with age. The ova are in different stages of ripeness, the ripest 

 being very large and yolky. They are shed into the body cavity 

 and passed forwards by contractions of the abdominal walls to 

 the front of the peritoneal space 

 where they enter the internal 

 opening of the oviducts. The latter 

 are large straight tubes, one on 

 each side of the body, attached to 

 the dorsal wall of the coelom. They 

 start from a common opening in the 

 suspensory ligament, not far behind 

 which each has a round swelling 

 known as the shell gland, by which 

 the shells of the eggs are secreted. 

 At the hinder end of the trunk they 

 enter the cloaca by a common opening 

 just behind the anus. The testes are 

 a pair of long organs slung by mem- 

 branes from the dorsal wall of the 

 coelom. Each communicates at its 

 front end with the kidney of its side 

 by several small vasa efferentia, the 

 sperms passing through these into 

 kidney tubules and thence to the 

 vas deferens or Wolffian duct, by 

 which it is conveyed to the urino- 

 genital sinus. A rudiment of the 

 internal opening of the oviducts is found in the suspensory 

 ligament of the male. Fertilisation is internal, and there is a form 

 of copulation in which the claspers are stiffened or erected under 

 the influence of adrenaline (p. i8) and inserted into the cloaca 

 of the female. It is possible that the semen is washed out of the 

 grooves of the claspers by sea water injected into them by the 

 ' siphon '—a muscular sac which lies under the skin of the ventral 

 surface in the pelvic region and communicates by two channels 

 with the grooves. Sperms are stored in the folds of the shell 

 gland. In the Bristol Channel the spawning season starts in 



Fig. 253. — An embryo dogfish 

 in its egg-case (' mermaid's 

 purse ') which has been cut 

 open to show the contents. — 

 From Thomson. 



d.f., Dorsal fin fold; e.g., 'external' 

 gills ; St., stalk of yolk-sac ; T., ten- 

 drils, prolongations of egg-case by 

 means of which it is moored to sea- 

 weed ; y.s., yolk-sac. 



