320 CHORD ATA. 



removing the dorsal peritoneum. Each has a fine duct, the 

 ureter^ leading from its inner lower border posteriorly to open 

 into the urogenital sinus. This sinus opens into the cloaca 

 by a small papilla. 



As already noted, the male skate has a pair of clasp ers^ 

 long firm organs strengthened by cartilages developed in 

 connection with the pelvic fins. They are deeply grooved 

 and have a large clasper-gland which opens into the groove 

 by a duct. 



Each testis discharges its sperms into its vas deferens 

 and thence into the sperm-sac in which they are mixed with 

 a secretion ; they then pass out of the cloacal aperture and 

 down the clasper-grooves. 



In the female, the ovaries are paired and occupy the 

 same position as the testes. They often contain large 

 partially ripe ova. The oviducts are paired tubes of large 

 size leading the whole length of the abdominal cavity. At 

 the anterior end they open by a common aperture into the 

 abdominal cavity^ and posteriorly each opens into the cloaca. 

 The anterior part is called the Fallopian tube which is thin- 

 walled and of small calibre ; the posterior part, sometimes 

 called the uterine portion, is thick- walled and wide ; at the 

 junction of these two parts is a large oviducal gland. (There 

 is a vestige of the epididymis.) The urinary organs do not 

 differ essentially from those of the male. 



The eggs on ripening are shed free into the abdommal 

 cavity, and thence pass down the oviducts. They are 

 fertilised in the Fallopian tubes and the oviducal gland then 

 secretes around them the egg-capsule or purse ; they are 

 laid singly through the cloacal aperture. 



If the skate be now turned upon its ventral surface, and 

 the skin removed from the head region, as far out as the 

 gills and backwards, the following structures can 

 ^^slnsory.'"'^ be recognised (Plate III). In the centre is the 

 cranium, the dorsal cartilaginous wall of which 

 may be carefully removed, when it will be seen to possess 

 a large central cavity containing the brain, a pair of anterior 

 cavities of the olfactory capsules and a pair of posterior cavi- 

 ties, those of the auditory capsules. Between these and the 

 olfactory capsules are the eyes. Hence the side of the head 

 in the skate bears three pairs of sense-organs, olfactory sacs. 



