64 MARKETABLE IJUITISH MARINE FISHES 



like a cigar pointed at both ends. It is burst when the little 

 fish arc well developed. Thus the eggs in the spiny dog-fish 

 develop inside the egg-tube, and when the young are born they 

 arc similar to their mother except in size, and are able to swim 

 about actively and vigorously. 



In the male dog-fish the generative organ consists of a flat 

 solid m.ass attached to the back of the body cavity on each side 

 of the intestine. It is similar in position to the ovary of the 

 female, but the surface is smooth, not bulged out with eggs. 

 This organ, the milt or testis, consists of a number of tubes, 

 in which the liquid milt is produced. The milt or semen does 

 not find its way to the exterior by a direct route, but by the aid 

 of a portion of the kidney through which it passes. The tubes 

 of the testis are in communication at the back of the body cavity 

 with the tubes of the foremost part of the kidne}-, and the latter 

 open into a single larger tube on each side, which like the egg- 

 tube in the female passes down the body to open into the sac at 

 the end of the intestine. 



The male dog-fish or skate is distinguished, as is well known, 

 by two long thick projections attached at each side of the vent, 

 and connected with the hinder paired fins, of which they form 

 the hinder part. During coition the male inserts these two 

 organs through the vent of the female, into her two egg-tubes, 

 and the milt is passed along them.. In the males of skates and 

 rays there is a patch of sharp-pointed spines on each wing, 

 which is absent in the females, but whether these are used in 

 the act of copulation or not is unknown. 



The eggs of the common spotted dog-fishes {Scylliuni 

 caniciila and catiiliis) are attached, when laid, to weeds or other 

 objects standing up on the sea-bottom, by means of their 

 tendrils, which are tightly wound round the support. The 

 manner in which this is effected has been observed in the 

 aquarium of the Plymouth Laboratory, for these species lay 

 their eggs freely in captivity. The lower tendrils project first 

 from the vent, and the female rubs herself against some fixed 

 body, swimming round and round about it. The tendrils soon 

 catch fast in some slight projection, and the o.^^ is thus dragged 

 out and firmly bound to the fixed object at the same time, and 

 there it remains while development proceeds. The large rough 

 dog-fi.sh, or nurse-hound, lays its eggs in this way in some places 



