348 MOSTLY MAMMALS 



probable that after the young fish are hatched they are 

 retained for some time by attachment to the walls of the 

 chamber. In the true pipe-fishes (Syngnathus), on the 

 other hand, the task of looking after the nursery falls to 

 the males, which are provided with a long pouch on the 

 under-surface of the tail, formed by a fold of skin arising 

 on each side, and the two meeting in the middle line. 

 How the eggs are conveyed into this pouch I am totally 

 unaware, but when once there, they are completely enclosed 

 by the junction of the edges of the two folds of skin, and 

 thus remain till they are hatched into minute eel-like pipe- 

 fish, which soon make their way into the world by thrusting 

 open the folds of the pouch. In the sea-horses the 

 development is carried one stage farther, the nursing- 

 pouch being completely closed along the middle line, and 

 only communicating with the exterior by means of a small 

 aperture at the anterior end, through which the eggs are 

 by some means or other introduced, and by which in due 

 course the young make their escape. Certain pipe-fishes 

 (Doryichthys) differ from the ordinary forms in that the 

 males have the pouch situated beneath the abdomen instead 

 of under the tail; and it is not a little remarkable that in 

 certain allied genera (Nerophis, etc.) the eggs are simply 

 attached to the lower surface of the abdomen of the male 

 without the development of a pouch. We have thus an 

 excellent instance of the evolution of a special organ, so 

 far as the abdominal pouch is concerned ; but it would seem 

 highly probable that the caudal pouch of the allied forms 

 must have been independently evolved, in which event 

 we should have a remarkable example of parallelism in 

 development. 



Although many fishes retain their eggs within their 

 bodies until the young are hatched and attain a consider- 



