THE LIFE-HISTORIES OF FISHES 155 



for the offspring. There are, however, certain 

 exceptions to this rule, and these are of much 

 interest to the naturalist. In some of the dog- 

 fishes of British waters (Mustelus, the smooth- 

 hound ; Galeus, the tope ; Acanthias^ the pike-dog), 

 and in some tropical rays, the development is 

 analogous to that which takes place among the 

 warm-blooded animals ; that is, a kind of placenta 

 may be formed, so that the development might be 

 called " intra-uterine." * Much the same kind of 

 thing takes place in the case of another British fish 

 (Zoarces viviparus, the viviparous blenny). In 

 these cases the young fish is born into the world 

 in a highly advanced condition. The very general 

 rule among fishes is, however, that unimpregnated 

 eggs are laid by the female ; the class is, with 

 the exceptions noted, and a few others, oviparous, 

 not viviparous. 



In some cases there is a kind of parental care of 

 the young. In the female of Solenostoma (a tropical 

 fish) the eggs are retained, after spawning, in a 

 brood-pouch which the female bears, and which is 

 formed by the coalescence of the edges of the 

 pelvic fins, and they remain there until the young 

 hatch out. An analogous provision exists in the 

 case of a British fish (Syngnathus, the pipe-fish), 

 where two cutaneous abdominal folds form, in the 

 male^ a similar brood-pouch. In the butter-fish 



1 The structures concerned in the nutrition of the embryos in the 

 two classes of animals are, of course, not homologous. 



