35B MARKETABLE BRITISH MARINE FISHES 



toothed carps, these fishes having teeth in the jaws, while the jaws of the 

 carps are toothless. They are fishes of quite small size, and occur in 

 south Europe, and in the warmer parts of the American continents. 



The fact that viviparous reproduction, or the internal development 

 of the eggs, is found to occur in several widely distinct families, shows 

 that its origin is due to circumstances which may affect any family of 

 fishes without reference to its relationship with other families in structure 

 and descent. What these circumstances are it is difficult to say. We 

 have good reason to believe that viviparous reproduction, in whatever 

 division of the animal kingdom it is evolved, is always a secondary 

 modification ; that at first eggs were always expelled from the body of 

 the mother to undergo their development, and that in certain cases the 

 expulsion is delayed so that development goes on in the ovary or in the 

 egg-tube. Internal fertilisation must necessarily precede internal develop- 

 ment of the ova, for the eggs cannot begin to develop until they are 

 fertilised. AVe know that the arrangement of the roes in the bony fishes 

 was in existence before internal fertilisation or internal development 

 occurred. A\'e know that in certain species, as has been mentioned in 

 the text, the males have developed a very definite interest in the female 

 and the eggs. There is evident in these cases an association 

 between the perception of the female and her eggs, and the expulsion of 

 the milt. It is not difficult to understand how this association might be 

 so developed that the milt in certain species came to be introduced into 

 the ovary, and so the eggs were fertilised internally. 



In animals in which internal fertilisation takes place the milt is 

 usually injected by some kind of intromittent organ acting as a penis, 

 and in viviparous bony fishes we find some such organ present in the 

 males. In the viviparous blenny it is merely an elongated papilla on 

 which the testicular canals open. In some Cyprinodonts there is in the 

 male a tube passing down the front edge of the ventral fin, and the milt 

 passes through this tube. The ventral fin is elongated, and with the 

 tube forms an intromittent organ. In the females of these species the 

 opening of the oviducts is covered by a special scale, which is free on 

 one side but not on the other. The male organ in some individuals is 

 turned to the right, in others to the left, and in some females the opening 

 beneath the special scale is to the right, in others to the left. Copula- 

 tion thus takes place sideways, a left-sided male pairing with a right-sided 

 female, and vice versa. 



In the viviparous blenny copulation and fertilisation take place in 

 September. The eggs during their formation, or growth to the ripe 

 condition, are contained, as in all bony fishes, in chambers in the 

 ovarian substance, Avhich chambers are called follicles. Whether 

 fertilisation occurs before the eggs have left the follicles is not stated,. 



