158 



FISHES. 



In tlie Cyelostomes the generative organ is single, and 

 fixed to or suspended from the median line of the back of 

 the visceral cavity by a duplicature of the peritoneum {meso- 

 arium) ; the testicle and ovary being distinguishable by 

 their contents only. These escape by dehiscence of the cells 

 or capsules and rupture of the peritoneal covering into the 

 abdominal cavity, and are expelled by reciprocal pressure of 

 the intertwined sexes through the porus genitalis, which is 

 sunk between two labia of the skin in Myxine, and produced 

 into a long papilla in Petromyzon. 



The ova of the Lampreys are small, globular, Uke those of 

 Teleosteans. Those of Myxine have a very peculiar shape 

 when mature ; they are of an oval form, about 15 millimetres 

 long and 8 millunetres broad, enveloped in 

 a horny case, which at each end is provided 

 with a bundle of short threads, each thread 

 ending in a triple hook. Whilst in the 

 mesoarial fold the eggs are attached to one 

 another by means of these hooks, and after 

 being expelled they probably fix themselves 

 by the same means to other objects. As in all 

 fishes producing ova of large size, the number 

 of ova matured in one season is but small. 

 In Teleosteans the generative organs are 

 Myxine giutinosa, comparatively large. In some families the 

 enlarged. ovaries are without closed covering and with- 



out oviducts, as in Salmonidce, Galaxiidce, Notopteridm, Murce- 

 nidce, and others. The surface of such an open ovary — 

 as, for instance, that of the Salmon — is transversely plaited, 

 the ova being developed in capsules in the stroma of the 

 laminae ; after rupture of the capsules the mature ova drop 

 into the abdominal cavity, and are expelled by the poms 

 genitalis. The ovaries of the other Teleosteans are closed 

 sacs, continued into oviducts. Frequently such ovaries 



Fig. 69.— Ovum of 



