Glas.f 2. Pisces. 



381 



It is quite correct to speak of a metamorphosis in many 

 Teleosteans, since the young one leaves the egg in a very imperfect 

 condition, differing much from the adult ; often the caudal extremity 

 of the vertebral column is still straight ; there is a continuous dorsal 

 and ventral fin, etc. (Fig. 311 A). It may often happen that the 

 transition from this state to the adult form is not a simple, gradually 

 advancing development ; but the larva not infrequently displays 

 special characters for a long time after leaving the egg, which do not 

 occur in the newly-hatched young, nor in the adult. Especially 

 in the Pelagic Fauna, abundant large-eyed 'I'eleostean larvae are 

 met with, possessing enormous spines, and fin appendages, struc- 

 tures which recall those observed in many pelagic crustacean larvEe, 

 €.gr., the Crabs (Fig. 312). For the peculiar development of the 

 Lampreys, see p. 382. 



The embryos of Selachians (Fig. 313) are distinguished by the 

 possession of a huge yolk sac, and also by the projection from the gill-slits of 

 numerous long gill-filaments, processes from the gill-lamelte. These 

 filaments are embryonic organs, and atrophy before birth. 



Most Fish are predaceous, only a few feed upon plants or mud. 

 The majority are marine, but many are freshwater (some species are 



- fc 



Fig. 313. .1 Bay embryo, B Shark embryo with exteinal gills (k). d yolk sac (not 

 completely drawn; removed in .4). 



both) ; a few wander upon land. They often migrate from one 

 place to another in the sea, or from the sea into fresh water and 

 back. They usually occur in shoals. Fish make their way through 

 the water by movements of the whole body, and by lateral movements 

 of the tail ; the Teleostei can also progress slowly by undulations of 

 all the fins (paired and unpaired).* 



* A very singular movement, 

 about by the powerful fore limbs. 



suggesting flight, occurs in the Rajidse, brought 



