102 VERTEBRATE ZOOLOGY 



16. A Swim-Bladder occurs in all but the Elasmobranchii, the 



Holocephali, and in a few degenerate teleosts. It is a gas- 

 filled bladder, derived from, and frequently connected with, 

 the pharynx. In some fishes it is used as an accessory lung, 

 but it is usually for hydrostatic purposes. 



17. Kidneys. — The nephridial system consists of elongated bodies 



situated in the median dorsal part of the coelom. The units 

 of the system are nephric tvhules that have nephrostomes, 

 funnel-like openings into the coelom. The functional Iddney 

 is a mesonephros. 



18. Gonads. — The ovaries and testes are simple sac-like structures 



that have ducts, oviducts and vasa deferentia, developed in 

 connection with the primitive nephridial ducts, as in other 

 groups. 



19. Eggs. — The eggs of different fishes range from large, heavily- 



yolked eggs with chitinous shells, as in the modern elasmo- 

 branchs, to small pelagic eggs of many modern teleosts. 

 The eggs are for the most part fertilized in the open water, but 

 many fishes of various orders practice internal impregnation 

 and are viviparous. 

 An understanding of the majority of these characters will doubt- 

 less be acquired in connection with the laboratory exercises that ac- 

 company courses in vertebrate zoology, but further comment on three 

 of the most significant characteristics of fishes, the fins, the respira- 

 tory organs, and the integument, seems to be necessary. 



The Fins of Fishes 



Of all characters of fishes the fins are, perhaps, the most distinctive 

 since they are adaptations for aquatic life. Analogous structures have 

 been secondarily developed by reptiles and mammals, such as the ex- 

 tinct ichthyosaurs and the porpoise. (Fig. 4). 



The median fim system appears primitively as a continuous median 

 fold supported by cartilaginous or bony rays, running from just back 

 of the head on the dorsal side, round the tail and ending behind the 

 vent on the ventral side. According to one view the median fin sys- 

 tem bears no relation to the paired fin system, which is thought of as 

 being derived independently from gill septa. The prevailing view 

 or "continuous fin-fold theory," however, holds that originally the 

 median fin system, instead of terminating back of the vent, bifur- 



