192 



BONY FISHES 



VII. 2- 



The tail differs from that of elasmobranchs in being outwardly 

 symmetrical, though internally there are still traces of the upturned 

 tip of the vertebral column (Fig. 118). Besides the typical caudal 

 'fish-tail', supported by bony rays, there are two dorsal fins and a 

 ventral fin, but the hinder dorsal fin differs from the others in having 

 no rays to support it and is called an adipose fin, because of its flabby 

 structure. The paired fins are rather small and it is from their struc- 

 ture that the whole group derives the name Actinopterygii or rayed-fin 

 fishes. There is no lobe projecting from the body and containing 



Fig. i i 6. Male and female brown trout (Salmo trutta) spawning. The male is quivering — a 



short sequence of rapid shudders of whole body which excites the female. 



(After J. W. Jones, The Salmon.) 



basal fin supports, as there is in the fin of lung-fishes. All the basal 

 apparatus of the fin is contained within the body wall and only the fin 

 rays project outwards, as a fan. The pelvic fin of bony fishes often 

 lies relatively far forward; in the trout, however, it is unusually far 

 back, just in front of the anus; in other types it may be level with the 

 pectoral fin, or even anterior to it (Fig. 118). The significance of the 

 shape of the body and fins in swimming will be discussed later (p. 244). 

 The skin consists of a thin epidermis and thicker dermis, the former 

 has stratified squamous layers but contains no keratin (Burgess, 1958). 

 It contains mucous glands. The mucus of some eels and other fishes 

 has remarkable powers of precipitating mud from turbid water. The 

 mesodermal dermis provides an elaborate web of connective tissue 

 fibres. It also contains smooth muscle, nerves, chromatophores, and 

 scales. The latter are thin overlapping bony plates, covered by skin, 

 that is to say, they do not 'cut the gum' as do placoid denticles. The 

 exposed part of each scale bears the pigment cells, which control the 

 colour of the animal, in a manner presently to be described. The bone 

 of the scales is absorbed at intervals by scleroclasts, making a series 

 of rings, which, like the growth-rings on a tree, are due to the fact 

 that growth is not constant but occurs fast in the spring and summer 



