354 METAZOAN PHYLA 



eel-like body permits the animal so shaped to explore crevices. Many- 

 fishes, however, show modifications, often very curious, that fit into none 

 of these general types. 



376. Scales. — Most fishes possess a soft epidermis, below which are 

 dermal scales. These are composed of bone and appear in several 

 different forms (Fig. 250). Placoid scales are characteristic of elas- 

 mobranchs, and the types known as cycloid and ctenoid are found in the 

 bony fishes. Scales of the two latter types overlap like shingles on a 

 roof. Cycloid scales are elliptical in shape, are marked by concentric 

 lines, and are found more frequently in the lower teleosts. In the 

 ctenoid type the portion not overlapped by adjacent scales is covered 

 with small, toothlike points. A fourth type of scale, the ganoid scale, 

 has a hard external enamel-like covering of ganoin which is produced 

 by the dermis. (True enamel is a product of the epidermis.) Ganoid 

 scales may overlap, but in some cases they are rhombic in form and are 

 arranged like tiles, meeting but not overlapping. In some fishes the scales 

 are small and completely hidden in the skin, or they may, as in the cat- 

 fishes, be entirely absent; in other cases, as in the trunk fishes, they form 

 a complete bony box, the fins being articulated into openings in this box. 



377. Fins. — The swimming appendages vary in number and precise 

 location, but they are always of two kinds: (1) unpaired median fins, 

 which include the dorsal, caudal, and anal fins; and (2) paired lateral 

 fins, which include the pectoral and pelvic, or ventral, fins. The principal 

 use of the fins is in locomotion, but various modifications for other 

 purposes occur. By some fish they are used in walking; by others, 

 such as the flying fish, they are used as gliding planes; or they may be 

 modified to form sucking discs. The pelvic, and rarely the caudal, 

 fins may be absent. 



378. Locomotion. — In a fish the caudal fin is the principal locomotor 

 organ, the paired fins being held closely against the side of the body 

 in rapid movement and the other median fins being spread to maintain 

 the vertical position. The body is relatively rigid anteriorly but toward 

 the base of the tail it is very flexible. In rapid swimming the tail is 

 carried from side to side in such a manner as to trace a figure 8, a path 

 of motion similar to that of an oar in the sculling of a boat. The course 

 taken by the fish is directed upward or downward and to either side by 

 modification in the strength of the strokes of the caudal fin. In quiet 

 maneuvering the paired lateral fins come into service, being used like 

 oars. When the body is at rest they also serve to maintain equilibrium. 

 If both paired fins are removed, a fish turns completely over with the 

 ventral side upward, and removal of those of one side causes the fish 

 to lie upon that side. 



379. Air Bladder. — A characteristic organ in bony fishes is the 

 air bladder, or swim bladder, which lies against the dorsal wall of the 



