VI EXTERNAL CHARACTERS 1 63 



median fins may, individually, all be absent through atrophy. 

 The pectoral fins are rarely absent : nevertheless, in certain 

 species of Syngnathidae, and in most Muraenidae, for example, 

 these fins are entirely vfanting. The pelvic fins are much less 

 constant and are often absent in entire families, as in the Pipe- 

 Fishes (Syngnathidae), the "Electric Eels" (Gymnotidae), and 

 the true Eels (Anguillidae), and in the Globe-Eishes and Porcu- 

 pine-Fishes {Tetrodon, Diodon), as well as in certain genera of 

 families where they are usually present, as in some of the 

 Blennies (Blenniidae) and in the Ophidiidae. Even when present 

 the pelvic fins are often reduced to mere vestiges in the shape of 

 filaments, as in some of the Gadoids (Gadidae) and Eibbon- 

 Fishes, or are represented only by a pair of defensive spines, as 

 in some Sticklebacks (Gastrosteus), or even by a single spine 

 (Balistidae). Complete suppression of the pelvic fins, or their 

 reduction to vestigial remnants, seems to be of frequent occur- 

 rence in Fishes which live in the mud, or are able to pass a 

 longer or shorter time in soil periodically dried during the hot 

 season, as in some Cyprinodontidae, and in species of such tropical 

 Teleostean families as the Ophiocephalidae, Galaxiidae, and 

 Siluridae. Suppression of the dorsal fin is apparent in the 

 Gymnotidae, and of the anal fin in the Eibbon- Fishes. In 

 some of the latter family, as in the rare British visitor the 

 Oar-Fish {Regalecus hanksii), and in the Sea-Horse (IRppocampus), 

 where the tail becomes a prehensile organ for coiling round sea- 

 weeds when the animal is not swimming, the otherwise remark- 

 ably constant caudal fin is absent. 



An initial stage in the degeneration of median fins is to be 

 seen in many of the Salmonidae and Siluridae, in which a 

 posterior division of the dorsal fin becomes reduced in size, loses 

 its fin rays, acquires much fat in its substance, and becomes an 

 " adipose fin." 



The " lateral line " is a notable feature Jn the external appear- 

 ance of most Fishes. Originally developed in the superficial 

 epidermis of the skin in the form of linear tracts of isolated 

 and often segmentally arranged masses of sense-cells, these organs 

 subsequently become imbedded for protection in the epidermic 

 lining of either an open groove or a closed canal extending along 

 each side of the trunk and tail, and prolonged on to the more 

 exposed parts of each side of the head in the shape of a more 



