v1 EXTERNAL CHARACTERS 163 
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 wanting. 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-Fishes and Porcu- 
pine-Fishes (Zetrodon, 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 Ribbon- 
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 Ribbon-Fishes. In 
some of the latter family, as in the rare British visitor the 
Oar-Fish (Regalecus banksii), and in the Sea-Horse (Hippocampus), 
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 1s 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 in 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 
