Xll EXTERNAL CHARACTERS. 



young fish, as salmonoids prior to the absorption of the umbilical vesicle, 

 the gills remain more or less uncovered by the opercles, and these fins are 

 constantly employed in assisting respiration. The six thicker lower rays of 

 the shanny are thus enlarged and stiff, as they are constantly employed 

 resting upon or against rocks during progression.. In rays and skates this 

 fin is very largely developed. 



The ventral .fins are those which are most frequently absent, not only 

 are they wanting in the large eel-like or apodal class, but occasionally their 

 absence may not even indicate the specimen being distinct from the form which 

 possesses them, as remarked upon respecting the ten-spined stickleback, 

 while their position when present is frequently found to be in widely 

 different localities on the body (see page ix ante). It is usually a narrow 

 fin consisting of but few rays, while these may be of most diverse appearance. 

 It may be restricted to a single spine useful for protective or aggressive 

 purposes as in the little sticklebacks (plate lxviii), or in the more developed 

 unicorn-fish, Triacanthus , of the Eastern seas ; or it may be a simple bony 

 ray having a broad extremity as in Bank's oar-fish (plate lxiv). Or its first 

 divided ray may be elongated into a tactile organ as in the- burbolt 

 (plate lxxxvii), or the two fins may be conjoined forming an adhesive sucker 

 as in the gobies (plate Hi), or reduced to stiff rays for progression as in the 

 blennies (plate lx). 



The anal fin. may be absent, or if present subdivided as the dorsal, it 

 likewise when possessing spinous rays has them in the anterior portion of 

 the fin, while its last few rays may be in the form of finlets as observed 

 of the dorsal fin. 



The caudal fin may be absent as in Bank's oar-fish (plate lxiv), indistinct 

 and often wanting as in some pipe-fishes (plate cxliv), single and with a 

 rounded posterior extremity, as in some gobies (plate lii), or with the central 

 rays rather elongated, giving it a lanceolate form as in some tropical gobies, 

 lunated or emarginate as in the grey mullet (plate lxvii), cut square at its 

 extremity as in the wrass (plate lxx). In most bony fishes the tail consists 

 of an upper and lower portion -which although rarely of exactly the same 

 size are still nearly so: it may be lobed as in the mackerel (plate xxxii), 

 while one or more of its rays may be prolonged : in one form of visitor to our 

 seas the fin is placed somewhat vertically at the end of the tail, as in the 

 vaagmaer (plate lxiii). In forms as Chondropterygians wherein the tail is 

 heterocercal the spine is continued into the prolonged upper lobe. 



Locomotion is doubtless the main use to which the. fins of fishes are put, 

 but even in locomotion other forces are frequently or rather generally brought 

 into play to assist the fins, whether such is for the purpose of swimming, as 

 in the generality of this class, walking as in the angler or frog-fish, leaping 

 as the salmon while ascending rapids when the muscles of the tail are of 



