THEIR GENERAL CHARACTER AND STRUCTURE 15 



stock is maintained, their strange structure, which at first 

 sight might seem to be a serious handicap in the struggle 

 for life, must be pronounced a successful departure from the 

 symmetrical scheme of Nature. 



The fins count for a great deal in determining the affinities 

 of genera and species among fishes. 



The two pairs of horizontal fins — the pectoral and the 

 ventral — have already been alluded to as the homologues of 

 Pectoral and ^^e anterior and posterior limbs of the higher 

 ventral fins, vertebrates, and their functions described as princi- 

 pally those of balancers, steerers, and regulators of speed. 

 They consist of a membrane, generally more or less trans- 

 parent, supported and extended by rays. Sometimes this 

 membrane of the ventral fins is brilliantly coloured, as in the 

 perch and roach. The position of the pectoral fins is con- 

 stant, just behind the gill-opening ; but that of the ventral 

 fins is very variable. In most British fresh-water fishes they 

 are situated far back on the abdomen, as in the Salmon, Carp, 

 and Roach Families ; in some they are on the thorax, as in the 

 perches ; in others on the jugular region, in advance of the 

 pectorals, as in the burbot and miller's thumb ; in others, 

 again, they are wholly absent, as in the eels and lampreys. 

 In the determination of genera and species, the number of rays 

 in the pectoral fins is held of little account ; but those in the 

 ventral fins are of more importance, and, together with the 

 rays of the dorsal fin, afford a sure guide to affinity in the 

 Acanthopterygii, or Spiny-finned Fishes. 



The unpaired, vertical fins are generally more conspicuous 

 than the others. The most elementary form has been described 

 The vertical ^y Dr. GUnther as " a simple fold of the skin sur- 



fins. rounding the extremity of the tail," and its gradual 

 extension forwards along the upper and under surfaces may 

 be traced in the development of fishes. Continuous in the 

 primitive form of fish, this vertical fin soon became interrupted, 

 and ranged itself in three principal masses — the caudal fin at 



