"Introduction. 



TJ^ISHES form the fourth class of vertebrate animals ; they are provided either with gills 

 -*- {branchice) , or gill-sacs {iiiarsipobranc/nce) , by means of which they are enabled to breathe 

 the air contained in the water in which they live; the heart, which consists of a single auricle 

 and ventricle, is present in almost all fish except in the sub-class Leptocardii, where certain 

 pulsating sinuses perform the functions of a heart, as in the curious little marine fish the 

 Lancelet (Branchiostomi lanccolatuui). The limbs in fishes, corresponding to those organs in 

 other vertebrates, occur, when present, in the form of fins. These fins are generally arranged 

 in pairs at the sides, when they represent the limbs of other vertebrata, or they may occur 

 singly on the back and abdomen ; the paired fins are the pectoral and the ventral ; the dorsal, 

 anal, and caudal fins are unpaired or asymmetrical. 



p u 



The arrangement of the fins will be readily seen in the accompanying woodcut of the 

 Bearded Mullet {Mullus barbatus), di is the first dorsal fin; di the second dorsal; v one of 

 the paired ventrals; p one of the paired pectorals; a the anal fin, and c the caudal fin or tail. 

 For the most part these fins are structurally similar; they consist of a fold of the skin, or 

 expansions of the integument, and are supported by bony or cartilaginous rays, pretty much 

 in the way, as Milne Edwards says, "that the wings of bats are supported by the fingers and 

 ribs." The pectoral fins, which are analogous to the fore-limbs of other vertebrates, are 

 attached by their base to a strong bony arch, which is itself fixed to the back of the skull, 

 or to the anterior part of the spinal column ; this arrangement may be readily seen by any 

 one who carves a Cod's head and shoulders at dinner; the ventrals, or the hind-limbs of 

 fishes, are fixed to an arch of bone — the representation of the pelvic arch of the higher 

 vertebrates — which is sometimes merely supported by the muscles, in cases where these organs 

 are placed far back as in the Pike, a complete skeleton of which is before me as I write; 

 but where the ventral fins are situated not far back, but in the vicinity of the psctoral, the 

 pelvic arch is united to the pectoral arch ; the unpaired or median, fins, as they are sometimes 

 called, are strengthened by osseous or cartilaginous rays, and are supported upon "interspinous 



