XIV 



.EXTERNAL CHARACTERS. 



body being turned upside down. There is one exception however to this, as 

 the sun-fish, wherein the dorsal and anal fins by means of* rapid undulations 

 assist the fish in locomotion ; while a similar action has been observed upon 

 as seen in the dorsal fin of pipe-fishes. 



Experiments made upon ordinary shaped osseous fishes have demonstrated 

 that should the dorsal and anal fins be removed they roll from side to side. ; 

 if one pectoral is cut off they fall over to the side from which it has been 

 removed; if both, the head sinks : should the pectoral and ventral of one 

 side be gone, equilibrium is lost ; while removal of the tail fin interferes 

 with progression. 



SKELETON. 



The skeleton or endoskeleton of fishes differs widely in the various 

 orders, the bones being less dense in their structure than are those of the 



* 



higher vertebrates. In some the- skeleton is cartilaginous, in others fibrous 

 or osseous, while two or more- varieties of structure may co-exist in one 

 fish. Among those. obtained from great depths in the ocean some at least 

 have their bony and muscular systems but feebly developed, and but loosely 

 connected together by ligaments. 



Fig. 2. Common Perch. 

 a, premaxillary ; b, maxillary ; c, dentary ; d, orbit ; e, cranium ; /, intei'opercle ; g, g,\ vertebral 

 column; h, pectoral fin; i, ventral fin; fc, first dorsal 'fin; I, second dorsal fin; m, anal fin; 

 n, ti, 1 caudal fin. 



If we examine the skeleton, of a teleostean fish', as a perch, we see a 

 framework consisting of a vertebral or spinal column, a skull and fins. The 

 vertebral column consists of a varying number of bones or vertebras, the 

 bodies of which are excavated at each end, rendering, them biconcave or 

 amphiccolous.* The cavity thus .produced by the apposition of two 



* The vertebra; of the exotic bony pike are " opisthocselous," or concave posteriorly and 

 convex anteriorly. 



