CHORD. \TA 



275 



tail fin ; such a tail is called diphycercal 

 (Figs. 305 and 307). Or, more com- 

 monly, the posterior end of the vertebral 

 column may be bent upwards, toward 

 the dorsal side, forming a heterocercal 

 tail ( Fig. 285); in this case the outline 

 of the caudal fin may be asymmetrical, 

 as in sharks, or it may be symmetrical 

 despite the shape of the vertebral col- 

 umn, and is then sometimes called a 

 homocercal tail (Figs. 297 and 298). 

 In addition to the unpaired fins there 

 are generally two pairs of paired fins, 

 an anterior pair, the pectoral, and a 

 posterior pair, the pelvic ; sometimes 

 the pelvic fins lie very far forward, even 

 anterior to the pectoral, but in such 

 cases the pectoral fins are the more 

 dorsal in position. Running longitudi- 

 nally along each side of the body is a 

 more or less distinct line ; this is called 

 the lateral line, and marks the position 

 of a series of sense organs, which also 

 extend over the head. 



The gills are delicate processes or 

 folds, rich in blood vessels, attached 

 to the walls of the gill clefts. They 

 are supported by skeletal structures, 

 the gill or branchial arches. In some 

 fishes the number of gill clefts visible 

 on the exterior of the body is the same 

 as in the wall of the pharynx, but in 

 most fishes there is a fold of the skin 

 which covers the gills, leaving only 

 a single external opening on each side 

 of the body. This fold is called the 

 operculum (Fig. 297) and is supported 

 by skeletal structures. While the fishes 

 are sharply marked off from the other 



FlG. 285. Mustelus antarcticus, 

 a dogfish. Lateral aspect with a 

 portion of the skin removed, an, 

 anus; c.d.f, caudal fin; d./.i, 

 d.f.2, dorsal fins ; e, eye ; ex.br. ap, 

 external branchial aperture ; /./, 

 lateral line ; ;;;///, mouth ; myc, 

 myocommata; mym, myomeres; 

 n.a, nasal aperture ; pet. /'. pectoral 

 fin ; pv. /', pelvic fin ; sp, spiracle ; 

 v./. ventral fin. (From Parker's 

 Biology.) 



