204 PISCES. 



the passage and exit of semen, and thus perform the function of ex- 

 ternal generative organs. 



The genus Amphioxus, like the whole order of Cyclostomi, is de- 

 void- of all extremities. In Lepidosiren there are found externally, 

 in the situation of the pectoral and ventral fins, two pairs of inarti- 

 culate filaments. The first pair rest upon a bone or cartilaginous 

 girdle forming their support ; and for the posterior rudiments of fins 

 there exists likewise a pelvic piece. 



MUSCULAR SYSTEM. 



THE Muscles of Fishes are characterized by their slight degree of 

 separation from each other, by the absence of long tendons (which 

 occur only in some anomalous muscles of the cranium, e. g. in the 

 Electric Ray), and by the softness of their fibres. The color of the 

 muscles is generally pale, being either white or having a tinge of 

 yellow ; occasionally, however, as in the Tunny (Scomber thynnus), 

 they are red like the flesh of Mammalia. Their microscopic struc- 

 ture does not differ from that of the other Vertebrata, nor are the 

 characteristic transverse striae wanting even in the whitest fibrillae. 



Peculiar muscles of the integument do not appear to exist ; still 

 the small superficial muscles which move the several rays of the 

 dorsal and anal fins are obviously analogous to the muscles that act 

 upon the feathers in Birds, and upon the ventral scales in Serpents. 

 Each fin-ray is constantly provided with a single superficial muscle 

 upon either side (right and left) of its basis ; they arise from the 

 integument, and wave the fin to and fro so as to maintain the act 

 of swimming. Besides these, there are other muscles more deeply 

 seated, and of some length, which cover the interspinous bones. 

 Each fin itself is provided with a pair of protractor and a pair of 

 retractor muscles, those that correspond upon either side being 

 separated by the interspines, and covered by the great lateral mus- 

 cles of the body. By means of these muscles the dorsal and anal 

 fins can be elevated and depressed. Those Fishes which, like Gas- 

 terosteus, Silurus, Lophius, and Balistes, are furnished with certain 

 strong but loose spines or fin-rays as instruments of defence, present 

 greatly developed and isolated fin-muscles. 



By far the largest portion of the fleshy mass of Fishes is made up 

 by the large lateral muscles of the body ; they consist of longitudinal 

 fibres, which are interrupted by numerous tendinous undulating 

 lines passing from the dorsal to the ventral aspect, and extend from 



