230 



VERTEBRATE ANIMALS. 



enclose any thoracic cavity, or protect the organs which are usually 

 contained in the chest — namely, the heart and breathing-organs. 

 The anterior or lower ends of the ribs of fishes are free, or are 

 rarely united to hard pioductions of the integument ; but there is 

 never any breastbone or xtvrmim, properly so called. 



The only i-emaining bones of the trunk projier are the so-called 

 " interspinous bones " (fig. 165, it). These are a series of pointed 

 dagger-like bones, embedded in the middle line of the body, between 

 the great lateral muscles which form the greater part of the body of 

 a fish. The inner ends or points of the interspinous bones are at- 

 tached by ligament to the spinous processes of the vertebrfe, and at 

 their outer ends they support the framework (rays) of the so-called 



Fig. 165.— Skeleton of the common Perch {Perm flltviatilis'). p Pectoral fin ; v One of 

 the venti-al tins ; a .\iial fin, supported upon inleispinony hones (i) ; c Caudal fin ; 

 tl First dor.sal fin ; d' Srt ond dorsal tin, both sujiported n]ion interspinous hones ; 

 ii Intersjinious hones; 7-Rihs; s Sj-inous processes of ^crfelirte ; h Hivnial pro- 

 ci-sscs of vi-r1chr;i'. 



"median" fins. As a rule there is only one interspinous bone to 

 each vertebra, but in the Flat-fishes (Sole, Turliot, &c.) there are two. 

 The limbs of fishes may be wholly wanting, or one pair may be 

 absent, but in no case is the number greater than the regular Verte- 

 brate type — namely, two pairs. When developed, however, the 

 limbs of fishes are very diff'erent from those of other Vertebrates, 

 consisting of e.\]iansiiins of the integument, furnished with- bony 

 or gi'i-stly supports (jr rays, and thus constituting what are called 

 '•fins "(fig. IfiCi). The pair of limbs which correspond to the arms 

 of man and to the foi-e-limbs of other Vertebrates are termed the 

 pectoral fins, and they are attached to a bony arch which is attached 

 either to the back of the the skull or to the spinal cc^lumn (fig. 165, 

 p, and 166, ji). The hind-limbs in fishes are known as the reiitral 



