208 



VERTEBRATE ANIMALS. 



vertebrae (Fig. 101, r) ; and they do not 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 productions of the integument ; but there 

 is never any breastbone or sternum properly so called. 



Fig. 101. — Skeleton of the common Percli {PercajlwEiatilis). p Pectoral fin; v One of the 

 ventral fins; a Anal fin, supported upon interspinous bones (i); c Caudal fin; d First 

 dorsal fin ; d' Second dorsal fin, both supported upon interspinous bones ; i i Interspinous 

 bones ; r Ribs ; s Spinous processes of vertebrffi ; h Hffimal processes of vertebMe. 



The only remaining bones of the trunk proper are the so- 

 called " interspinous bones " (Fig. 101, i i). These are a series 

 of pointed, dagger-like bones, imbedded 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 bonps are attached by ligament to the 

 spinous processes of the vertebrae, and at their outer ends they 

 support the framework (rays) of the so-called " median " fins. 

 As a rule there is only one interspinous bone to each vertebra, 

 but in the flat-fishes (Sole, Turbot, etc.) there are two. The 

 limbs of fishes may be wholly wanting, or one pair may be ab- 

 sent, but in no case is the number greater than the regular 

 vertebrate type — namelj', two pairs. When developed, how- 

 ever, the limbs of fishes are very different from those of other 

 Vertebrates, consisting of expansions of the integument, 

 furnished with bony or gristly supports or rays, and thus con- 

 stituting what are called "fins" (Fig. 102). The pair of limbs 

 which correspond to the arms of man and to the fore-limbs of 

 other Vertebrates are termed the pectoral fins, and they are 



