VERTEBRATA : FISHES. 



463 



this fish, however, the vertebral column is composed of " opis- 

 thocoelous " vertebrae that is, of vertebrae, the bodies of which 

 are concave behind and convex in front. The entire spinal 

 column is divisible into not more than two distinct regions, an 

 abdominal and a caudal region. The abdominal vertebrae pos- 

 sess a superior or neural arch (through which passes the spinal 

 cord), a superior spinous process (neural spine), and two trans- 

 verse processes to which the ribs are usually attached. The 

 caudal vertebrae (fig. 248) have no marked transverse processes ; 



Fig. 248. Skeleton of the common Perch (Percafluviatilis). p One of the pectoral 

 fins ; v One of the ventral fins ; a Anal fin, supported upon interspinous bones (z) ; 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 vertebrae ; h Haemal 

 processes of vertebrae. 



but, in addition to the neural arches and spines, they give off 

 an inferior or hcemal arch below the body of the vertebrae, and 

 the haemal arches carry inferior spinous processes (haemal 

 spines). 



The ribs of a bony fish are attached to the transverse pro- 

 cesses, or to the bodies of the abdominal vertebrae, in the form 

 of slender curved bones which articulate with no more than 

 one vertebra each, and that only at a single point. Unlike the 

 ribs of the higher Vertebrates, the ribs do not enclose a thoracic 

 cavity, but are simply embedded in the muscles which bound 

 the abdomen. Usually each rib gives off a spine-like bone, 

 which is directed backwards amongst the muscles. Inferiorly 

 the extremities of the ribs are free, or are rarely united to der- 

 mal ossifications in the middle line of the abdomen ; but there 

 is never any breast-bone or sternum properly so called. 



The only remaining bones connected with the skeleton of 

 the trunk are the so-called interspinous bones (fig. 248, / /). 



