FISHES. Ill 



out life. In others the skeleton remains permanently car- 

 tilaginous; in others it is partially cartilaginous and partially 

 ossified ; and, lastly, in most modern fishes it is entirely 

 ossified, or converted into bone. Taking a bony fish (fig. 

 487) as in this respect a typical example of the class, the 

 following are the chief points in the osteology of a fish which 

 require notice : — 



The vertebral column in a bony fish consists of vertebrae 

 which are hollow at both ends, or biconcave, and are techni- 

 cally said to be " amphiccelous." The cup-like margins of 

 the vertebral bodies are united by ligaments, and the cavities 

 formed between contiguous vertebras are filled with the gela- 

 tinous remains of the notochord. This elastic gelatinous 

 substance acts as a kind of ball-and-socket joint between 

 the bodies of the vertebrae, thus giving the whole spine the 

 extreme mobility which is requisite for animals living in a 

 watery medium. The ossification of the vertebrae is often 

 mvich more imperfect than the above, but in no case except 

 that of the Bony Pike {Lepidosteus) is ossification carried to 

 a greater extent than this. In this fish, however, the verte- 

 bral column is composed of " opisthocoelous " vertebrae — that 

 is, of vertebne the bodies of wdiich are concave behind and 

 convex in front. The entire spinal column is divisible into 

 not more than two distinct regions, an alclominal and a 

 caudal region. The abdominal vertebrae possess a superior or 

 neural arch (through which passes the spinal cord), a superior 

 spinous process (neural spine), and two transverse processes 

 to which the ribs are usually attached. The caudal vertebrae 

 (fig. 487) have no marked transverse processes; but in ad- 

 dition to the neural arches and spines, they give off an 

 inferior or hmmal arch below the body of the vertebrae, and 

 the haemal arches carry inferior spinous processes (ha?mal 

 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 



