358 



Vertebrata. 



is a bony rod, wliicli is jointed, i.e., is divided transversely into a 

 number of short pieces connected by connective tissue ; moreover, 

 it is more or less deeply split at its apex into several brancbes, 

 wbich are also jointed. Some, However, do not sbow tbis splitting, 

 and in otbers (Pig. 296 w') the jointing is limited to the tip of 



the ray, or is absent altogether. Each 

 soft ray consists of two symmetrical halves, 

 closely apposed, and corresponding the 

 one with the other. The spinose rays are 

 stiff, pointed, unsegmented bony rods ; 

 they also are composed of two halves, 

 which are either closely connected or fused. 

 Intermediate forms also occur, which, 

 though not jointed, are yet flexible. Whilst 

 soft rays are present in all Pisces, and 

 especially in those with bony rays, the 

 spinose rays only occur in some of them, 

 and then are almost invariably confined to 

 the front of the fin ; they are absent from 

 the caudal fin. The bony rays are capable 

 of depression and erection, the membrane 

 being folded together and then expanded. 

 Sometimes the skin is absent from several successive rays (free 

 rays). For the endoskeleton connected with the unpaired fins, 

 interspinal bones, etc., see below. Along the edge of the limbs, the 

 pectoral and pelvic fins, which are usually very short, 

 there is a fin border which is of similar structure to that of the 

 unpaired fin, furnished in the Selachians with horny rays,* in Ganoids, 

 Dipnoans, and Teleosteans with bony rays (soft or spinose). Spiny 

 rays are very rarely present in the pectoral, but are more abundant 

 anteriorly in the pelvic fin, and in this case occur also in dorsal and 

 anal fins. For the sense organs of the skin (lateral line, etc.), see 

 below. 



Phosphorescent organs ocoxu- in some Kshes ; they are pecuharly 

 modified portions of skin which look like larger or smaller spots ; their structure 

 is as yet not quite understood. Such organs ai-e fairly common in the Deep-sea 

 Fish, but are also present in some pelagic foi-ms. 



Jn many Pisces (Cyolostomes and Selachians), the skeleton, 

 with the exception of the notochord, consists entirely of cartilage, 

 which is, however, usually to some extent calcified, i.e., calcareous 

 salts are absorbed by the intercellular substance ; in others (Ganoidei, 

 Dipnoi, Teleostei), the cartilage which originally forms the whole 



Fig-. 296. Portion of a fin 

 with spiny rays (s), and 

 soft rays {iv w') ; t inter- 

 spinal bones ; diagrrammatic. — 

 Orig. 



* The edge of the fin and the horny rays are specially weU developed in the paired 

 fins of the Sharks. In the Eajidse, on the other hand, the cartilaginous rays belongingf 

 to the limb skeletons to be mentioned below, reach aimost to the edge of the fin, and 

 in connection with this the homy rays in the pectoral und pelvic fins are little 

 developed, or altogether absent. 



