360 



Vertehrata. 



surrounds tlie cliorda^ is not divided into vertebrsBj and carries tlie 

 upper arches (tte arclies may even be absent, e.g., tlie Hag-fis]i 

 Myxine). Usually, however, this tube is broken up into a number of 

 pieces, the vertebrse, united by connective tissue. The vertebrte 



are short tubular bodies, thickened so that 

 the lumen is narrowed centrally and thinner 

 at each end, where there are concavities, 

 connected with each other by a small median 

 opening (like that of an hour-glass) : they are, 

 therefore, biconcave (amphicoelous) ver- 

 tebrae. They surround the notochord which 

 is much constricted intravertebrally, so that 

 it resembles a string of beads. In some 

 Sharks, in which the vertebral column is 

 not divided into vertebraB, the intravertebral 

 constrictions are already indicated by ring- 

 like thickenings on the inner side of the 

 continuous cartilaginous sheath (Fig. 298 B). 

 Dorsally, each vertebra usually bears a 

 neural arch* which is often produced 

 into a neural spine ; often, also, on the trunk 

 vertebrae, there are transverse pro- 

 cesses, which bend down at the beginning of the tail to form 

 the haemal arches, united just as are the dorsal ones. In the 

 Selachians the vertebrae consist of cartilage often partially calcified; 

 in the bony Ganoids and Teleosteans they consist entirely of bone ; 

 or partly of bone, partly of cartilage. 



The part of the vertebral column at the end of the tail, and its 

 relation to the caudal fin, deserves special consideration. In a small 

 number of Fish (Cyclostomi, Dipnoi) the hinder end of the vertebral 

 column is straight and there is an almost equal portion of the tail fin 

 above and below it, the upper portion being congruent with the lower ; 

 the tail is then said to be 4 Jphy car c_ al. In most Fish, on the 

 other hand, the posterior vertefce turn upwards; the lower portion of 

 the fin is then usually better developed than the upper, and the tail 

 is called h e t ev ji cerea l. In most Sharks, the cartilaginous 

 Ganoids and Lepidosteus, this condition is very evident. In the 

 Teleostei (Fig. 299 D) it also obtains, but the turned up 

 portion consists, not of separate vertebrae, but of a single bony or 

 cartilaginous piecet surrounding the end of the notochord, and often 

 uniting with the last or last few lower arches, to form the most 



A b 



Kg. 298. Longitudinal 

 sections through the verte- 

 bral column of various 

 Fish ; schematic. In A and 

 B a continuous cartilaginous 

 tube is stiH present, in C 

 this is divided into centra 

 (%), g boundary of two 

 centra, ch notochord. — Orig. 



* The spaces between the arches are filled in by cartilaginous intercalaria in 

 Selachians and the cartilaginous Ganoids, and thus the tube round the spinal cord is 

 completed. 



t In some Teleostei (Pig. 299 C) the curved portion is longer, and includes several 

 vertebrje besides the rod-like part. 



