34 



VERTEBRATE SKELETON 



in Lepidoskus, intercalated cartilages, but differing from the inter- 

 calaria of other groups. 



The neural arches of the adult (separated from the centra in a 

 few forms) are closed by neural spines, and the same holds for the 

 haemal arches in the tail. There is usually a regular transition from 

 the caudal haemal arches to the haemal ribs of the trunk, these latter 

 lying just beneath the peritoneum and being usually articulated to 

 basal stumps, the latter placed higher on the sides of the centra 

 further forwards, even on the neurapophyses near the head. The 

 form and length of the haemal ribs varies considerably, even in the 

 same fish. In some Pleuronectids complete 

 haemal arches occur on the ventral side of the 

 trunk vertebrae, no haemal ribs being present. 

 Lophobranchs and Plectognaths lack both kinds 

 of ribs. 



The neural arches bear pre- and postzygapo- 

 physes, and in the tail similar interlocking 

 structures may occur on the haemal arches. 

 These make the column more firm and the 

 whole is strengthened by longitudinal ligaments 

 between the zygapophyses. In some Scombrids 

 the zygapophyses of the two sides meet dorsal to 

 the spinal cord, adding to the roof of the spinal 

 canal. Dry skeletons frequently show two 

 canals in the neural arch, the normal spinal 

 canal, and dorsal to this, one for the longitudinal 

 ligament (p. 19). 

 The number of vertebrae in Teleosts is variable, the larger number 

 occurring in the eel-like fishes, while some Teleosts may have as few 

 as twenty-three in the trunk. Very commonly the anterior vertebrae 

 are fused, and the resulting bone may fuse with the cranium, or the 

 connexion between column and skull may be strengthened by the 

 spinous process of the first vertebra. 



Besides the haemal ribs, some fishes like the shad and herring have 

 numerous slender bones of membrane origin among the trunk mus- 

 cles in the myosepta and horizontal septum. They have no common 

 name, but are grouped according to position (fig. 37) as epimerals, 

 epicentrals and hypomerals. They are frequently forked, one 

 branch coming into close relations with the vertebral column. 



Fig. 37. — B ones of 

 anterior caudal region of 

 Alosa (Butschli, ' i o ) . 

 c, vertebral centrum; h, 

 haemal base; hr, htemal 

 rib; i, hypomeral; m, 

 epicentral; s, epineural. 



