SKELETON 273 



ossified in the Teleostomes. True bone is present in the earlier and 

 more primitive forms ; but it may become much modified later, and 

 in many Teleostei may lose all trace of bone-cells (p. 355). A 

 return to the cartilaginous condition may, on the other hand, have 

 taken place in certain degenerating groups (Chondrostei). 



The vertebral column differs markedly in structure from that 

 of other fish, more especially the Elasmobranchs, in that the 

 notochordal sheaths remain throughout growth intact (Fig. 338). 

 However much the notochord may be constricted, the mesoblastic 

 cells never pass through the elastica externa to invade the fibrous 

 sheath [265, 471a, 130]. The early and primitive Teleostomes 

 may have possessed permanently unconstricted notochords, as is 

 still the case with the living Chondrostei ; but even amongst the 

 Devonian genera annular bony vertebral bodies are found. The 

 vertebral centra, whether simple or complex, are formed in connec- 

 tion with the neural and haemal arches in the connective-tissue 

 skeletogenoiis layer outside the notochordal sheaths, and are dis- 

 tinguished as perichordal centra from the chordal centra of the 

 Elasmobranchs (p. 100). 



As a rule, the neural and haemal spines form a very complete 

 series, and fuse with the corresponding arches (p. 352). The 

 median fin-radials are present either in equal number or a multiple 

 thereof. As already mentioned (p. 109), the radials (somactidia), 

 both in the paired and the unpaired fins, project as a rule but little 

 into the fin -fold, which becomes more and more completely 

 supported by the dermotrichia. The endoskeletal radii, then, 

 diminish and retreat to the base of the fins as the lepidotrichia in- 

 crease in importance. At the same time, each radial becomes more 

 definitely related to individual lepidotrichia, in the dorsal and anal 

 fins only, of the higher Teleostomi (p. 321 ; Bridge [56], 

 Harrison [196], Goodrich [175]). Whereas in the lower forms 

 the dermal rays are much more numerous than the endoskeletal, 

 in the higher fish they come to correspond in position and 

 number, each double (right and left) lepidotrich forming an 

 elaborate articulation with its supporting radial (Figs. 301 A, 311). 

 The median fins of the Actinopterygii are thus beautifully adapted 

 to folding, being capable of erection or depression at the will of 

 the animal. Similar articulations for the lepidotrichia occur in the 

 paired fins. 



Lepidotrichia are found on the fins of all Teleostomi ; but the 

 presence of small actinotrichia at the edge of all the fins in the 

 adult is quite characteristic of the group (Harrison [196], Goodrich 

 [175]) (Figs. 186, 187). These delicate unjointed horny fibres are 

 the only dermotrichia in the embryonic fins. They closely resemble 

 the ceratotrichia of Elasmobranchs, with which they are in all 

 probability homologous (p. 122). The actinotrichia develop in the 



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