XIII 



PHYLUM CHORDATA 



229 



Endoskeleton. In the Sturgeon the vertebral column (Fig. 886 

 WS.) consists of a persistent notochord with cartilaginous arches, 

 and is fused anteriorly with the cranium. In the remaining 

 orders bony vertebrae are present ; the centra are biconcave, except 

 in' some Eels, in which the anterior face is flat or even convex, 

 and in Lepidosteus, in which the anterior face is distinctly convex. 

 Vertebrae of this form, i.e. having the centrum convex in front and 

 concave behind, are called opisthoccelous. Ribs are usually present : 

 in Polypterus each vertebra has two pairs, a dorsal pair (Fig. 885, 

 R, I V) of considerable length, running between the dorsal and 

 ventral muscles, and a short ventral pair (f) between the muscles 

 and the peritoneum: the former answer to the ribs of Elasmobranchs, 



-IT 



..V 



FIG. 885. Anterior end of vertebral column of Polypterus. PS. parasphenoid ; R. I V, dorsa 

 ribs ; WK, centra ; t, ventral (pleural) ribs. (From Wiedersheim's Comparative Anatomy.) 



the latter to the ribs (pleural ribs) of the remaining Teleostomi, 

 which are always placed immediately beneath the peritoneum. 

 There may be one or more sets of intermuscular bones, attached 

 either to the neural arch (epineurals), to the centrum (epicentrals), 

 or to the ribs (epiphurals), not preformed in cartilage, but 

 developed as ossifications of the intermuscular septa. The posterior 

 end of the vertebral column is turned up in the Sturgeons, 

 Lepidosteus, and Amia, resulting in a heterocercal tail-fin: in 

 Amia, however, the fin-rays are so disposed that the fin appears 

 almost symmetrical. Among Teleostei the tail-fin is very usually 

 homocercal, as in the Trout, with a more or less disguised asymmetry : 

 in many cases in the adult the development of the large, fan- 

 shaped, posterior haamal arches completely hides the upturned end 

 VOL. II P 



