EXDOSKELETOX 



435 



the dorsal body-wall is tunnelled, from end to end, by a 

 median longitudinal neural cavity, in which the central 

 nervous system is contained. The greater part of the 

 cavity is narrow and cylindrical, and contains the spinal 

 cord (Figs, in and 117, 

 sp. cd) : its anterior or 

 cerebral portion is dilated, 

 and contains the brain. 



Skeleton. Embedded in 

 the body-wall and extend- 

 ing into the fins are the 

 various parts of the endo- 

 skeleton. This character- 

 istic supporting framework 

 is mainly composed, as in 

 the tadpole and in embryos 

 of Vertebrates generally, 

 of cartilage, which ma}' be 

 more or less impregnated 

 with lime salts (p. 46), so 

 as to have, in part, the 

 appearance of bone but 

 differing in structure from 

 true bone : this calcifica- 

 tion is well seen in the 

 vertebral centra (p. 439). 



The entire skeleton con- 

 sists of separate pieces of 

 cartilage, calcified or not, 

 and connected with one 

 another by ligaments (p. 57) : as in the frog, it is 

 divisible into skull, vertebral column, and skeleton of the 

 paired fins, with their arches or girdles ; in connection 

 with the skull are certain cartilaginous visceral arches, 

 forming the upper and lower jaws and supporting the 



at.V 



IlG. in. Diagrammatic transverse 

 section through the trunk of a 

 female dog fish (compare Fig. 5). 

 The ectoderm is dotted, the endo- 

 derm radially striated, the meso- 

 derm evenly shaded, and the coelo- 

 mic epithelium represented by a 

 beaded line. ( y. J.) 

 Card. I', cardinal vein ; Ccel. coelome; 

 Cael. Epthm. parietal, and Ccel. 

 Epthm'. visceral layer of coelomic 

 (peritoneal) epithelium ; D. Ao. 

 dorsal aorta ; Der. Epthm. epi- 

 derm ; Derm, derm ; Derm. F. /?. 

 dermal fin-ray ; D. F. dorsal fin ; 

 /. int. I", intra-intestinal vein ; Int. 

 intestine ; K. kidney ; Lot. V. 

 lateral vein ; M. myomeres ; .V. A ; 

 neural arch ; A', cce. central canal 

 of spinal cord ; \pH. nephridium ; 

 Ovd. oviduct ; Ovy. ovary ; Sp. Cd. 

 spinal cord ; L'r. ureter; I'. Cent. 

 vertebral centrum. (From Parker's 

 Elementary Biologv.) 



