ENDOSKELETON 



419 



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. 

 102 and 1 08, sp. cd) : its 

 anterior or cerebral portion 

 is dilated, and contains the 

 brain. 



Skeleton. — Imbedded in 

 the body-wall and extend- 

 ing into the fins are the 

 various parts of the endo- 

 skekton. This character- 

 istic supporting framework is 

 mainly composed, as in the 

 tadpole and in embryos of 

 Vertebrates generally, of 

 cartilage, which may be more 

 or less im]i)regnated with 

 lime salts, so as to have, in 

 part, the appearance of bone, 

 but in most parts differing in 

 structure from true bone and 

 consisting merely of calcified 

 cartilage (p. 46). 



The entire skeleton con- 

 sists of separate pieces of 

 cartilage, calcified or not, 

 and connected with one 

 another by ligaments (p. 57) : 

 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 



E E 2 



Fig. 102. — Diagrammatic trailbvtrae 

 section through the trunk of a 

 female dogfish. The ectoderm is 

 dotted, the endoderm radially 

 striated, the mesoderm evenly 

 shaded, and the ccelomic epithelium 

 represented by a headed line. 

 Card. I', cardinal vein ; Cccl. coilome ; 

 CteL Epthtu. parietal, and Cixi. 

 Eptliiu' . visceral layer of coslomic 

 (peritoneal) epithelium ; D. Ao. 

 dorsal aorta ; Derm, derm ; Derm. 

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

 fin; /. hit, r. ventral intra- intes- 

 tinal vein ; /«/. intestine; K. kid- 

 ney ; Lat. P'. lateral vein ; HI. 

 myomeres ; N. .4 . neural arch ; A' 

 a£. central canal of spina! cord ; 

 JVp/z. nephridium ; 07'd. oviduct ; 

 Oz'j/. ovary ; Sp. Cd. spinal cord ; 

 Ur. ureter; ^'. Gvz/. vertebral cen- 

 trum. (From Parker's Ekntcntary 

 Biology.') 



as in the frog, it is divisible 



