ENDOSKELETON 



435 



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. Imbedded 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 may be more 

 or less impregnated with 

 lime salts, so as to have, in 

 part, the appearance of bone, 

 but differing in structure 

 from true bone and con- 

 sisting merely of calcified 

 cartilage (p. 46). 



The entire skeleton con- 

 sists of separate pieces of 

 cartilage, calcified or not, 

 and connected with 



one 



FIG. in. Diagrammatic transverse 

 section through the trunk of a 

 female dogfish (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. 



Card. V. cardinal vein ; CceL coelome ; 

 Cccl. Epthm. parietal, and Coel. 

 Epthiri ' . visceral layer of coelomic 

 (peritoneal) epithelium ; D. Ao. 

 dorsal aorta ; Derm, derm ; Derm. 

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

 fin ; /. int. V. ventral intra- intes- 

 tinal vein; Int. intestine; K. kid- 

 ney ; Lat. V. lateral vein ; M. 

 myomeres ; N. A . neural arch ; N. 

 coe. central canal of spinal cord ; 

 Nph. nephridium ; Ovd. oviduct ; 

 Ovy. ovary ; Sp. Cd. spinal cord ; 

 Ur. ureter ; V. Cent, vertebral cen- 

 trum. (From Parker's Elementary 

 Biology.') 



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 



F F 2 



