426 



COMPAEATIYE ANATOMY. 



of their connection with the internal skeletal parts. They not only 

 form a dorsal shield on the dorsal surface^ bat a ventral one on 

 the ventral surface (plastron). In the dorsal shield we can make 

 out a median row of bones, which are fused with the spines of the 

 vertebrEGj and project from them. At the sides there are larger 

 plates, which are fused with rib-like processes, and in addition to 

 these there are special marginal plates around the edge of the 

 shield. These are wanting in Trionyx. Four paired pieces and one 

 unpaired piece can be made out, as a rule, in the plastron. All of 

 these parts are variously developed in different families of the 

 Chelonii. 



Although the dermal bones of the Eeptilia may probably be 

 rightly regarded as derived from the bony carapace of Fishes, we 

 must regard the ossifications which are found in the Edentata as 

 independent arrangements, which have had their origin in fresh 

 adaptive modifications. 



Internal Skeleton. 



§ 327. 



The internal skeleton is of greater morphological importance than 

 the skeletal structures formed from the integument ; it is connected, 

 on the one hand, with arrangements which are found in the Inver- 



tebrata, and on the other, and by a long- 

 series of very varied arrangements, it can 

 be followed out through all divisions of 

 the Vertebrata. 



At first the internal skeleton has the 

 form of a rod-like structure which tra- 

 verses the whole length of the body, and 

 is, when simplest, made up of indifferent 

 cells, and surrounded by a cuticular 

 structure which is formed from a secre- 

 tion of these cells. This primitive organ 

 of support is the chorda dorsalis or 

 notochord ; we have already met with it 

 in the Tunicata (cf. § 303). The invest- 

 ment formed by it is the chordal 

 sheath {cs). 



The eai'liest rudiment of the noto- 

 chord is placed just below the central 

 nervous system; it has not always the 

 same relations to the germ-layers, al- 

 though it must be derived either directly 

 or indirectly from the mesoderm. The compact, and, in all cases, 

 primitively unjointed condition of the notochord speaks to its having 

 been inherited from an unjointed condition of the oi'ganism, and this 

 is what might be supposed from its early appearance in the embryo. 



Fig. 221 a. Section through 

 the vertebral column of Am- 

 moccetes. Ch Chorda. cs 

 Chordal sheath. m Spinal 

 chord, a Aorta, v Veins. 



