70 



ZOOLOGY 



SECT 



sped 



C.C 



fins, or fore- and hind-limbs, are moved by longitudinal muscles 

 derived from those of the trunk. All the voluntary or body-muscles 

 of Crania ta are of the striated kind. 



The ccelome is lined by peritoneum (C, pr.), a membrane consisting 

 of an outer layer of connective-tissue, next the muscles, and an 

 inner layer of ccelomic epithelium bounding the cavity, and thus 

 forming the innermost layer of the body-wall. In Fishes the 

 ccelome is divided into two chambers, a large abdominal cavity 

 containing the chief viscera, and a small f orwardly-placed pericardia! 

 cavity (A, pc.) containing the heart, and lined by a detached portion 



of peritoneum 

 known as the peri- 

 cardium. In Mam- 

 mals there is a 

 vertical muscular 

 partition, the dia- 

 phragm, dividing the 

 ccelome into an an- 

 terior chamber or 

 thorax, containing 

 the heart and lungs, 

 and a posterior 

 chamber or abdomen 

 containing the re- 

 maining viscera. 



Skeleton. The 

 hard parts or sup- 

 porting structures 

 of Craniata fall into 

 two categories the 

 exo skeleton and the 

 endoskeleton. The 

 exoskeleton con- 

 sists of bony or horny 

 deposits in the skin, 

 and may be either 

 epidermal or dermal, 

 or both, but is never, 

 like the armour of an Arthropod or the shell of a Mollusc, cuticular. 

 The epidermal exoskeleton is always formed by the cornification or 

 conversion into horn of epidermal cells, and may take the form of 

 scales as in Reptiles, feathers, hairs, claws, nails, horns, and hoofs. 

 The dermal exoskeleton occurs in the form of either bony or horn- 

 like deposits in the derm, such as the scales and dermal fin-rays of 

 Fishes, and the bony armour of the Sturgeon, Crocodile, or Armadillo. 

 The endoskeleton, or " skeleton " in the ordinarv sense of the 



sh.ncTi- 



p.c.l 



el.rn 



FIG. 770. Semi-diagrammatic transverse section of the vertebral 

 column of a Craniate embryo, c. c. central canal ; el. m. ex- 

 ternal elastic membrane ; h. r. haemal ridges ; n. c. neural tube ; 

 nch. notochord ; nch. c. notochordal cells ; p. c. t. perichordal 

 tube ; sh. nch. sheath of notochord ; sk. c. skeletogenous cells 

 migrating into notochordal sheath ; sk. I. skeletogenous layer ; 

 sp. cd. spinal cord. (Modified from Balfour and Gadow.) 



