70 



ZOOLOGY 



SECT 



sped 



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

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

 of Craniata are of the striated kind. 



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

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

 inner layer of coelomic 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 pericardial 

 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 

 coslome 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 



n.c 



c.c 



jo.o.t 

 r 



_ 7 _ 

 el.rn 



endosJceleton. 

 exoskeleton 



The 

 con- 



Jfia. 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 ; deDOsits in the skin 

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



tube ; sh. nch. sheath of notochord ; sk. c. skeletogenous cells and may be either 

 migrating into notochordal sheath ; sk. I. skeletogenous layer ; . j _ i i i 



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



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, a'nd 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 



