72 ZOOLOGY SECT. 



between the skin and the skeleton, presently to be referred to : in 

 the tail, however, there is found a hwmal canal (h. c.) containing 

 connective-tissue, and representing a virtual backward extension 

 of the ccelome. The 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 striped kind. 



The coelome is lined by peritoneum (C, pr.), a membrane con- 

 sisting 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 forwardly-placed peri- 

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

 portion of peritoneum known as the pericardium. In Mammals 

 there is a vertical muscular partition, the diaphragm, dividing the 

 ccelome into an anterior chamber or thorax, containing the heart 

 and lungs, and a posterior chamber or abdomen containing the 

 remaining viscera. 



Skeleton. The hard parts or supporting structures of Craniata 

 fall into two categories the exoskeleton and the endoskeleton. The 

 exoskeleton consists 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 con- 

 version 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 ordinary sense of the 

 word, forms one of the most complex portions of the body, and 

 presents an immense range of variation in the different classes and 

 orders. As in Amphioxus, the axis of the entire skeletal system 

 is formed by the notochord (Fig. 760. nch.), an elastic rod made of 

 peculiar vacuolated cells (Fig. 761, nch.), resembling the pith of 

 plants, and covered by a laminated sheath (sh. nch.), with an 

 external elastic membrane (el. m.) around it. The whole sheath is a 

 cuticular product of the superficial notochordal cells (nch. c.), i.e., 

 is developed as a secretion from their outer or free surfaces. The 

 notochord lies in the middle line of the dorsal body-wall between 

 the cerebro- spinal cavity above and the ccelome below : it is 

 usually developed, as in the lower Chordata, from a median 

 longitudinal outgrowth of the dorsal wall of the gut. Posteriorly 

 it extends to the end of the tail, but in front it always stops short 

 of the anterior end of the head, ending near the middle of the 

 brain immediately behind a peculiar organ, the pituitary body 



