64 ZOOLOGY SECT, xin 



In the trunk, as shown by a section of that region, the muscles 

 form a definite layer beneath the skin and enclosing the ccelome 

 (Fig. 715, A and C, cod.). The muscular layer, as in Amphioxus, is 

 not of even diameter throughout, but is greatly thickened dorsally, 

 so that the coelome is, as it were, thrown towards the ventral side. 

 Its dorsal portion, moreover, is excavated by a canal, the neural or 

 cerebro-spinal cavity (c. s. c), in which the central nervous system is 

 contained, and the anterior portion of which is always dilated, as 

 the cranial cavity, for the brain. Thus a transverse section of the 

 trunk has the form of a 'double tube. In the head, neck, and tail, 

 (B, D), the coelome is absent in the adult, and the muscles occupy 

 practically the whole of the interval between the skin and the 

 skeleton, presently to be referred to : in the tail, however, there is 

 found a hcemal canal (Ji. 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 ccelome 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 pen- 

 car dial cavity (A. pc.) containing the heart, and lined by a de- 

 tached 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,. 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 fin-rays of Fishes, and 

 the bony armour of the Sturgeon, Crocodile, or Armadillo. Some 

 recent researches tend to show that the dermal exoskeleton may 

 be ectodermal and not mesodermal in its ultimate origin. 



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 



