322 ZOOLOGY. 



organs. Metamerism on the contrary is much more evident 

 from the internal than from the external organs. There are 

 usually two pairs of lateral appendages for support and loco- 

 motion : the thoracic at the anterior end of the trunk, and the 

 pelvic, ordinarily occurring near the union of the trunk and 

 tail. These are variously modified as to their form and in- 

 ternal structure (e. g., fins, legs, arms, wings), but are looked 

 upon as homologous. In many water forms there are median 

 appendages (dorsal, ventral, and caudal fins) also assisting in 

 locomotion. The ccelom or body cavity is well represented in 

 the trunk region, and arises by a splitting of the mesoderm into 

 an inner layer which comes to unite with the digestive tract 

 and an outer layer which unites with the ectoderm (Fig. 154). 

 In this the visceral cavity beside the mesoderm-covered 

 digestive tract to which reference has already been made, lie 

 the principal organs of respiration, of excretion, of circulation, 

 and of reproduction. Dorsal to the notochord the nervous 

 system occupies a cavity within the mesoderm, which is not, 

 however, a part of the ccelom. This is described as the dorsal 

 or neural cavity and is protected by a sheath of cartilage or 

 bone. In the anterior region this is much enlarged to accom- 

 modate the brain. This condition of -a dorsal and ventral 

 cavity is very characteristic of vertebrates. In mammals the 

 ventral cavity is further divided by the diaphragm into an 

 anterior or thoracic and a posterior or abdominal cavity. 



338. Protective and Supportive Structures the Integu- 

 ment. Covering the body of vertebrates is the skin, which 

 consists of two layers; the outer, or epidermis, which is de- 

 rived from the ectoderm, and the dermis or true skin which 

 is mesodermal in origin. The epidermis consists of from two 

 to many layers of cells in thickness, and in the higher forms 

 the differentiation into layers becomes very pronounced (Fig. 

 153, E). The outermost cells of the epidermis frequently be- 

 come hardened for the better protection of the parts within. 

 This is especially true of the terrestrial forms. .The inner layer 



