326 ZOOLOGY 



an inner layer which comes to unite with the digestive tract 

 and an outer layer which unites with the ectoderm (Fig. 156). 

 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 coelom. 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. 



346. 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. 

 155, 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 

 of the epidermis is usually columnar in form, and from this 

 layer the outer cells are renewed, and all special epidermal 

 growths arise (Fig. 155, c.e.). The dermis consists largely 

 of connective tissue, but contains in addition nerves and blood 

 vessels beside such ingrowths from the epidermis as glands, 

 hair-follicles, etc. Fat is deposited in the lower layers of the 

 dermis in many vertebrates. 



347. Special Products of the Integument often occur in the 

 form of outgrowths or ingrowths. Glands are examples of the 

 latter, and are frequent in connection with the epidermis. They 

 may be simple and unicellular (mucous glands in fishes) or 

 multicellular, penetrating deep into the dermis (sweat and oil 

 glands. Fig. 155, sg). The mammary glands of Mammalia are 

 possibly modified forms of the sweat glands. The outgrowths 



