134 



ANIMAL BIOLOGY 



of the coelom, known as the peritoneum, forms the innermost 

 layer of the body wall, covers the organs, and in certain regions 

 forms broad folds, or mesenteries, in which they are suspended. 

 In the Mammals the organs of the chest, or thorax, are separated 

 from those of the abdomen by a muscular partition, or diaphragm. 

 (Figs. 106-109.) 



In aquatic forms thin extensions from the trunk and tail form 

 median fins. Paired fins, developed from the trunk, comprise the 

 pectoral fins, situated near the junction of head and trunk, and 

 the pelvic fins, just lateral to the anus. The pectoral and pelvic 

 fins, or the fore limrs and hind limbs which replace them in all 

 forms above the Fishes, are the only lateral appendages typically 

 found in Vertebrates. 



B. Skin 



The surface of the body which comes in direct contact with the 

 environment is covered by an integument, or skin, which, though 



Epidermis j 

 Dermis. 



Sebaceous 

 gland 



Subcutaneous 

 tissue 



Hair follicle 



Opening of sweat gland 



Horny layer 

 Epithelium' 

 Nerve ending 



Blood vessel 



Muscle 

 to hair 



Connective 

 tissue 



Fat 

 Sweat gland 



Blood vessel 



Fig. 96. — Diagram of section through the human skin, highly magnified. 



primarily protective and sensory in function, takes part to a greater 

 or less degree in respiration, excretion, and secretion. Scales, 

 feathers, claws, horns, hoofs, nails, teeth, etc., are derivatives of 

 the skin. The skin, unlike that of the Invertebrates, is formed of 



