OF THE SECRETIONS. 



129 



usually termed the vascular layer. The 

 superficial layer (c) is the epidermis, 

 or cuticle. The cells of which it is com- 

 posed are distinct at its inner portion, 

 but become dried and flattened as they 

 are pushed outwards. It is supplied with 

 neither vessels nor nerves, and, conse- 

 quently, is insensible. Between these 

 two layers, and more especially con- 

 nected with the cuticle, is the rete muco- 

 sum, (&,) a very thin layer of cells, some 

 of which contain the pigment which 

 gives the complexion to the different 

 races of men and animals. The scales 

 of reptiles, the nails and claws of 



Fig. 94. 



mammals, and the solid coverings of the Crustacea, are 

 merely modifications of the epidermis. On the other hand, 

 the feathers of birds and the scales of fishes arise from the 

 vascular layer. 



269. Of all the Excretions, if we except that from the 

 Lungs, the bile seems to be the most extensive and im- 

 portant ; and hence a liver, or some analogous organ, by 

 which bile is secreted, is found in animals of every depart- 

 ment ; while some, or all, of the other glands are want- 

 ing in the lower classes of animals. In Vertebrates, the 

 liver is the largest of all the organs of the body. In mol- 

 lusks, it is no less preponderant. In the gasteropods, like the 

 snail, it envelopes the intestine in its convolutions, (Fig. 52 ;) 

 and in the acephala, like the clam and oyster, it generally 

 surrounds the stomach. In insects it is found in the shape of 

 long tubes, variously contorted and interlaced, (Fig. 51.) In 

 the Radiata, this organ is largely developed, especially 

 among the echinoderms. In the star-fishes it extends into 



