376 



SKIN STEXJCTUKE AND EXCKETION 



be SO thin, in the earthworm, as to permit of the action of 

 respiration, while, in the insects or crustaceans, it may be so 

 thick, or become so loaded with salts of lime, as to form 

 a hard external skeleton for the animals. When we speak 

 of an insect's shedding its skin, we mean that it sheds this 

 outer secretion or exoskeleton. The underlying layer or 

 hypodermis remains and serves to secrete the new layer. 

 This hypodermis is also found to contain glands of various 

 kinds, and may even show layers of nerve and mtiscle 

 fibers. The hard layer of the insect and lobster is evi- 

 dently not a satisfactory substance for permitting the 

 excretion of wastes, and very little excretion or respiration 

 takes place through the skin of such forms. 



Body coverings of vertebrates. — All vertebrates possess 

 a dermis and epidermis similar in structure and function 

 to that of man. The epidermis always consists of epi- 

 thelial cells only, while the dermis is made up mainly of 

 connective tissue cells and fibers, and may also inclose 

 muscle fibers. While the various glands of the skin are 

 composed of epithelial cells they are found most often 

 embedded in or under the dermis, and have their opening 

 by ducts on the surface of the epidermis. The epidermis 

 is in two layers, as in man; the outer being composed of 



Fig. 173 — Skin of llsb (diagram); a, dermis with yertieal fibers, 6; c, gristly layer; 

 dy surface scales; d, layer containiug limestone; /, scale pit. 



dead cells or cuticle, with an underlying layer of living 

 ceUs. In some vertebrates this outer layer, or cuticle, 



