INTEGUMENT 53 



The shell of molluscs such as the snail, clam, and oyster is 

 also the product of the activity of cells at the surface of the body. 

 The secretion consists of organic matter, conchiolin, richly 

 impregnated with calcium carbonate. The shell increases in 

 size by additions at its edge and, unlike the cuticle of arthropods, 

 is not shed. 



The hard integument of the starfish and sea urchin consists 

 of calcareous plates developed in the subepidermal region of the 

 skin. The epidermis is a thin simple epithelium overlying 

 the plates except where points of the plates project. These 

 plates act as an armor protecting the internal organs and con- 

 stitute an exoskeleton though differing in origin and development 

 from the exoskeleton of the arthropods. 



General. — A comparison of the integument of invertebrates 

 with the integument of vertebrates brings out the following 

 points: (1) In both groups an epidermal layer of cells, repre- 

 senting the primitive covering of the body, is present. Among 

 invertebrates, the epidermis may secrete a cuticle of variable 

 hardness and chemical composition which serves as a protective 

 covering and also provides points of attachment for body muscles 

 and other internal organs. Among vertebrates, on the other 

 hand, the surface layer of the body remains cellular, though the 

 cells may be highly transformed, as in the outer layer of the shell 

 of turtle which is epidermal in origin. In snakes the hardened 

 outer layer of the skin is the stratum corneum of the epidermis. 

 (2) The subepidermal layer of the invertebrate skin is as a rule 

 poorly developed. An exception is found in Echinodermata, 

 such as the starfish, in which the hard calcareous plates of the 

 exoskeleton develop below the epidermis. The corium of 

 vertebrates is a well-developed layer that provides the skin with 

 its tough and at the same time flexible character. Leather is the 

 chemically treated corium of hides. In those vertebrates in 

 which the skin takes on the form and consistency of an exo- 

 skeleton, such as in many fishes, turtles, alligators, crocodiles, 

 and some mammals (armadillo), the condition is the result of the 

 development of scales or bony plates in the corium. 



