"WOEMS AND ARTHROPODS 375 



Sponges and ccelenterates. — In these animals the body 

 is composed of only two layers of cells, and for the most 

 part the action of these cells is on the same plane as that ■ 

 of the protozoan. In the sponge, the variation between 

 the inner and outer cells results in division of labor, and 

 the outer cells are more protective and less digestive in 

 their action. 



In some of the ccelenterates, however, we find a simple 

 form of covering which approaches closely in structure 

 the skin of man. In certain of the polyps, for example, 

 we find the outer ectoderm layer of cells more or less com- 

 pletely divided into two layers. Where this occurs the 

 outer layer is composed of cells very similar to those of 

 the epidermis, while the irmer layer may contain muscle 

 fibers, nerves, touch organs, and certain simple glands. 

 Such a covering is evidently much more than a protective 

 form, and in fact performs several functions identical in 

 nature with those of the human skin. 



Mollusks. — In the oyster, clam, snail, and other members 

 of this gTOup we find the covering of the body composed 

 of a true two-tissued layer. The outer layer is com- 

 posed of epithelial cells, while the underlying part is made 

 up of connective tissue in which are found muscle fibers, 

 blood vessels, and nerve ends. This skin also acts very 

 effectively in the discharge of carbon dioxide from the 

 body. It differs from other forms of skin by the presence 

 of special shell-secreting glands and thus is the source of 

 the hard exoskeleton which we call the shell. 



Worms and arthropods. — In the members of these two 

 great classes we find also a true skin. The outer body 

 layer, or hypodermis, as it is called in these groups, has 

 the power of secreting a thin layer of material, which may 



