i EPITHELIAL TISSUE 39 



epithelial cells by means of the protrusion of pseudopodia-like or 

 amoeboid processes is an important function of these cells, which serve 

 exclusively for purposes of nutrition. In many lower Metazoa the 

 amoeboid character of one or of all the cells of the intestinal epithelium 

 is so marked that they sever themselves from the complex of epithelial 

 cells and float about independently in the intestinal cavity. 



Formations similar to the contractile vaeuoles of many Protozoa 

 are found in the so-called excretory cells of the Turbellaria. In their 

 protoplasm, and in that of their processes, small drops (products of 

 metabolism) collect, and these may mingle to form one large drop 

 (vacuole). The drop is emptied into the lumen of the cell (see Fig. 

 109, p. 152), and thence expelled by means of the excretory ducts. 



In certain Protozoa the ectoplasm gives rise to stinging cells. 

 In most Cvlcnterata the production of such stinging capsules, the 

 so-called nematoeysts, is always the chief function of very many 

 ectodermal cells (cnidoblasts) which do no other work for the 



organism. 



In the Protozoa a membrane or shell for the protection of the 

 unicellular organism is provided by the secretion of a resistent external 

 envelope ; so also in the Metazoa the epithelial cells, and naturally 

 the cells of the outer body epithelium especially, provide a protection 

 or covering for the body by the formation of outer eutieular 

 membranes. Such eutieular formations arise by the mingling of the 

 secretions of different cells, or the transformed products of the proto- 

 plasm, to form a layer. They may vary very much chemically and 

 physically, and stand in just the same relation to the epithelial cells 

 which produce them as do the glandular secretions to the gland cells. 

 To these eutieular formations belong the chitinous integuments of the 

 Annulata and Arthropoda, which sometimes by calcareous deposit become 

 carapaces as hard as stone ; these integuments are secreted by the 

 epithelium (hypodermis) which underlies them. 



The Cuticle is often penetrated by fine perpendicular pores, which 

 probably owe their origin to the fact that, as the cuticle increases in 

 thickness, the cell protoplasm remains connected with the eutieular 

 surface by fine processes. A fine cuticle is also to be found in 

 ciliated epithelium, in which case the cilia pass through the pores to 

 the exterior. 



There are, however, among the Metazoa a series of cells with 

 special adaptations and functions which are wanting in the Protozoa. 

 The aggregation of a large number of cells which can adapt themselves 

 to the most various activities, presents much more favourable conditions 

 for a far-reaching division of labour than are offered in the case of 

 unicellular organisms. In the first place, we have in the epithelium 

 the most various glandular cells cells which are distinguished by 

 peculiarities of form and structure, and possess a protoplasm capable 

 of transforming the nutritive substances provided by the body into 

 different sorts of secretions, or of assimilating from the body super- 



