62 



INVERTEBRA TE ZOOLOGY 



motion are only infrequently found in adult motazoans but do 

 occur in the flatworms (Fig. 62), the rotifers (Fig. 72), and 

 molluscs (Fig. 91). More frequently, however, cilia and flagella 

 on epithelia of Metazoa produce movements of the medium (P'ig. 

 37) rather than affect locomotion. Thus in the sponges the 

 flagellated cells produce the water currents which carry the food 

 supply. The same is true of the ciliated mantle and gills of the 

 mussels. 



Protective Epithelium. — Because of their location on external 

 surfaces epithelia are frequently modified for a protective func- 

 tion. Stiff fibrils in the cytoplasm (Fig. 38) and thickenings of 





Fig. 37. — Glandular and ciliated 

 epithelium showing unicellular glands 

 in entoderm of earthworm, Eisenia 

 rosea. (After Schneider, courtesy of 

 Giittar Fischer). 



Fig. 38. — Protective epithelium 

 from a flatworm, Planocera folium. 

 (After Schneider, courtesy of Gustav 

 Fischer) . 



the outer margin of the cell are two means whereby mechanical 

 protection is secured. Frequently, an external epithelium may 

 form extracellular materials which are deposited in a layer or in 

 a succession of layers over the surface of the body. The cuticula 

 characteristic of so many invertebrates is thus produced by an 

 underlying epithelium which is frequently termed a hypodermis. 

 In many instances, inorganic salts are deposited in this cuticula 

 until a shell-like armor of resistant plates is formed. 



Glandular or formative epithelium represents one of the com- 

 monest modifications of this class of tissue. Even in the gastrula 

 stage the entoderm of the embryo has begun to be specialized as 

 a glandular tissue and the cytoplasm and its inclusions render the 

 entoderm obviously different from the ectoderm. Cells scattered 

 singly through an epithelium may become specialized as gland 

 cells, as in the case of the mucous cells (Fig. 37) of the earthworm. 

 Frequently, these unicellular glands become enlarged and through 



