INTRODUCTION TO THE METAZOA 



63 



elongation the single cells extend down into the underlying tissues 

 much deeper than the ordinary epithelial cells which surround 

 them. 



Flat surfaces or large areas of an epithelium may become 

 differentiated for a glandular function, as in the foot of Hydra, 

 but in most instances specialized glands require an increased sur- 

 face for their functioning as secretory or excretory organs, and 

 consequently areas of glandular epithelium become invaginated 

 either as a simple tube or as a series of branched tubes and cham- 

 bers the walls of which are composed of gland cells. A duct 

 usually keeps such a compound gland in communication with the 

 surface from which the gland cells were originally invaginated. 



Sensory or Communicative Epithelium. — Naturally, the only 

 communication which an organism may have with the out- 

 side world is through its surfaces. Consequently, any sensory 

 mechanism must be associated with 

 the surface of the body and is there- 

 fore epithelial in origin. Sensory cells 

 located in the epithelium have com- 

 munication with nerve endings through 

 which they are able to transmit stimuli 

 to the central nervous system. Hairs 

 or bristles on the surface of epithelial 

 cells render them especially susceptible 

 to touch stimuli, so isolated cells or 

 groups of cells thus modified form 

 various tactile organs. An association 

 of pigment with epithelial cells having 

 rich nerve supply usually signifies an 

 optic organ of some type and is des- 

 ignated as a retina. Balancing organs 



or statocysts (Fig. 39) are usually modified epithelia specialized 

 for receiving tactile stimuli through the movements of a body 

 called a statolith or an otolith. 



Germinal Epithelium. — Germ cells of all Metazoa have their 

 origin in an epithelium. In many of the groups, the presence of 

 a germinal epithelium is readily observable. In some of the 

 simple Metazoa, as, for example, the coelenterates, the germ 

 cells may arise as modifications of epithelial cells which are not 

 even necessarily grouped to form a gonad. The gonads of many 

 groups are bounded by an epithehum, the cells of which trans- 



Ofohfh 



Pr'imarjj 

 vesicle 



Fig. 



Secondary 

 v&s/cls 



39. — Statocyst of Goni- 

 onemus. (Orig.) 



