Chap. 23 COELENTERATES SIMPLE MULTICELLULAR ANIMALS 473 



which move into it from the true cell layers; in sea anemones, it is a tough 

 fibrous tissue. 



Epidermis. The epidermis is composed of epithelial tissue containing sup- 

 porting cells, epithelio-muscular, glandular-muscular and glandular cells, sen- 

 sory nerve cells, formative and stinging cells (Fig. 23.8). The supporting cells 

 protect and support other cells. The outer ends of the epithelio-muscular cells 

 are likewise protective but their inner ends are drawn out into contractile 

 strands which extend along the mesoglea lengthwise of the body. When these 

 strands contract, the tentacles and body shorten and widen. Glandular cells 

 are crowded about the mouth and in the basal disk along with epithelio-mus- 

 cular cells. Hydras attach themselves to objects by means of a sticky secretion 

 and the contraction of epithelio-muscular cells. Gas is also secreted in the basal 

 region; a bubble of it caught in the mucus often buoys an animal up beneath 

 the surface film (Fig. 23.2). 



The neurosensory cells reach to or near the outer surface and their processes 

 extend to the nerve plexus close to the mesoglea. These are the receptors of 

 touch and other stimuli, called neurosensory cells because they look so much 

 like nerve cells. Cells of the nerve plexus or "net" rest against the processes of 

 the epithelio-muscular cells (Fig. 23.7). The neurosensory, nerve and epi- 



B. Nerve cells 

 Hydra 



A. Nerve cells 

 Sea anemone 



Fig. 23.7. Nerve cells. A, sea anemone. A layer of nerve cells from the oral 

 disk, more elaborate but similar to the layer of nerve cells in the body wall of 

 hydra. Note the lack of continuity of the cells. B, hydra, part of the ring of nerve 

 cells in the base (pedal disk) of the body. Note that these cells are not regularly 

 continuous. (Courtesy, Hyman: The Invertebrates, vol. 1. New York, McGraw- 

 Hill Book Co., 1940.) 



