PHYLUM COELENTERATA 151 



regular, slender, neuroepithelial cells which are joined into a net 

 by intercellular processes. These cells fit between the others and 

 are either sensory or motor in function, thus receiving external 

 stimuli and also causing contraction of the contractile cells at 

 proper times. 



Beneath the ectoderm and embedding the bases of the cells is a 

 very thin layer of noncellular substance called mesogloea. It is 

 produced by the cell layers and serves as attachment for them, par- 

 ticularly for the fibrils of the epitheliomuscular cells. In some of the 

 other coelenterates, this layer is exceedingly thick and heavy. 



The inner, thicker cell layer of the wall is the endoderm which lines 

 the lumen of the gastrovascular cavity. The most conspicuous cells 

 here are the nutritive-muscular cells which are long, vacuolated struc- 

 tures attached to the mesogloea by fibrils which extend in it parallel 

 to the circumference of the animal. By contraction these cells in- 

 crease the length of the animal by reducing its circumference. These 

 cells often possess flagella at the free margin and at times engulf 

 particles of partially digested food like an amoeba. It is seen then, 

 that they serve both as muscles and as digestive cells. Glandular 

 cells are also present in this layer. Being slender, they wedge 

 themselves between the nutritive-muscular cells and secrete what 

 is probably a digestive fluid into the gastrovascular cavity. Neuro- 

 epithelial and interstitial cells are also interspersed among the 

 other cells of this layer. The general morphology of the adult 

 animal is very similar to the gastrula stage of the developing em- 

 bryo of more complex metazoans. 



Metabolism 



The food of hydra consists of small insect larvae, minute worms, 

 small bits of organic matter in the water, water fleas, and other 

 small Crustacea. Ingestion of the food has been described already. 

 Upon entering the mouth the morsel of food is moved some distance 

 down in the cavity by successive wavelike contractions of the column 

 progressing from distal to proximal. Such serial contractions are 

 usually called 'peristaltic contractions. Here in the upper half of the 

 enteron digestion takes place. The wall possesses many more of the 

 gland cells in the endoderm, and the food material disintegrates into 

 smaller particles here in this region. The digestion which occurs 

 here is spoken of as intercellular digestion and is brought about by 

 enzymes produced by the secreting cells of the endoderm. The dis- 



