Chap. 23 COELENTERATES SIMPLE MULTICELLULAR ANIMALS 



475 



A 



Tentacle 



Cnidoci 

 (sensitive 



D 



Nematocys 

 (stinging so 



Nucleus 



Fig. 23.8. The stinging capsules (nematocysts) of hydra. A, a bit of tentacle 

 magnified to reveal the batteries of stinging capsules. B, tail bristle of Cyclops with 

 the stinging capsules thrown upon it during its capture by hydra. C, Cyclops, a 

 favorite food of hydra, is only a white speck to the naked eye. Note its single eye, 

 the eggs it carries and the tail bristles. D, a stinging capsule highly magnified 

 within the cell that formed it. E, the stinging cell with the thread unloosed and 

 poison discharged. (A and B, courtesy, Hyman: The Invertebrates, vol. 1. New 

 York, McGraw-Hill Book Co., 1940. D and E, after Schneider. Courtesy, Dahl- 

 gren and Kepner: Animal Histology. New York, The Macmillan Co., 1908.) 



enter the cell layers at some other point. Wherever they enter they finally 

 lodge in the ectoderm. Not all of them migrate; some remain in the ectoderm 

 where they developed. Their structures and functions are entirely different 

 from those of the wandering cells (macrophages) of mammals that pick up 

 foreign substances in the human body, yet both types move about in similar 

 ways. Each illustrates the flexibility of form and function that is highly charac- 

 teristic of living matter. 



Endoderm. The gastrodermis of the enteron and its extensions in the ten- 

 tacles is in general similar to the epidermis. It is composed of epithelial tissue 

 and contains glandular, sensory, and nerve cells — the latter less frequent than 

 in the epidermis. There are fewer formative cells and no stinging cells except 

 those that migrate into it (Figs. 23.6, 23.8). 



Nutritive muscular cells are the predominant cells of the gastrodermis. Their 

 bases are extended into muscular processes which run in a circular direction 

 opposite to the processes in the epidermis but like them rest against the 



