COELENTERATA AND CTENOPHORA 



hydras all have four types (Fig. 10.4): a large globular kind with conspicuous 

 spines on the enlarged basal portion of the long thread, a small globular type 

 with its thread always spirally coiled after discharge, and two small elongate 

 types with long, straight threads. Examination of the bodies of animals par- 

 alyzed by hydras indicates that these nematocysts have different functions. 

 The threads of the first type penetrate the body of the prey, even piercing 

 the exoskeleton of small crustaceans; it is now apparent that this puncturing 

 of the prey is very significant in the further feeding activities of the hydra 

 (p. 295). Threads of the spiral type wrap themselves about fine projections, 

 such as the bristles of a water flea. In some manner the nematocyst exerts a 

 paralyzing influence on the prey. This action has been attributed to a poison, 

 which is variously considered as being ejected from the free end of the hollow 

 thread or as simply coating the outside of the everted thread. The exact 

 nature of this poison has not been ascertained, although the name "hypno- 

 toxin" has been applied to it. It is apparently composed of several different 

 substances, and is probably not the same throughout the phylum. 



The gastrodermis is the inner layer of cells lining the coelenteron. It con- 

 sists chiefly of large cells which may bear one or two flagella (Fig. 10.3). 

 These cells are capable of extending pseudopodia at their free ends to ingest 

 particles of food from the coelenteron. As in the epidermis, these cells are 

 epitheliomuscular in character; it was pointed out earlier that the arrange- 

 ment of their muscle processes gives them an action antagonistic to that of 

 the epidermal cells in contraction. In Chlorohydra the gastrodermal cells are 

 crowded with green bodies regarded as unicellular plants. These cells live 

 within the gastrodermis of the hydra and presumably pass from one genera- 

 tion to the next by transfer in the hydra's eggs. Similar plant cells, unicellu- 

 lar brown algae, or dinoflagellate protozoans inhabit the cells of many marine 

 coelenterates, particularly some of the corals. The presence of such cells 

 seems advantageous to the hydra, for in their autotrophic nutrition these 

 simple plant or plant-like cells consume carbon dioxide and inorganic wastes 

 and produce oxygen and organic compounds which are useful to the coelen- 

 terate host. Similarly, an advantage would seemingly accrue to the plant 

 cells from the abundant raw materials produced as metabolic wastes by the 

 cells of the host, as well as from the protected environment afforded the 

 plants. A relationship of this .sort, in which both organisms involved may be 

 thought to benefit from the association, is termed mutualism. This is a par- 

 ticular manifestation of the broader phenomenon of symbiosis, interpreted as 

 any situation in which the existence of one organism is closely involved in 

 the existence of another. Mutualism may be contrasted with another kind of 

 symbiosis termed parasitism, in which the advantage of the association is all 

 on one side, and with commensalism, in which animals are associated merely as 

 "messmates" without obvious advantage or disadvantage to either. The 

 ciliated protozoans Tncliodina and Kerona, often seen living on the outer 

 surfaces of hydras, are probably commensals and not parasites. 



In addition to the large gastrodermal epitheliomuscular cells, there are 



291 



