COELENTERATA AND CTENOPHORA 



products of this extracellular phase of digestion are absorbed directly by the 

 gastrodermal cells. Finely divided particulate matter is ingested by pseudo- 

 podia formed by the large gastrodermal cells and comes to lie in food 

 vacuoles within their cytoplasm. Here the intracellular phase of digestion 

 occurs, which is presumably entirely comparable with the process as it occurs 

 in an amoeba. The indigestible residues are cast off by the gastrodermal 

 cells and, together with the resistant parts of the food mass in the coelenteron, 

 are expelled through the mouth by a series of violent contractions of the body. 

 The bottom of a culture dish near a vigorous and well-fed hydra may often 

 be littered with the egested exoskeletons of the water fleas upon which the 

 animal has been feeding. 



Although there is no circulatory system for the transfer and distribution of 

 nutrients, it will be noted that no part of the body of the hydra is far removed 

 from the source of food in the coelenteron. The wave-like contractions of 

 the body tube, together with the activities of the gastrodermal flagella, main- 

 tain a circulation of the products of digestion within the coelenteron, which 

 in recognition of its dual function is often termed the gastrovascular cavity. 

 In colonial hydrozoans this distributive function of the common coelenteron 

 pervading the entire branching colony is very important in the nutrition of all 

 the component individuals. Soluble products of digestion undoubtedly reach 

 epidermal regions by diffusion across the short intervening distances. 



Oxygen reaches all the cells by diflTusion from the external medium. As in 

 all animals it is used in cellular metabolism, which releases energy and pro- 

 duces as by-products carbon dioxide, water, and nitrogenous wastes. In 

 animals as small as the hydra, excretion probably occurs over the general 

 body surface; there are no specialized organs of excretion. 



Responsiveness. All movements of the hydra are the result of contractions 

 by the longitudinal and circular muscle processes of epidermal and gastro- 

 dermal cells. The varied positions and shapes which the animal may assume 

 indicate that these processes can contract and relax locally as well as over the 

 entire body, and that they react in a coordinated fashion. In addition to 

 extensions and contractions of body and tentacles as a whole, there are 

 peristaltic movements; these may be very slow, or they may be rapid, as when 

 egesta are violently expelled, or when food is quickly moved from one region 

 of the coelenteron to another. The circular muscle processes function as 

 sphincters about the mouth and at the base of each tentacle. 



Locomotion occurs in a variety of ways. A hydra may move by imper- 

 ceptible degrees, gliding upon its base. It may extend itself horizontally 

 until the tentacles are in contact with the surface, release the base, which is 

 then drawn to a new point of attachment, and extend the tentacles once more, 

 repeating the process. Again, the hydra may attach the tentacles, release the 

 base, and move by a series of slow somersaults (Fig. 10.5). It can also walk 

 clumsily upon its tentacles with the body free and contracted. The habit of 

 some hydras of floating at the surface with the base clinging to the surface 

 film has already been mentioned. 



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