112 



COLLEGE ZOOLOGY 



to maximum elasticity. The mesoglea here 

 reaches its greatest thickness; and both mus- 

 cle layers are well developed, particularly 

 the longitudinal. 



The basal disk 



The epidermis of the foot consists of 

 columnar epitheliomuscular cells. Their 

 outer portions are filled with refractive 

 globules, which store the sticky mucus these 

 cells elaborate. Their bases are provided with 

 muscle fibers which radiate from the center 

 of the disk. 



The mesoglea is lackmg over a small area 

 at the center of the disk, and here epidermis 

 and gastrodermis come in direct contact. 

 This is the region of the aboral pore, which 

 is opened when the hydra suddenly releases 

 its hold on the substratum, but is completely 

 closed during attachment. The probable 

 function of the pore is to enable the hydra 

 to "blast" itself loose from the sticky mucus 

 secreted by the foot. This is accomplished by 

 a "peeling" action of the cells at the pe- 

 riphery of the foot, plus a simultaneous 

 strong expulsion of water from the gas- 

 trovascular cavity through the aboral 

 pore. 



Physiology 



Food 



The food of the hydra consists principally 

 of small animals that live in the water, such 

 as Cyclops, annelids, and insect larvae. Large 

 specimens may ingest aquatic animals as big 

 as young fish and tadpoles. Bits of meat may 

 be ingested when offered to them in a lab- 

 oratory aquarium. The hydra normally rests 

 with its basal disk attached to some object, 

 and its body and tentacles extended into 

 the water. In this position it occupies a 

 considerable amount of hunting territory. 

 Any small animal swimming within touch 

 of a tentacle is at once shot full of pene- 

 trants, affixed by glutinants, or grappled by 

 volvents. 



Ingestion 



The tentacle which has captured the prey 

 bends toward the mouth with its load of 

 food. The other tentacles not only assist in 

 this, but may use their nematocysts in quiet- 

 ing the victim. The mouth often begins to 

 open before the food has reached it. The 

 edges of the mouth gradually enclose the 

 organism and force it into the gastrovascular 

 cavity. The body wall contracts behind the 

 food and forces it down. Frequently organ- 

 isms many times the size of the hydra are 

 successfully ingested. 



Reactions to food 



It is common to find hydras that will not 

 react to food when it is presented to them. 

 This is due to the fact that these animals 

 will eat only when a certain interval of time 

 has elapsed after their last meal. The phy- 

 siologic condition of the hydra, therefore, 

 determines its response to the food stimulus. 

 The collision of an aquatic organism with 

 the tentacle of the hydra is not sufficient to 

 cause the food-taking reaction, since it has 

 been found that not only a mechanical 

 stimulus, but also a chemical stimulus must 

 be present. A very hungry hydra will go 

 through the food-taking movements when 

 it is excited by a chemical stimulus alone, 

 such as beef juice. 



Digestion 



Immediately after the ingestion of food, 

 the gland cells in the gastrodermis show 

 signs of great activity; their nuclei enlarge 

 and become granular. This is accompanied 

 by the formation of enzymes which are dis- 

 charged into the gastrovascular cavity and 

 begin at once the digestion of the food. The 

 action of the digestive juices is made more 

 effective by the churning of the food as the 

 animal expands and contracts. The flagella 

 extending out into the central cavity also 

 aid in the breakdown of the food by creat- 

 ing currents. This method of digestion dif- 

 fers from that of the amoeba, paramecium, 



