THE SPONGES AND THE TWO-LAYERED ANIMALS 



145 



Fig. 8-9. Regeneration in hydra. 



The presence of food in the coelenteron 

 stimulates the gland cells to secrete diges- 

 tive enzymes into the cavity where the soft 

 parts of the ingested animal are partially 

 digested (extracellular digestion). The 

 hard outer coverings are indigestible and 

 are eventually regurgitated through the 

 mouth. Thus a single opening functions 

 both as a mouth for the entrance of food 

 and as an anus for the exit of undio-ested 

 food. The breakdown of food molecules is 

 probably not as thorough as in higher 

 forms, for it has been observed that many 

 of the endoderm cells take in particles of 

 food by an amoeboid process. Food vacu- 

 oles are formed and intracellular dio;estion 

 proceeds, the same as in the Protozoa. This 

 might be expected, since this animal is not 

 so far removed from its protozoan an- 

 cestors. The cells of the epidermis and other 

 portions of the body receive their food 

 supply from the endodermal cells. 



Hydra is sensitive to light and seeks out a 

 suitable illumination. The intensity sought 

 is usually that in which food is most likely 

 to be found. The colorless hydra seeks a 

 lower intensity of light than its green rela- 

 tive (Hydra viridis). The latter has tiny 

 green algae in its endodermal cells, which 

 require more light for photosynthesis ( Fig. 

 5-9). Hydra prefers cool, clear water and 

 seeks it out. If exposed to various concen- 

 trations of injurious chemicals, it avoids 

 each with regularity. If unfavorable condi- 

 tions are forced upon it, such as desiccation, 

 it undergoes a series of regressive changes 



called "depression." The tentacles and 

 body begin to disintegrate and this con- 

 tinues until the animal is destroyed. Under 

 favorable conditions, however, it may re- 

 cover at almost any stage in its disintegra- 

 tion. 



In a suitable environment, hydra repro- 

 duces asexually by forming one or more 

 buds along the body wall where cells are 

 congested with a surplus of stored food 

 (Fig. 8-10). This is accomplished by a pro- 

 liferation of the cells pushing a part of the 

 body wall outward. Small blunt tentacles 

 develop and a mouth breaks through. Food 

 obtained by the parent hydra circulates 

 into the coelenteron of the bud during its 

 formation, thus providing means for rapid 

 growth. Eventually the bud constricts at 

 the point where it joins the parent and di- 

 vorces itself from the latter to carry on its 

 own existence. Sometimes several buds 

 form simultaneously, indeed, buds may 

 have buds upon themselves. This is very 

 close to a colonial form, such as obelia, an- 

 other hydrozoan, which will be considered 

 a little later. 



Since buds form so readilv from almost 

 any part of its body wall, it should follow 

 that hydra could perhaps be made to repro- 

 duce itself experimentally by simply cutting 

 off small pieces of the body. This idea 

 apparently occurred to a Swiss naturalist, 

 Abraham Trembley, around the middle of 

 the eighteenth century ( 1744 ) . He did just 

 that and gave us our first experiments on 

 regeneration in animals. Trembley was em- 



