GENERAL ZOOLOGY 



club-shaped gland cells, with the smaller end attached to the basement mem- 

 brane and the larger end exposed to the digestive cavity (Fig. 10.3). These 

 are located principally in the distal third of the body (the basal end is 

 considered proximal) and presumably are the source of enzymes effective in 

 extracellular digestion occurring in the coelenteron. A circlet of gland cells 

 just within the mouth secretes something that seems to activate the other 

 gland cells, indicating that even in this simple animal a rather complicated 

 secretory cycle may exist. Scattered sensory cells, nerve cells, and some 

 interstitial cells also occur in the gastrodermis. 



The cells described in the epidermis and gastrodermis collectively constitute 

 the somatic cells of the hydra. Cell specialization in this animal gives rise 

 to different kinds of somatic cells, but it mxust be noted that the aggregation 

 of these types into well-defined tissues has not progressed to any great extent. 

 For example, the epitheliomuscular cells of the epidermis and gastrodermis 

 combine the functions of epithelia and of contractile tissues, which in more 

 advanced animals become the properties of separate and distinctly modified 

 aggregations of cells. Organs are similarly primitive: only the tentacles may 

 be termed organs in the accepted sense. Thus it may be stated that in the 

 hydra there are different kinds of specialized cells, but that the differentiation 

 of tissues and of organs is at a very low level. 



The interstitial cells of the hydra constitute a sizable complement of ap- 

 parently totipotent cells, capable of specialization in a variety of ways through- 

 out the life of the individual. For example, interstitial cells are the source 

 of cnidoblasts required for the replacement of discharged nematocysts. They 

 are also important in the differentiation of a new hydra arising by budding, 

 and in the regeneration of lost parts, for which these animals have a con- 

 siderable capacity. Interstitial cells are, in addition, the source of the germ 

 cells which appear at certain seasons. For all its simple organization, the 

 hydra, with its several kinds of somatic cells, shows a great advance in cell 

 specialization over such forms as Volvox, in which all the somatic cells are 

 alike. 



Metabolism. The small animals serving as food for the hydra, after being 

 paralyzed and held fast by the nematocysts, are brought to the mouth by the 

 tentacles and are ingested by engulfing movements of the hypostome. The 

 mouth is capable of a surprising degree of distension to accommodate large 

 objects of food. Soon after its ingestion the food is shifted by peristaltic 

 contractions of the body to a position in the distal half of the coelenteron, 

 where the early stages of digestion occur. Although no structural differentia- 

 tion exists other than the abundance of gland cells in the distal region, there 

 is apparently a physiological division of the coelenteron into gastric and 

 intestinal regions; the food mass is never found in the more proximal or basal 

 part of the cavity. 



The process of digestion in the hydra is twofold. Enzymes released from 

 the gland cells bring about the disintegration of the softer parts of the food 

 mass, liquefying it and hastening its breakdown into particles. The soluble 



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