PHYLUM COELENTERATA (cNIDARIA). SIMPLE TISSUE ANIMALS 



107 



Interstitial cells- 

 Nematocyst- 



Sensory cell- 



Supporting 

 cell nucleus- 

 Interstitial cells- 

 Nerve eel 



Longitudinal 

 muscle fiber- 



Mesoglea- 



Nufritive cell 

 Gland cell 



Sensory cell 

 -Gland cell 



Young gland cell 



— Food 

 vacuole 



Food being 

 engulfed 



Pseudopodium 



Figure 52. Longitudinal section of the body wall of the hydra, highly magnified to show the 

 structure of the epidermis, mesoglea, and gastrodermis. Note that the mesoglea is free from 

 nerve cell bodies although nerve cell fibers pass through it. (Redrawn from a drawing by Justus 

 F, Mueller; made expressly for this text.) 



postome; sparsely in the stalk and basal 

 disk. 



The interstitial cells are small rounded 

 cells with clear cytoplasm and a relatively 

 large nucleus containing one or two nucleoli. 

 Their cytoplasm lacks specialized structure; 

 hence they are undifferentiated. Mitotic 

 figures are frequent. 



It is thought by some that the interstitial 

 cells represent a sort of embryonic tissue 

 carried over into the adult, that they can 

 differentiate into any of the specialized cells 

 of the hydra, and that hence they are the 

 chief agents in reconstructing tissues in 

 growth, budding, regeneration, etc. The in- 

 terstitial cells also form the primordial germ 

 cells of the gonads, and replace worn-out 

 gland cells in the gastrodermis; but whether 

 they have any other significance is a de- 

 bated point. It has been shown conclusively 

 that regeneration, as well as bud formation, 

 takes place by a rearrangement of the differ- 

 entiated epitheliomuscular cells of both lay- 

 ers, and that the interstitial cells, at least 



locally, play only a subordinate part in the 

 process. Whether cnidoblasts, germ cells, 

 and gland cells arise from a common stem 

 cell or from several types of interstitial cells 

 is not known. 



The muscular system 



The muscular system of the hydra con- 

 sists primarily of two layers of contractile 

 fibers applied to opposite surfaces of the 

 supporting mesoglea. The outer muscle layer 

 is longitudinal and is formed by the con- 

 tractile fibers of the epidermal cells, while 

 the inner muscle layer is circular and is de- 

 rived from the contractile fibers of the 

 gastrodermal cells. 



The circular muscle layer contracts slowly, 

 performing movements of a peristaltic char- 

 acter, while the external longitudinal layer is 

 capable of rapid response. Thus the two 

 muscle layers of hydra already foreshadow in 

 a dim way the visceral and skeletal muscles 

 of higher animals. 



