GENERAL ZOOLOGY 



Epidermis 



Gastrodermis 



Gastrodermis 



Epidermis 



Muscle fibrils 

 of epidermis 



Nematocysts 



Gastrodermis 



Epidermal 

 gland cells 



Gastrodermal 

 Food vacuoles muscle fibrils 



Myoneme or muscle fibril 



Fig. 10.3. Details of cellular structure in the hydra, as shown by cross sections through dif- 

 ferent regions of the body. A, in the stalk region, with very vacuolated gastrodermal cells. 

 B, through the hypostome, showing glandular gastrodermal cells. C, through the pedal disk, 

 with glandular epidermal cells. D, through the "stomach" region, showing digestive gland 

 cells and food vacuoles. /:, detail of a large epidermal epitheliomuscular cell. (A-D, re- 

 drawn from L. H. Hyman, The Invertebrates: Protozoa through Ctenophora, copvright 1940 bv 

 McGraw-Hill Book Co., Inc., printed by permission; E, after J. von Gelei.) 



to the entire organism. In hydrozoan medusae, or jellyfish, this layer is repre- 

 sented by a thick, watery jellv without cells, termed the mesogloea ("middle- 

 jelly"). 



Cellular Structure and Function. The outer cell layer, or epidermis, is 

 composed principally of large epitheliomuscular cells (Fig. 10.3). These are so 

 called because, in addition to their epithelial function of covering a surface, 

 they also serve the function of contractility and make possible the movements 

 of the animal. Each cell possesses contractile processes extending along the 

 supporting lamella. In the epidermis, the contractile processes run lengthwise 

 of the body tube, so that their coordinated contraction produces a shortening 

 of the body. The corresponding muscle processes of similar cells in the 

 gastrodermal layer extend along the circumference of the tube, at right angles 

 to the epidermal processes, and their contraction lengthens the body by 

 decreasing its diameter. The many changes of shape and position exhibited 

 by the hydra are all produced by the coordinated and localized contractions 

 of this simple muscular system. At the base of the body the epidermal cells 

 are represented by glandular cells. These produce the secretion by which the 



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