PHYLUM COELENTERATA ( CNIDARIA ) . SIMPLE TISSUE ANIMALS 



105 



hydra in the summer or autumn. The stimu- 

 lus for the formation of gonads in some 

 species appears to be a sudden change in 

 temperature, either rising or falhng. Both an 

 ovary and testes (Fig. 50) are produced on 

 a single individual in some species; the for- 

 mer is knoblike, occupying a position about 

 one-third the length of the animal above 

 the basal disk. The testes, usually several to 

 many in number, are conical or rounded 

 elevations located near the tentacles of the 

 animal. The stalk never has gonads on it. 



Histology 



The body wall 



The hydra consists of two cellular layers: 

 an outer thin layer, the epidermis; and an 

 inner layer, the gastrodermis, about twice as 

 thick as the outer (Fig. 52). Formerly the 

 terms ectoderm and endoderm were applied 

 to these layers and are still retained in some 

 textbooks, but these terms are strictly ap- 

 plicable to the germ layers of an embryo, 

 and therefore cannot properly be used to 

 designate the differentiated epithelial tissues 

 of an adult animal. Both layers are com- 

 posed primarily of epitheliomuscular cells. 

 A thin space containing a jellylike material, 

 the mesoglea, separates the epidermis from 

 the gastrodermis. Although in many coelen- 

 terates the mesoglea constitutes a large part 

 of the bulk of the body, in the hydra it is 

 thin, especially toward the oral end of the 

 body and in the tentacles, while in the cen- 

 ter of the basal disk it is lacking altogether. 

 It serves as a basement membrane for the 

 epithelial cells and a place for attachment of 

 their muscle processes. Hence in the hydra 

 it serves as a supporting layer. Both body and 

 tentacles are hollow, the single central space 

 being known as the gastrovascular cavity. 



The following outline shows the cellular 

 elements of the several layers: 



Epidermis 



1. Epitheliomuscular cells. 



2. Sensory ccl-s. 



3. Other nerve cells. 



4. Interstitial cells. 



5. Cnidoblasts. 



6. Germ cells (eggs and sperms). 



Supporting Layer (Mesoglea) 



This layer is nonccllular, but is traversed by 

 migrating cells and crossed by intercellular 

 bridges and nerve cell processes (fibers). 



Gastrodermis 



1. Epitheliomuscular or nutritive cells (var- 

 iously differentiated in different regions). 



2. Gland cells (several types). 



3. Sparse interstitial cells, which migrate from 

 epidermis as needed, and transform into 

 gland cells between stalks of nutritive cells. 



4. Sensory cells. 



5. Other nerve cells. 



6. Cnidoblasts (temporary migrants through 

 this layer on the way to their final location) . 



The primary component of both epider- 

 mis and gastrodermis is the epitheliomus- 

 cular cell, which extends the full height of 

 the epithelial layer and supports the other 

 elements. Its proportions vary greatly with 

 the expansion or contraction of the animal. 



The epitheliomuscular cells of the epider- 

 mis have their polygonal outer surfaces ce- 

 mented together in wavy borders to form 

 a continuous membrane over the animal, in- 

 terrupted only where stinging or sense cells 

 come to the surface. 



The epitheliomuscular cells of the gastro- 

 dermis line the entire wall of the gastro- 

 vascular cavity. The character of these cells 

 is subject to wide variation in different re- 

 gions, but since they are all concerned with 

 either digestion or absorption of food mate- 

 rial, they are nutritive in function, and 

 therefore are called nutritive cells. The 

 stomach cells form pseudopodia, flagella, 

 and food vacuoles at their free ends; their 

 bases are drawn out into extensions, and 

 often contain contractile fibers. 



On the center of the hypostome, the 

 gastrodermal cells are either filled with small 

 secretion granules, or they have a fine spongy 

 texture, the two conditions alternating in 



