DIVISION OF LABOR IN THE COELENTERATES IJil 



tain chemical stimuli that apparently cause a change of osmotic pres- 

 sure within the cell, thus forcing out its threadlike portion. After a 

 nematocyst is protruded, the cnidoblast dies and is soon replaced by 

 another. 



stinSina 



"nerve 

 cell 



"muscular 



absorbing 

 cell ^ 



.flagellum 



-sensory 

 cell -^ 



cell ® 



cxxnthmd 

 cell 



The Endoderm and Its Functions 



By cutting a section through the body of a Hydra its similarity to a 

 two-walled sac is evident. Between the ectoderm and the inner layer 

 of cells (endoderni) a thin, structureless layer called the mesoglea 

 forms as a secretion 

 from the cells of the «^toclerm j e«dod^m 



inner and outer layers. 

 Mesoglea forms much 

 of the bulk of other 

 coelenterates like the 

 jellyfishes. The endo- 

 derm consists principally 

 of large vacuolated cells 

 that have flagella at the 

 free or inner end, al- 

 though they are also 

 capable of developing 

 pseudopodia at this 

 end. Circular contrac- 

 tile fibers are developed 

 at their basal end. Thus 



they are endothelial-muscular cells. In the third of the body nearest 

 the basal end, gland cells develop, which secrete digestive enzymes. 

 Nerve and sensory cells are also found in the endoderm. 



For a simple animal, the Hydra seems to have many kinds of cells. 

 What is the use of so many ? The answer is found in the way it gets 

 food, ingests it, and finally absorbs it into the body cells. By watch- 

 ing a hydra in the aquarium it will be seen that its tentacles are con- 

 stantly moving as if seeking food. If a tiny bit of raw beef is placed 

 within reach, the animal will bend over and carry the meat to the 

 mouth, the edges of which soon close around it, forcing it inside. If 

 the piece is too large to be taken in, the Hydra actually turns inside 

 out in an attempt, usually successful, to put the meat inside the 

 gastrovascidar cavity. Once inside the cavity, digestive enzymes from 

 the glandular cells act upon the food, gradually breaking it down into 



Sections through the body wall of hydra showing 

 the two layers of cells separated by the striated 

 lamella secreted by the basal parts of the ecto- 

 dermal and endodermal cells. 



