CHAPTER XVII. 



THE COELENTERATA 



The Coelenterata (Gr. koilos==hollow+enteron intestine) are all 

 aquatic (mostly marine) animals, possessing a single system of internal 

 chambers called a gastro-vascular-cavity, having a single opening which 

 serves both as a mouth and a vent for egestion and excretion. In other 

 words, digestion and circulation all occur in this single tubular cavity. 

 In all the higher forms of animal life there is a coelom ( ), 



that is, a cavity between the intestinal tract and the body wall. This 

 was observed in the frog where all the viscera ( ) are 



inside the body but outside the intestinal tract. 



In the Coelenterata there is a radial symmetry as contradis- 

 guished from the bilateral symmetry of the frog. 



The animals belonging to this phylum are diploblastic, that is, they 

 have gone through the gastrula stage in developing and remained sta- 

 tionary at the end of that stage, with this exception, that they just begin 

 forming a third layer which, however, never becomes a regular tissue. 

 The entoderm and ectoderm are separated from each other by a thick 

 mucilaginous mesoglea ( ) or mesenchyme 



( ). The point of value here is that in the higher 



forms this midlayer becomes an actual tissue by forming a very definite 

 sheet of cells called the mesoderm, while in the Coelenterata the layer 

 does not become cellular. The midlayer here acts as though it were 

 about to form into a tripoblastic animal but has not succeeded. 



There may be a few migratory cells found in the mesoglea, but as 

 a whole it is non-cellular. 



The phylum is further distinguished by the fact that in practically 

 all its members there are stinging cells [(sometimes called nettle-cells 

 or nematocysts ( )]. 



Nerve cells (sensory) and muscle cells both occur. 



Reproduction by non-sexual methods is the more common, though 

 sexual methods may alternate, forming individuals of quite unlike ap- 

 pearance. 



HYDRA FUSCA 



The classic coelenterate for laboratory study is this little animal 

 (Fig. 152), found in ponds and streams attached by its basal end to vari- 

 ous types of aquatic vegetation. It is from 2 to 20 mm. long; conse- 

 quently it can be seen by the naked eye. 



The entoderm contains the brown bodies from which the animal 

 receives its name. The animal itself has a mouth opposite the basal disk. 

 About the mouth, there is a varying number of tentacles, usually four 



