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. This cavity 

 has a single opening which serves both as a mouth and a vent for eges- 

 tion 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 contradistin- 

 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. 



A few migratory cells may be 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 with non-sexual to form individuals of 

 quite unlike appearance. 



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 

 various types of aquatic vegetation. It is from 2 to 20 mm. long; con- 

 sequently 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. 



