Some of the Lower Metasoans 383 



Whether this simplicity of structure represents true primitiveness 

 or degeneration is debatable. If it is truly primitive, it represents an 

 amazing intermediate stage between colonial protozoans and true multi- 

 cellular forms. 



Fig. 126. — A mesozoan. 



THE PHYLUM COELENTERATA 



Due to the comparative simplicity of their structure, the members 

 of this phylum are considered to be the most primitive of the branch 

 Eumetazoa. Although when compared with others of this branch the 

 coelenterates do seem primitive, they are much advanced over the 

 sponges. The sponges represented a cellular level of organization ; the 

 coelenterates definitely are of a tissue grade, that is, cells are combined 

 into tissues which act together to perform certain functions. They are 

 directly dependent upon one another. This dependence can be demon- 

 strated by separating the cells as was done with the sponge. Here, how- 

 ever, the animal will not reassemble to reform an individual. Although 

 the coelenterate cannot reassemble in this way, most of them do demon- 

 strate remarkable powers of regeneration. 



The animals of this phylum are either cylindrical forms with ten- 

 tacles at the upper end or bell-like forms with tentacles on the margin of 

 the bell. The cylindrical forms are know^n as polyps or hydroid jorms, 

 the others as jellyfish or medusae. 



Characteristics of the Phylum. — Many of the coelenterates, like 

 the sponges, are attached forms; others are quite motile. The sessile 

 forms, however, are more efficient in their food getting than are the 

 sponges. They do not depend upon sifting large amounts of water, 

 rather they have stinging cells and tentacles which actively capture 

 small animals which are then transferred to the central gastrovascular 

 cavity. This cavity serves both for digestion and distribution. 



