384 The Animal Kingdom 



The coelenterates are often considered to have but two true body 

 layers ; thus are called diploblastic. The epidermis which is derived from 

 the embryological ectoderm is protective in function. It is differentiated 

 into various types of cells. The inner epithelum lining the gastrovascular 

 cavity is derived from the embryological entoderm and is known as the 

 gastrodermis. These cells, too, are differentiated into many different 

 types. Between these two layers is a third region, the mesoglea. Meso- 

 glea is a gelatinous material which gives support to the animal. In 

 some forms this contains no cellular elements, and the animal is truly 

 diploblastic. In others, cells invade from the ectoderm and make this 

 a true third cellular layer. For this reason, it is not entirely correct 

 to characterize the members of this phylum as "diploblastic." 



The opening of the central gastrovascular cavity in the polyp form 

 is surrounded by the tentacles. This cavity has but a single opening, 

 the mouth. Food is conveyed to the mouth by the tentacles, and un- 

 digested materials are ejected through this same opening. 



Radial or hiradial symmetry is one of the most characteristic 

 features of the members of the phylum. The main axis of symmetry 

 extends from the mouth to the base and the parts are arranged around it. 



The tissue level of organisation of the members of this phylum is 

 further evidenced by the numerous special cells which unite in function- 

 ing. Both the epidermis and gastrodermis contain special supporting 

 cells, epithelial muscular cells, glandular cells, nerve cells, and in- 

 terstitial cells. In addition, the gastrodermis has special gland cells 

 for digesting the engulfed food and some of the cells have special 

 flagella. 



The individual nerve cells are bi- or multi-polar cells located in 

 both the epidermis and gastrodermis. They are connected in such a 

 way that the whole body surface is coordinated. This netlike arrange- 

 ment is characteristic of the animals of this phylum. Nerve impulses 

 are conducted in either direction over any one nerve cell as these cells, 

 unlike those of the vertebrates, are nonpolarized. The connections 

 between the individual cells are synaptic in nature. Stimulation of any 

 part of the animal results in a generalized response of the whole animal. 

 In some coelenterates, there is a concentration of these nerve cells 

 around the mouth. 



The nematocysts or stinging capsules are found only in the members 

 of this phylum ; thus they constitute a unique character. These re- 

 markable structures develop inside special interstitial cells which are 

 termed cnidoblasts. Inside the nematocvst is a coiled hollow thread 



