228 CWLENTERATA. 



PHYLUM III. CCELENTERATA (CNIDARIA, JSTEMATO- 



PHOKA). 



The animals belonging to the coelenterates were formerly called 

 Zoophyta (plant-animals). They were united by Cuvier with the 

 Echinoderma to form the type Radiata, a union which Leuckart, the 

 father of the name Ccelenterata, set aside because a special intes- 

 tine and a special body cavity occur in the Echinoderma, while in 

 the Coelenterata there is but a single system of cavities in the 

 body. Each of the three names indicates certain important 

 characters of the group. 



(1) The name Zoophyta was selected with regard to the gen- 

 eral appearance. Most coelenterates, like the plants, are fixed 

 and form bush-like or mossy colonies by incomplete budding. This 

 resemblance, is but superficial, for in any accurate investigation 

 there cannot be the slightest doubt of the animal nature of any 

 Ccelenterate. The name therefore must not be understood to 

 imply that these are doubtful forms which stand on the border 

 between plants and animals. Besides, there are not only fixed but 

 free-moving forms which swim in the water with great ease. 



(2) Most Coelenterata are radially symmetrical. There is a 

 main body axis one end of which passes through the mouth and 

 the other through the blind end of the digestive tract, and the 

 organs of the body are radially arranged around this so that the 

 body may be divided into symmetrical halves by numerous planes. 

 In the higher Coelenterata this may be replaced by a biradial 

 symmetry or even by bilaterality (Ctenophora, many Anthozoa). 



(3) The term Coelenterata is given these animals because they 

 contain a single continuous coelenteron or gastrovascular cavity. 

 In the simplest species this is a wide-mouthed sac into which food 

 passes for digestion. The single opening into it serves at once as 

 mouth and anus; the sac itself is the alimentary tract. Frequently 

 lateral diverticula or branched canals are given off from the central 

 sac which distribute the nourishment to the peripheral parts of the 

 "body, and thus functionally replace the vascular system of higher 

 forms. 



Since this gastrovascular system is primarily for nourishment, 

 it is an error to call it a body cavity and to say that the coelenterates 

 are stomachless. On the other hand, the term ' coelenteron, ' that 

 is a cavity which is at once gastric and ccelomic (p. 158), is perfectly 

 defensible, since in many higher animals which possess a true body 



