198 



MESOZOA THROUGH ENTOPROCTA: 



ates have only two germ layers, the middle layer of 

 the adult is strongly suggestive of the developmental 

 mesoderm of the Eumetazoa. 



COELENTERATA ( = CNIDARIA) 



Diagnosis: symmetry radial, biradial in some 

 Anthozoa (corals and sea anemones); two tissue lay- 

 ers with some semblance of connective tissue be- 

 tween; no head or segmentation; tentacles surround 

 the mouth, which leads into a sac-like gastrovascular 

 cavity that may be separated by membranes; two 

 body types, the tubular-bodied polyp with mouth and 

 tentacles at one end and attachment at the other, and 

 the umbrella-shaped, usually free-swimming, medusa 

 with a jelly-like body, having tentacles on the margin 

 and a mouth on a central projection of the under- 

 surface; both medusae and polyps are variously modi- 

 fied and both body types appear in the life cycle of 

 many species; when both body types occur, one type 

 is small, frequently microscopic, and inconspicuous; 

 solitary as polyps and medusae, or colonial as polyps, 

 individual polyps of a colony usually minute; thread 

 cells are present for defense and capturing prey; 

 many look plant-like, some flower-like, others re- 

 semble jelly-like umbrellas; many are brightly 

 colored. 



Some members of this phylum may be confused 

 with plant-like Ectoprocta or Entoprocta, and 

 some with algae (similar-appearing algae possess no 

 minute "cups" which indicate individuals of a 

 colony). The individual Ectoprocta and Entoprocta 

 are bilaterally symmetrical (usually not obvious 

 without appropriate magnification), usually white or 

 whitish, and usually rough to the touch. Coelenter- 

 ates are found in both marine and fresh waters. 



The tissue basis of organization in coelenterates 

 implies that the tissues, as such, carry on body func- 

 tions and definite organs are not formed. Division 

 of labor is found in some colonial forms; difiTerent in- 

 dividual animals within a single colony are 

 specialized for various difTerent functions, such as 

 feeding, defense, and reproduction. Here one might 

 consider individuals as taking the place of definite 

 organs. 



These animals possess a common plan of structure 

 that is expressed in two ways, the polyp or the me- 

 dusa (Figure 12.5). In both types, at one end a single 

 opening functions as both mouth and anus. This 



fennale 

 colony 



settles and 



starts new 



colony 



Rgure 12.5 Life cycle of the marine colonial hydroid, Obe/jo, show- 

 ing both medusa end polyp stages and alternation of generations. 

 Variably enlarged and highly diagrammatic. 



opening leads into a central cavity within both 

 types of animals. Functionally, the central cavity 

 is called the gastrovascular cavity because it is the site 

 of some of the digestive processes and its large size 

 and branchings give it the effect of a circulatory 

 system. The mouth is often at the end of a cone of 

 tissue and, except in a very few cases, is surrounded 

 by a ring of tentacles. 



