96 INVERTEBRATE ZOOLOGY 



involves a rather conspicuous metamorphosis. The eight arms 

 mentioned above mark off eight radii of the mature jellyfish. 

 The four radii passing through the angles of the mouth are known 

 as the perradii, the other four as the interradii. Axes falling 

 between the bases of the eight arms of the ephyra are designated 

 as the adradii. By more rapid growth of the adradial zones the 

 notches of these regions become filled up, thus giving the young 

 jellyfish a slightly crenated circular outline. The marginal 

 tentacles make their appearance in the adradial regions. 



Though alternation of generations described for Aurelia is the 

 customary type of development among the Scyphozoa, in some 

 genera a free-swimming medusa develops directly from the ferti- 

 lized egg. True asexual reproduction does not occur in the 

 medusa of Scyphozoa. 



Body Form. — There is great diversity in form of body among 

 the Scyphozoa. Aurelia has a flattened, saucer-shaped bell 

 while some of the genera from the tropics have almost cubical 

 bodies (Cubomedusae). The cubical appearance is heightened 

 by the presence of only four tentacles on the margin of the bell. 

 Some of the relatives of the Aurelia lack marginal tentacles 

 altogether and in the members of the suborder Rhizostomae a 

 series of folds around the manubrium bear eight small mouths in 

 addition to the one at the end of the manubrium. Members of 

 the genera Haliclystus and Lucernaria have the aboral surface 

 drawn out as a peduncle by which the animal becomes attached. 

 In these same genera, marginal tentacles are lacking but the bell 

 is drawn out into eight arms each of which is provided with a 

 cluster of diminutive tentacles. 



Class Anthozoa 



All organisms belonging to this class conform to a polyp form 

 of organization. The anthozoan polyp differs fundamentally 

 from the characteristic hydrozoan polyp in the presence of an 

 ectodermal esophagus and of longitudinal partitions called 

 septa or mesenteries (Fig. 55) partially dividing the gastrovascular 

 cavity. Well-developed bands of muscles are found in connec- 

 tion with the septa. The mesoglea contains numerous cells, 

 thus having the appearance of a simple connective tissue and 

 supplying firmness and a fleshy consistency to the body in many 

 forms. Both solitary and colonial forms occur in this exclusively 

 marine group of almost universally sessile organisms. Examples 



