Fig. 10.1. Pebnatohydra nlioactu in its normal 

 attitude, attached to a submersjed leaf, with 

 tentacles extended. Note the attenuated, 

 stalk-like basal portion of the trunk. (Re- 

 drawn from L. H. Hvman, 1930, Transaclions 

 of ihe American Microscopical Society, vol. 49.) 



diploblastic. The typical radial symmetry has in some cases become biradial, 

 with two centers of symmetry rather than one. Coelenterates are always pro- 

 vided with stinging capsules, the nematocysts, structures which are not known 

 to be produced by any other animal group. The organs which occur in this 

 phylum are simple in structure and function. As compared with sponges, 

 coelenterates exhibit more definite form and symmetry and a more advanced 

 level of organization. 



The phylum includes three classes: the class Hydrozoa, containing the 

 hydras, hydroids, hydromedusae or hydroid jellyfishes, and hydroid corals; 

 the class Scyphomedusae, including jelly-fishes of a more advanced type; and 

 the class Anthozoa, the sea anemones, sea pens, sea fans, and true stony corals. 



Coelenterates are predominantly marine animals, occurring at all depths in 

 the ocean and under a wide range of conditions, since the phylum contains 

 both attached and free-swimming forms. Typically, however, they are in- 

 habitants of the shallow waters along shore and of the upper layers of the 

 deeper ocean. In their feeding habits they are strictly carnivorous, capturing 

 other animals by means of tentacles armed with nematocysts, from which 

 paralyzing threads are emitted. Most coelenterates are attached for a con- 

 siderable part of the life cycle, during which they commonly reproduce by 

 budding, and colonies of innumerable individuals may be produced. In 

 Hydrozoa a free-swimming sexual phase may alternate with the attached, 

 asexual stage; in Scyphomedusae the free-swimming jellyfish stage constitutes 

 the major part of the life cycle; but in Anthozoa both asexual and sexual 

 reproduction occurs during the attached phase, for no free-swimming repro- 

 ductive individuals are produced. The radial symmetry characteristic of coel- 



285 



