72 



SYSTEMATIC ZOOLOGY 





different in many respects from the polyps and medusae of the 

 Hydrozoa. Inasmuch as it is necessary to divide this class into 

 two subclasses, one consisting entirely of polyps, and the other. 

 in the adult condition, exclusively of medusae, it will be simpler 

 to consider the structure of the polyps and the medusae under 

 their respective subclasses. 



SUBCLASS I. ANTHOZOA 



The Anthozoa (dr. civOos, a flower, and £&>oi\ animal), some- 

 times called the Actinozoa (Gr. a/c-m. ray, and £&W, animal), con- 

 sist of polyps only, and include those 

 animals which are known popularly 

 as corals and sea anemones. They 

 differ from the hydrozoan polyps in 

 two important points; first, the mouth 

 docs not open directly into the coel- 

 enteron, but into a tubular gullet or 

 oesophagus, lined with ectoderm, and 

 the ectoderm and entoderm meet at 

 the inner end of this tube; second, 

 the coelenteron is not bounded by a 

 smooth wall, but has a number of 

 longitudinal folds projecting into it 

 from the wall ; these folds are called 

 mesenteries (Figs. 56, 57, and 58). 

 Another difference, of less funda- 

 mental importance, is the fact that, 

 instead of a basement membrane, a 

 mesenchyme is present, containing 

 cells in great abundance. The mouth, which is somewhat elon- 

 gated instead of circular as in the Hydrozoa, lies in the center 

 of a disc, which has at its margin from one to several rows of 

 tentacles. 



The oesophagus is held in position in the coelenteron by means 

 of some of the mesenteries, which extend from the body wall to 

 it, and are pierced by a circular opening near the mouth, or 

 oral, end of the oesophagus, so that the spaces between the mes- 

 enteries are all in communication with each other. The free 



Fig. 56. Diagrammatic cross-section 

 through the cesophagea ■>( a 



sea anemone. Tl ted cavity 



in the center is the i im >phagus. 

 six pairs of primary mesenteries meet 

 the wall of the oesophagus. Tl 

 pairs of secondaries are shorter; 

 the twelve pairs of tertiaries are the 

 shortest of all. In some cases a 

 fourth series may develop, twenty- 

 four paii's, occupying the spaces on 

 each sidt of each pair of tertiaries, 

 (Drawn by the author.) 



