BRANCH CCELENTERATA: THE POLYPS, ETC. 93 



remaining fixed. In localities near the seashore many 

 animals belonging to this great group can be readily 

 found and observed. The beautiful sea-anemones with 

 their slowly-waving tentacles, the fine many-branched 

 truly plant-like hydroids with their hosts of little buds, 

 and the soft colorless masses of jelly, the jellyfishes, which 

 are cast up on to the beaches by the waves are all animals 

 belonging to the branch Coelenterata. 



General form and organization of body. The general 

 or typical plan of body-structure for the Coelenterata, 

 these animals which come next to the sponges in degree 

 of complexity, can best be understood by imagining the 

 typical cylindrical or vase-like body of the simple sponges 

 to be modified in the following way: The middle one of 

 the three layers of the body- wall not to be composed of 

 scattered cells in a gelatinous matrix, but to be simply a 

 thin non-cellular membrane; the body-wall not to be 

 pierced by fine openings or pores, but connected with the 

 outside only by the single large opening at the free end, 

 and this opening to be surrounded by a circlet of arm-like 

 processes or tentacles, which are continuations of the 

 body-wall and similarly composed. Such a body-struc- 

 ture, which we saw well shown by Hydra, is the funda- 

 mental one for all polyps, sea-anemones, corals, and 

 jellyfishes. The variety in shape of the body and the 

 superficial modifications of this type-plan are many and 

 striking, but after all the type-plan is recognizable through- 

 out the whole of this great group of animals. 

 C The two chief body-shapes represented in the branch 

 are those of the polyps on the one hand, and the jelly- 

 fishes or medusae oTT the other. The polyp-shape is that 

 of a tube with a basal end blirrS^or closed, attached to 

 some firm object in the water ana with the free end with 

 an opening, the mouth-opening. At this mouth-end 

 there is a circlet of movable, very contractile tentacles. 



