II 



I'XIDA El A GENERAL 



73 



structure and in the absence of all organs at the alioral side of the body (exumbrella), 

 i.e. at tliat part of the body by which the stationary forms are attached ; and further 

 in the fact that very many Hydromedusos and many Scyphomcdusce actually pass 

 tin ( nigh such a stage of attachment in the course of their development. As for the 

 ' '' :i/il<firi~i, their descent from attached forms, and, generally, their position among 

 the Cnidaria, seems very doubtful. The presence of complicated sensory bodies at 

 the aboral pole, and the constant occurrence of 8 rows of rowing plates with corre- 

 spondingly arranged gastro-canals, and the marked departure from the strictly radial 

 type, point to the fact that in their case the adoption of the swimming manner of 

 life dates very far back. 



I. General. 



We shall better understand the varied organisation of the Cn i< lri" 

 if we keep clearly in mind that they can in all cases be referred to 

 one of the three following forms : 

 (1) the Hydropolyp or HyJritla, (2) 

 the Scyphopolyp or Scyphula, (3) the 



< '!< 1/l'jJlOfl'. 



1. The simplest form (from which 

 all others can be deduced) is that of 

 the Hydropolyp or Hydrula. 



A Hydropolyp (Fig. 64, A} (type : 

 Hydra) is a pouch -shaped gastrula 

 attached by its aboral pole, and 

 possesses, round its mouth, hollow 

 tentacles as evaginations of the body 

 wall. The gastric cavity is continued 

 into the tentacles. Between endo- 

 derm and ectoderm there is a struc- 

 tureless supporting membrane (si). 



A more highly developed form, 

 the Ci-aspcdote Medusa (Fig. 64, B) \ 

 may proceed from the Hydropolyp 

 by adaptation to a free -swimming , 

 manner of life. The aboral portion 

 of the Hydropolyp body (from the 



attached pole to the tentacles) (longitudinal section); B, of a Craspedote 



spreads out like an umbrella or bell, f^^\ s i,' s^o^Lgi^^, /jeuy'be- 



and becomes the exumbrella of the tween ectoderm and endoderm ; rk, radial 



Mi'tlt/xtt. The Oral portion of the canal ' 3l ' vascular Amelia or cathammal 

 if plate ; v, velum; rik, circumferential canal. 



body (from the tentacles to the 



mouth) also widens out, deepens, and becomes the subumbrella. We 

 thus have a convexo-concave body, on whose circular margin the 

 tentacles are radially arranged (Fig. 65, A). The mouth lies in the 

 middle of the concave side, and generally on the summit of a pro- 

 jection (gastric peduncle, gastric tube). 



The supporting membrane of the Hydropolyp thickens very much, 



FIG. 64. A, Diagram of a Hydropolyp 



