CNIDARIA GENERAL 



73 



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

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

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

 through such a stage of attachment in the course of their development. As for the 

 Ctcnophora, 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. 



si 



I. General. 



We shall better understand the varied organisation of the Cnidaria 

 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 Hydrula, (2) 

 the Scyphopolyp or Scyphula, (3) the 

 Ctenophora. 



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 Craspedote 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 FIG. 64.-A, Diagram of a Hydropolyp 

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



spreads out like an umbrella or bell, *\5ttJS ilSfti 



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



Medusa. The oral portion of the canal; gl > vascular lamella or cathammai 



i i / < .-, , , 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, 



3' 



