vi THE VOYAGE OF H.M.S. CHALLENGER. 



ectoderm. Nearly all Medusae are gonochoristie, only a few hermaphrodite. As regards 

 the phylogeny of the Medusae, their comparative anatomy and ontogeny points out the 

 class of Polyps as their original ancestral group. 



§ 2. Definition of the Polyp. The class of the Polyps, from which the class of Medusa? 

 are phylogenetically derived, and which, therefore, furnishes the key to our morphological 

 knowledge of them, is at the same time the common ancestral group of all Cnidariae. 

 At present the fresh- water Polyp (Hydra), and the closely allied genera Clava, Coryne, &c., 

 must be considered their simplest and most primitive representatives. Their developed 

 organism is immediately allied to that of Gastrula (phylogenetically Gastraea), and has the 

 same simple primitive intestine with primitive mouth, whose wall is formed from the two 

 primary germinal layers. The Polyp is chiefly distinguished from the Gastrula by begin- 

 ning histological differentiation of the two primary germinal layers, and also by the fact 

 that the aboral pole of the principal axis serves as a peduncle for adhesion, whilst a corona 

 of feelers or tentacles is developed round the mouth at its oral pole. This corona of ten- 

 tacles forms the boundary between the strongly-arched aboral part of the body(cup, "calyx") 

 and the flat or even depressed oral part (oral disk, " peristomium "). 



§ 3. Medusa and Polyp. The most essential difference between the organisation of the 

 Medusa and the Polyp consists in the formation of the characterstic swimming organ of 

 the former, the umbrella, and of the cathamma or partial fusions between the aboral wall 

 of the cup (" calyx ") and the oral disk (" peristomium "), by means of which the peripheric 

 part of the simple gastral cavity is divided into radial pouches or canals, which regularly 

 surround its simple central space. The central gastral cavity of the Medusa does not 

 therefore correspond to the whole simple gastral space of the Polyps (or to the primitive 

 intestine of the Gastraea) but only to the central part of it, whilst on the other hand its 

 peripheric part is homologous with the radial (usually four-rayed) coronal intestine 

 of the Medusa (" pouch corona or canal corona "). The central oral opening is identical 

 in both animal classes, arising from the primitive mouth of the Gastrula, and when it is 

 prolonged in the Polyps into an oral cone or proboscis, the latter corresponds to the 

 freely projecting oesophagus or oral peduncle of many Medusa?. The peristomium of the 

 Polyps (or the slightly concave oral disk) is homologous with the " subumbrella " of 

 the Medusa (or the more strongly depressed lower surface of the umbrella). In the same 

 way the convex outer surface or the dorsal cup-wall (calyx) of the Polyp corresponds to 

 the more depressed dorsal wall or " exumbrella " of the Medusa. The cup-margin of the 

 Polyps (with the insertion of the corona of tentacles) is therefore homologous with the 

 umbrella margin of the Medusa, but in the latter, differentiated organs of sense are 

 developed beside or from the tentacles, whilst this is not the case in the former. As 

 regards histological differentiation, the older and lower Polyp form remains far behind 

 the younger and higher Medusa form. The latter has arisen from the former by adaptation 

 to a swimming mode of life, and has thereby become perfected. 



