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



gression. On the other hand, it rarely happens that the central part is four, and the 

 peripheric part of the umbrella six {e.g., Polyclonia frondosa). That the central umbrella 

 usually retains the fundamental number by inheritance, whilst the peripheric part 

 varies in several ways, is explained by the fact that the latter is subject in a higher 

 degree to adaptation. (The fundamental number of all Rhizostomae, for example, 

 remains 4, with four oral pillars, four genitalia, &c, although they have all eight 

 arms and from eight to twelve sense clubs, as well as a very variable number of marginal 

 lobes.) 



§ 27. Radii of the first to the fourth order. The radial structure of the Medusae (like 

 that of most radiata) is caused by the division of the growth of the central body (originally 

 uniaxial in the gastrula) into different meridian planes. As the growth is more energetic 

 in definite meridian planes, or radial planes (which touch in the common central prin- 

 cipal axis), and leads to the development of new organs, the interlying radial planes con- 

 tinue indifferent or opposed, and usually in the middle between these energetic rays of 

 growth. In this way an antithesis arises first of all between radii of Order I. and Order 

 II., which we shall designate shortly " perradii " and " interradii." Special organs very 

 often arise in the middle between the perradii and the interradii, and these then lie in the 

 radii of Order III., the "adradii." Finally, we can distinguish in many cases radii of 

 Order IV., or subradii, which lie in the middle between the eight adradii and the eight 

 principal radii. In the "principal radii" we include the four perradii and the four 

 interradii, whilst, in contrast to these, we term all other possible radii " succursal or 

 secondary radii." Our distinction of these four orders of radii is of great importance, not 

 only for the architecture of the Medusae, but for the promorphology of most other 

 " radiata" ; it allows us to designate, in a single word, the most important conditions of 

 position and relation of the organs with mathematical sharpness and precision. For 

 example, Ephyra, the important ancestral form of all Disconiedusaa (fig. A), has four 

 perradial oral lobes and limbs of the oral cross (as), four interradial genitalia (s) and 

 filaments or phacelb (/), eight adradial tentacles (ta), and sixteen subradial marginal 

 lobes (/). 



§ 28. Parameres and antimeres. As in all Acraspedse, and in the majority of the 

 Craspedotse, 4 is the normal typical fundamental number of the body, the latter conse- 

 quently consists of four parameres or " radiate parts," which touch in the common vertical 

 principal axis of the body. These four parameres are originally congruent, so that a prin- 

 cipal organ comes on each paramere, — an oral lobe, a quadrant of the stomach, a radial 

 canal or a radial pouch, a septum or cathamma between the pouches, a primary tentacle, and 

 so forth. Each paramere has a cbpleuric fundamental form (or a strictly " bilaterally 

 symmetrical " form), and therefore again consists of two equally symmetrical halves, the 

 "counterparts" or antimeres. These two antimeres or counterparts comport themselves 

 the same as the symmetrical halves of the body of all higher (dipleuric) animals. We 



