xxii THE VOYAGE OF H.M.S. CHALLENGED 



This fundamental form has become predominant in the Medusoid personae of the 

 Siphonophora colonies. It appears also in the parasitic Mnestra, and is sometimes 

 faintly indicated in some Cubomedusae (Charybdea). 



§ 32. Perradii, or transverse axes of Order I. In the ordinary regularly quadripartite 

 Medusa?, the four perradii or " upper radii " lie in the two transverse axes of the 

 quadrate pyramid perpendicular to each other, in the sagittal middle hne of the four 

 parameres, between these two antimeres. In most Craspedotae the four oral angles, or 

 oral lobes, the four radial canals, the four primary tentacles, and the four genitalia lie in 

 the four perradii (or with dichotomised genitalia in the middle line of the perradii between 

 the halves). In all Acraspedse the four limbs of the oral cross and of the gastral cross 

 (fig. B, ivx) correspond to the four perradii, also the four oral lobes and the four oral 

 pillars or primary oral arms (ab), and also the middle line of the four lobed primary 

 radial pouches : in the Staurornedusae and Peromedusae in addition to these the four 

 tentacles Lie in the four perradii, and in the Cubomedusae and Discomedusae the four 

 primary sense clubs (fig. B, op). In all Medusae the perradii are originally the zones of 

 the most active life, of the strongest growth, and most complicated differentiation, with 

 preponderating tendency to centrifugal development of the organs. 



§ 33. Interradii, or transverse axes of Order II. In the quadripartite Medusae the 

 four interradii lie exactly in the middle between the four perradii, and, therefore, intersect 

 the latter at an angle of 45 degrees; they therefore lie at the same time in the boundary hne, 

 between the four parameres. In the quadrate Craspedotae the four interradii form the 

 geometrical middle line of the four injected oral archings (between every two perradial 

 oral lobes), and also of the four broad cathammal plates (between every two radial canals) ; 

 in the octonemal Craspedotae (with eight tentacles) the four primary tentacles He in the 

 perradii, the four secondary in the interradii. In all Acraspedae the four injected oral 

 angles and oral columns lie in the four perradii, also the four fundamental taeniola or 

 gastral ridges, and the four primary gastral filaments (or groups of filaments), and, finally, 

 the four important cathamma (the four primary septal nodes or septal ridges). The 

 interradii, moreover, form the middle line of the four genitalia in most Acraspedae (fig. 

 B, s), and also of the four pair of genitalia in such Acraspedae in which the latter are 

 divided into two halves. In all Medusae the interradii are next to the perradii, the 

 zones of the most intense growth and of the most important differentiation, but the 

 tendency to development is predominantly centripetal with them, whilst with the 

 perradii it is centrifugal. 



§ 34. Adradii, or transverse axes of Order III. The eight adradii lie in the middle 

 between the four perradii and the four interradii of the quadripartite Medusa ; they 

 halve the angles between the former and the latter, and intersect the two at an angle 

 of 22|> degrees. They consequently lie at the same time in the median planes of the 

 eight antimeres. If, on the one hand, we designate the four radii of Orders I. and II. 



