231 



Echinoderms, are only the peculiar way in whicli our general formula 

 of radiation is carried out- In Polyps, we have hollow spherical 

 wedges, the sides of which form projecting partitions; — in Acalephs, 

 these spherical wedges are solid with the exception of the radiating 

 tubes representing the chambers of the former; — in Echinoderms, 

 this is still differently carried out by the condensation of the mass 

 toward the periphery, leaving tubes and digestive cavities with dis- 

 tinct walls hanging freely in the envelope. Whether our formula for 

 Polyps has a limited number of radiating partitions, as in Halcyonoids^ 

 or a large number, as in Fungidce, this will make no difference ; the 

 transformation from the Polyp into the Acaleph or the Echinoderm 

 can always be made. Even in Acalephs we have such forms as 

 jEquorea, in which the number of radiating tubes is as great as that 

 which we find in many of the Polyps. Whether the Acaleph is a flat 

 disc, Kke Aurelia, or assumes the shape of aBeroe; — whether the 

 Echinoderm is a flat Starfish, a spherical Sea-Urchin, or a tubular 

 Holothurian, the transformation can still be made. The spherical 

 Beroe presents only this peculiarity, that the radiating tubes, tending 

 first horizontally, branch up and down at the periphery. The Sea- 

 Urchin can be transformed into a Starfish simply by slitting it open 

 through the middle of the interambulacral space, and stretching the 

 abactinal area to the extent necessary to cover the sj)ace above the 

 ambulacral zone. In Crinoids, the abactinal area has the upper 

 hand, forming the calyx and stem, while in Holothurians it is reduced 

 to a minimum. 



It has already been stated that the plan of structure of Kadiates 

 consists of spherical wedges symmetrically arranged round a vertical 

 axis, but this does not imply that in their natural attitudes as living 

 animals all Radiates stand either mouth upward or downward. We 

 have differences in this respect even among members of one and the 

 same class, as, for instance, among Crinoids as compared to Starfishes 

 or Sea-Urchins ; — or among members of the same order, as, for in- 

 stance, in Pleurobrachia as compared to Bolina. Nor does it always 

 happen that the mouth is either above or below ; the structural ver- 

 tical axis may assume a horizontal position, as in Holothurians, and 

 it is a great mistake, therefore, to believe that the mouth in Holothu- 

 rians is in a homologous position to the mouth of worms, or that the 

 sides of a Holothurian are homologous to the sides of a worm. It 

 is contrary to the plan of their structm-e to speak of a back and a 

 ventral side in Holothurians or in Spatangoids ; the sides on which 

 they rest are truly homologous to the vertical sides of a genuine 

 Echinus or to those of an Actinia. Nor is the position of the mouth 

 in Holothurians the extreme of the tendency observed among Spa- 

 tangoids to bring the mouth forward. The side on which Holothu- 



