THE GENERAL SHAPES OF ANIMALS. 171 



individualities merged into an individuality of a higher 

 order, must be named the compound Infusoria. The 

 cluster of Vorticellce in Fig. 144, will sufficiently exemplify 

 them ; and the striking resemblance borne by its individuals 

 to those of a radially-arranged cluster of flowers, will show 

 how, under analogous conditions, the general principles of 

 morphological differentiation are similarly illustrated in the 

 two kingdoms. 



§ 246. Hadial symmetry is usual in those aggregates of 

 the second order that have their parts sufficiently differentiat- 

 ed and integrated to give individualities to them as wholes. 

 The Ccelenterata offer numerous examples of this. Solitary 

 polypes— hydroid or helianthoid — mostly stationary, and 

 when they move, moving with any side foremost, do not by 

 locomotion subject their bodies to habitual contrasts of con- 

 ditions. Seated with their mouths upwards or downwards, 

 or else at all degrees of inclination, the individuals of a 

 species taken together, are subject to no mechanical actions 

 affecting some parts of their discs more than other parts. 

 And this indeterminateness of attitude similarly prevents 

 their relations to prey from being such as subject some of 

 their prehensile organs to forces unlike those to which the 

 rest are subject. The fixed end is differently conditioned 

 from the free end, and the two are therefore different ; but 

 around the axis running from the fixed to the free end the 

 conditions are alike in all directions, and the form therefore 

 is radial. Again, among many of the simple free- 



swimming Hydrozoa, the same general truth is exemplified 

 under other circumstances. In a common Medusa, advanc- 

 ing through the water by the rhythmical contractions of 

 its disc, the mechanical reactions are the same on all sides ; 

 and as, from accidental causes, every part of the edge of the 

 disc comes upwards in its turn, no part is permanently af- 

 fected in a different way from the rest. Hence the radial 

 form continues. 



