188 MORPHOLOGICAL DEVELOPMENT. 



the one head in § 218, may here be instanced afresh under 

 the other. Further, among these secondary aggregates in 

 which the units, only physically integrated, have not had their 

 individualities merged into an individuality of a higher 

 order, must be named the compound Infusoria. The cluster 

 of VorUcellce 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 mor- 

 phological differentiation are similarly illustrated in the two 

 kingdoms. * 



§ 246. Eadial symmetry is usual in low aggregates of 

 the second order which have their parts sufficiently differen- 

 tiated 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 



