192 MORPHOLOGICAL DEVELOPMENT. 



Calenterata, presenting clusters of individuals which ai 

 severally homologous with the solitary individuals last deall 

 with, we have to note both the shapes of the individuals thi 

 united, and the shapes of the aggregates made up of them. 



Such of the fixed Hydrozoa an&Actinozoa as form branched 

 societies, continue radial; both because their varied attitudes 

 do not expose them to appreciable differences in their rela- 

 tions to those surrounding actions which chiefly concern 

 them (the actions of prey), and because such differences, even 

 if they were appreciable, would be so averaged in their effects 

 on the dissimilarly-placed members of each group as to be 

 neutralized in the race. Among the tree- 

 like coral-polypedoms, as well as in such 

 ramified assemblages of simpler polypes 

 as are shown in Figs. 149, 150, we have, 

 indeed, cases in many respects parallel 

 to the cases of scattered flowers (§ 233), 

 which though placed laterally remain 

 radial, because no differentiating agency 

 can act uniformly on all of them. Meanwhile, in the 



groups which these united individuals compose, we see the 

 shapes of plants further simulated under a further parallelism 

 of conditions. The attached ends differ from the free ends 

 as they do in plants; and the regular or irregular branches 

 obviously stand to environing actions in relations analogous 

 to those in which the branches of plants stand. 



The members of those compound Ccelenterata which move 

 through the water by their own actions, in attitudes that are 

 approximately constant, show us a more or less distinct two- 

 sidedness. Diphyes, Fig. 259, furnishes an example. Each 

 of the largely-developed and modified polypites forming its 

 swimming sacs is bilateral, in correspondence with the bi- 

 lateralness of its conditions; and in each of the appended 

 polypites the insertion of the solitary tentacle produces a 

 kindred divergence from the primitive radial type. The 



aggregate, too, which here very much subordinates its mem- 



