176 



MORPHOLOGICAL DEVELOPMENT. 



metrical. The Physcdia, Fig. 152, is an example. Here the 

 relations of the integrated group of individuals to the environ- 

 ment are indefinite ; and there is hence no agency tending 

 to change that comparatively irregular mode of growth that 

 is probably derived from a primordial 

 type of the branched Hydrozoa. 



So various are the modes of union 

 among the compound Coelenterata, that 

 it is out of the question to deal with 

 them all. Even did space permit, it 

 would be impracticable for any one but 

 a professed naturalist, to trace through- 

 out this group the relations between 

 shapes and conditions of existence. The 

 above must be taken simply as a few of 

 the most significant and easily-interpret- 

 able cases. 



§ 248. In the sub-kingdom Mollm- 

 coida, we meet with examples not wholly 

 unlike the foregoing. Among the types assembled under 

 this title there are simple individuals or aggregates of the 

 second order, and societies or tertiary aggregates produced 

 by their union. The relations of forms to forces have to be 

 traced in both. 



Solitary Ascidians, fixed or floating, carry on an inactive 

 and indefinite converse with the actions in the environment. 

 Without power to move about vivaciously, and unable to 

 catch any prey but that contained in the currents of water 

 they absorb and expel, these creatures are not exposed to 

 sets of forces that are equal on two or more sides ; and their 

 shapes consequently remain vague. Though there are in 

 them traces of symmetrical arrangement, probably due to 

 their derivation, yet they are substantially asymmetrical. 

 Fig. 156 is an example. Among the composite 



Ascidians, floating and fixed, the shape of the aggregate, 



