194 MORPHOLOGICAL DEVELOPMENT. 



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-interpretable cases. 



248. In the sub-kingdoms Polyzoa and Tunicata we meet 

 with examples not wholly unlike the foregoing. Among the 

 types assembled under these names there are simple indivi- 

 duals 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 which are equal on two or more sides ; and their 

 shapes consequently remain vague. Though internally their 

 parts have a partially-symmetrical arrangement, due to their 

 derivation, yet they are substantially unsymmetrical in that 

 part of the body which is concerned with the environment. 

 Fig. 156 is an example.* Among the composite 



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

 partly determined by the habitual mode of gemmation and 

 partly by the surrounding conditions in each case, is in 

 great measure indefinite. We can say no more about it than 

 that it is not obviously at variance with the laws alleged. 



Evidence of a more positive kind occurs among those com- 

 pound Molluscoida which are most like the compound 

 Coslenterata in their modes of union the Polyzoa. Many of 

 these form groups that are more or less irregular spread- 

 ing as films over solid surfaces, combining into sea-weed- 

 like fronds, budding out from creeping stolons, or growing 

 up into tree-shaped societies; and besides aggregating 



* Should it be proved that the Ascidian is a degraded vertebrate, then the 

 argument will be strengthened; since loss of bilateral symmetry has gone 

 along with change to asymmetrical conditions. 



