THE GENERAL SHAPES OF ANIMALS. 175 



plants further simulated under a further parallelism of con- 

 ditions. The attached ends differ from the free ends as they 

 do in plants ; and the regular or irregular branches obvious- 

 ly 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- 

 bers, exhibits the same conformity of structure to circum- 

 stances. It admits of symmetrical bisection by a plane pass- 

 ing through its two contractile sacs, or nectocalyces, but not 

 by any other plane ; and the plane which thus symmetrically 

 bisects it-, is the vertical plane on the two sides of which its 

 parts are similarly conditioned as it propels itself through 

 the water. 



Another group of the oceanic Uydrozoa, the PhysophoridcB, 

 furnishes interesting evidence not so much in respect of the 

 forms of the united individuals, which we may pass over, as 

 in respect of the forms of the aggregates. Some of these 

 which are without swimming organs, have their parts sus- 

 pended from air-vessels which habitually float on the surface 

 of the water ; and the distribution of their parts is asyra- 



