THE GENERAL SHAPES OF ANIMALS. 



193 



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 Hydrozoa, the Physoplioridce, 

 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 

 are without swimming organs, and have their parts sus- 

 pended from air-vessels which habitually float on the surface 

 of the water. Hence the distribution of their parts is asym- 

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

 relations of the integrated group of 

 individuals to the environment are in- 

 definite; and there is thus no agency 

 tending to change that comparatively 

 irregular mode of growth which is pro- 

 bably derived from a primordial type 

 of the branched Hydrozoa. 



So various are the modes of union 

 among the compound Ccelenterata, 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- 



59 J& 



