THE GENERAL SHAPES OF ANIMALS. 



189 



comes uppermost in its turn, no part is permanently affected 

 in a different way from the rest. Hence the radial form con- 

 tinues. 



In others of this same group, however, there occur forms 

 which show us an incipient bilateralness ; and help us to see 

 how a more decided bilateralness may arise. Sundry of the 

 Medusidce are proliferous, giving origin to gemma? from the 

 body of the central polypite or from certain points on the 

 edge of the disc; and this budding, unless it occurs equally 

 on all sides, which it does not and is unlikely to do, must 

 tend to destroy the balance of the disc, and to make its 

 attitude less changeable. In other cases the growth of a 

 large process [a much-developed tentacle] from the edge of 

 the disc on one side, as in Steenstrupia, Fig. 257, constitutes 

 a similar modification, and a cause of further modification. 

 The animal is no longer divisible into any two quite similar 

 halves, except those formed by a plane passing through the 

 process ; and unless the process is of the same specific gravity 

 as the disc, it must tend towards either the lowest or the 

 highest point, and must so serve to increase the bilateralness, 

 by keeping the two sides of the disc similarly conditioned 

 while the top and bottom are differently conditioned. Fig. 258 

 represents the underside of another Medusa, in which a more 

 decided bilateralness is produced by the presence of two such 

 processes. Among the 



simple free-swimming Acti- 

 nozoa, occur like deviations 

 from radial symmetry, along 

 with like motions through the 

 water in bilateral attitudes. 

 Of this a Cydippe is a familiar _ 

 example. Though radial in 

 some of its characters, as in 

 the distribution of its meridi- 

 onal bands of locomotive paddles with their accompanying 

 canals, this creature has a two-sided distribution of tentacles 



