172 



MORPHOLOGICAL DEVELOPMENT 



Iii 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 gemmae 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 

 from the edge of the disc on one side, as in Steenstrupia, Fig 

 257 — a process that is perhaps the morphological equivalent 

 of one of the gemmae just named — constitutes a similar modi- 

 fication, and a cause of further modification. The existence 

 of this process makes the animal no longer divisible into any 

 two quite similar halves, except those formed by a plane 

 passing through the process ; and unless the process is 

 exactly 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, b}^ 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 bi- 

 lateralness is produced by the 

 presence of two such process- 

 es. Among the simple 

 tree-swimming Actinozoa, occur 

 like deviations from radial sym- 

 metry, 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 

 meridional bands of locomotive paddles with their accompany- 

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

 tentacles and various other parts, corresponding with its two- 

 sided attitude in moving through the water. And in other 



