806 DR. E. B. WILSON ON THE DEVELOPMENT OF RENILLA. 
at an early age. The possibility still remains that some other members—for instance, 
the feeding polyps or the nectocalyces—may be the direct descendants of fully- 
developed functional individuals which have become adapted to different functions in 
the physiological division of labour, 
The possibility must be borne in mind that the various members of a compound 
organism are not necessarily of morphological equivalence — which is simply a 
convenient term to denote identity of ancestral origin—and that, according as the 
members are or are not equivalent, different forms of polymorphism are to be 
distinguished. In some cases, as among some Hydroida, the polymorphism seems 
clearly the result of a physiological division of labour among members which were 
originally completely and similarly developed as individuals. Such communities alone 
can be regarded as polymorphic in the sense in which this term was originally applied 
by Levuckart to the Siphonophora. The polymorphism of Renilla and other Penna- 
tulid colonies has probably had in part a different origin and such cases must be 
clearly distinguished from typical polymorphism. For example, in some Pennatulids 
two distinct forms of secondary polyps may be recognised, viz.: feeding polyps 
possessing tentacles and sexual polyps destitute of tentacles. These two forms are 
probably to be regarded as differently modified descendants of sexual polyps like those 
of Renilla, in which the functions of nutrition and reproduction were united. To 
this extent the colony is therefore polymorphic in the ordinary sense. The remaining 
members of the community, viz.: the zooids, have however, probably had a different 
origin, since they are buds which never attain to complete development and never did 
so in the past. 
The zooid is in every respect—physiological as well as anatomical—identical with 
the young bud which is destined ‘to form a sexual polyp. Moreover the zooid may 
in some Pennatulids under some circumstances actually develop into a polyp, as 
KO6LLIKER states, and I have myself observed. The zooid is to be regarded therefore 
as a bud in a state of arrested development, which has however acquired the power 
of asexual multiplication. 
We must therefore consider the difficult question as to the agency which originally 
caused the arrest of development in the buds. How, it may be asked, can in the first 
place a bud have been produced identical in all respects with the buds which are to 
form mature polyps, and yet incapable of full development ? 
It is perhaps impossible to give a complete answer to this question, but the key to 
the solution of the problem lies possibly in the fact that the zooid, although in an 
embryonic state, possesses nevertheless the power of asexual multiplication. As 
pointed out on a preceding page, the secondary zooids of a group are to be regarded 
as offspring of the primary zooid and not directly of the sexual polyp on which they 
are placed. We may therefore explain their rudimentary structure as the result of 
inheritance from the primary zooid, and hence have only to consider how the latter can 
have been produced. 
