804 
DR. E. B. WILSON ON THE DEVELOPMENT OF RENILLA. 
and their environments would throw further light upon the influence exercised by the 
environment upon the mode of budding and thus upon the symmetry of the colony. 
§ 21. Polymorphism of Ilenilla. 
Polymorphism has been definitely recognised as existing in the Pennatulacea since 
the publication of Kolliker’s great work so often cited in the foregoing pages, but the 
existence of “ rudimentary individuals ” was observed in Renilla by Yerrill many 
years earlier. 
We may distinguish in Renilla at least four kinds of individuals, viz. : a, the 
axial polyp ; b, the secondary sexual polyps ; c, the exhalent zooid ; and d, the inhalent 
zooids. Possibly two classes of the latter should be recognised, viz. : zooids which 
possess a pair of calyx-teeth and those which are devoid of these structures. 
The question now arises whether these various forms of individuals are to be 
regarded as morphologically equivalent—that is, whether all are to be considered as 
the direct descendants of originally similar individuals which have become modified in 
various directions for the physiological division of labour. There can be no doubt 
concerning the nature of the secondary sexual polyps, for these are identical in all 
essentials with the axial polyp. With the various forms of zooids, however, the case 
is different; for we have here to consider whether these are the aborted and 
rudimentary descendants of sexual polyps or are new formations which have never 
had a more highly organised structure than at present. To put the question in a 
concrete form we may inquire : Did the zooids during their past history ever possess 
tentacles, mesenterial filaments, and reproductive organs which were gradually lost as 
the polyps became specialised for the performance of a single function only, or had the 
zooids, when first developed in the colony, the same imperfect polypoid structure as at 
present ? 
The problem is the same as that presented by the Siphonophora, and in the latter 
case has given rise to the two totally different views with which everyone is familiar. 
On the one hand Leuckart, Yogt, Haeckel, Claus, and others regard the various 
parts of the Siphonophora ( Nectocalyces , Polypites, Hydrophyllia, &c.) as the variously 
modified direct descendants of individuals which were once fully developed, though 
organically connected together. On the other hand we have the view especially 
urged by Huxley and Metschnikoff, that these parts are only organs which never 
existed as fully formed individuals. 
At first thought it would appear tolerably clear that the zooids of Renilla must 
have acquired their present structure simply through having degenerated from 
individuals resembling the sexual polyps. They agree closely with the latter m 
general structure, the differences consisting for the most part in the absence of organs 
* “ Revision of the Polyps of the Eastern Coast of the United States,” Bull. Mus. Comp, Zool., 
Cambridge: 1864. 
