DR, E. B. WILSON ON THE DEVELOPMENT OF RENILLA. 
805 
like the tentacles or mesenterial filaments, which could be of no use as the polyp 
gradually became exclusively adapted to the performance of a single function (taking 
in or discharging water). There are some structural details in the rudimentary zooid 
which seem scarcely explicable if not due to direct inheritance from a fully developed 
polyp. Such characters are the absence of a calyx-tooth from the ventral chamber 
and the presence of two long calyx-teeth on the ventro-lateral chambers. In some of 
the Pennatulids, according to Kolliker, the zooids even possess a pair of mesenterial 
filaments on the two dorsal septa, and the presence of such rudimentary organs in the 
zooids would seem to be a strong indication of their descent by degeneration from 
sexual polyps. 
A moment’s consideration shows however an insurmountable difficulty in the way 
of this view. The zooids are far too numerous to have ever been represented by full- 
sized polyps, for there would not have been room for them on the colony. The dorsal 
zooids on a single polyp number from 20 to 70 or more in different species of Renilla , 
and it is obvious that even a far smaller number of full-sized polyps could not possibly 
have stood upon the dorsal side of a single individual. The same difficulty exists 
in many other Pennatulids, as in Veretillum or in some species of Pennatula ( e.g ., 
P. rubra), where almost the entire ventral surface is covered with closely set zooids. 
Hence the sexual polyps and the zooids cannot be regarded as equivalent members 
of the community, for they are not divergent modifications of identical ancestral forms. 
The zooids are new formations, acquired after the rest of the colony was established. 
In this case the question as to the “ individuality ” of the zooid is merely a verbal 
one; for if descent be made the criterion we cannot consider them such, and yet they 
are absolutely indistinguishable from young polyps. The interesting point is that 
buds may appear in a colony which never attain full development as ordinary indi¬ 
viduals but are arrested at an early stage, before they have acquired all of their 
organs, and made to play a part in the physiological division of labour. If poly¬ 
morphism thus produced may occur in the Pennatulid community, there is no reason 
why it may not occur in the Siphonophora, and it is possible that some of the mem¬ 
bers of the latter organism may have had such an origin. These members may be 
called “individuals” or “organs which simulate individuals,” according to our fancy, 
the distinction being merely verbal. 
Such a view would perhaps partially reconcile the conflicting views respecting the 
nature of Siphonophora referred to at p. 804. It is admitted by the advocates of the 
polymorphism theory that some of the structures of the Siphonophora—as, for instance, 
the tentacles—are not to be regarded as modified individuals (“Persons ” of Haeckel) 
but are simply organs belonging to the true individuals, though they cannot be 
distinguished from the latter by their ontogenetic development. It is not improbable 
that other members of the organism—for instance, the hydrophyllia or pneumatocysts— 
may have the significance of imperfectly developed buds which owe their origin not 
to degeneration from more highly organised individuals but to arrest of development 
MDCCOLXXXIII. 5 L 
