DR. E. B. WILSON ON THE DEVELOPMENT OF RENILLA. 
803 
attachment of the latter at the base a.ncl the gradual acquisition of a radiate structure 
as a result of the equality in all directions of the lateral conditions. This theory 
leaves unexplained the bilateral symmetry which appears in a greater or less degree in 
all polyps, and Haacke (“ Blastologie der Korallen,” l.c.) has endeavoured to explain 
this as the result solely of the formation of colonies. This author holds that in solitary 
forms like the Actiniae the bilateral symmetry is due to descent from colony-building 
species, and he believes that the paired development of the septa is thus to be 
explained, though in precisely what manner he is unable to say. 
Without accepting in all details Haacke’s views, which are only a special applica¬ 
tion of the environment theory of Haeckel and Spencer, it appears to me highly 
probable that the nature of the environment of the individual polyps in the colony 
will satisfactorily explain their bilateral structure. It is, of course, impossible to 
explain exactly how the bilaterality of the various organs is related to the bilaterality 
of the environment on account of our imperfect knowledge of the functions of these 
organs and of the laws of growth. But we cannot admit that the perfect corres¬ 
pondence between structure and environment is due to mere accident, and the only 
alternative is to regard it as the result of adaptation in the organism. 
An obvious objection to this view is that it may be putting the cart before the 
horse; for there may be laws of nutrition or of growth, dependent upon a bilateral 
structure already existing, which limit the production of buds to the sides of the 
axial polyp. But we have seen that the zooids—which are undeveloped buds—are 
produced in the dorsal sides of the polyps in Renilla, and Kolliker has shown that 
in other Pennatulids the zooids may appear anywhere upon the polyps. All parts of 
the sexual polyps therefore possess equally the power of producing buds, and hence 
the circumstance that each polyp is laterally united with two other individuals depends 
on the general form of the colony and not upon any limitation existing in the laws of 
growth in the individual. 
Passing now to a consideration of the symmetry of the colony, we find here the 
same general conditions as in the individual. The young colony, as we have seen, 
(p. 785) assumes a definite position as soon as it begins its sedentary life, and this 
position is maintained during the entire existence of the organism. In this habitual 
position of the colony, with the peduncle rooted in the sand and the disc expanded 
upon the surface, the dorsal and ventral sides are quite differently conditioned, while 
the sides are similarly conditioned. The conditions of nutrition within the colony 
being equally distributed, the rate of growth must tend to be equal upon the two 
sides, and any modifying agency must, so far as we can see, tend to be equal upon both. 
There seems to be no reason to doubt that such an equality of lateral conditions, if 
maintained for a long period of time, would ultimately produce as perfect a bilaterality 
as that of Renilla. It is unfortunate that so little is known of the habits of other 
Pennatulids in which the bilateral symmetry is less marked than in Renilla. It is 
probable that a study of these forms with reference to the relation between them 
