802 DR. E. B. WILSON ON THE DEVELOPMENT OF RENILLA. 
In the community produced by the asexual multiplication of the individual, the 
bilateral symmetry is very nearly perfect. Such departures from perfect symmetry as 
do exist are inconstant, and appear to be due simply to slight inequalities of growth pro- 
duced by varying conditions of nutrition. In the adult colony a middle plane is clearly 
marked by the form of the disc, position and internal structure of the axial polyp, 
position of the exhalent zooid and of the “ keel,” and the insertion of the peduncle. 
On either side of this axis the polyps and zooids are disposed with great regularity. 
Each sexual polyp has its exact counterpart on the opposite side of the disc, the axis 
of the two polyps making the same angle with the long axis of the colony. The 
groups of zooids also correspond pretty closely on the two sides, though less perfectly 
than the polyps. : 
The budding of the colony is at first strictly bilateral with surprisingly small 
variation ; and this is true both of the polyps and of the zooids. In later stages the 
polyps assume a radiating arrangement, as may be seen in figs. 187 and 189, and a radial 
symmetry is therefore feebly indicated in the disc. This is however due simply to the 
cessation of growth in the long axis—z.e., the axial polyp—and stands in no relation 
whatever to the radial symmetry of the individual. In this case we have a slightly 
marked secondary radial symmetry superimposed upon a primary bilateral form ; and 
in this respect the symmetry of *the colony exactly reverses the symmetry of the 
individual, 
It seems clear therefore that the symmetry of the colony has been acquired inde- 
pendently of the symmetry of the individual, and it will be advantageous to consider 
separately the origin of the symmetry in the two cases. 
If we examine the position of the individual polyps, we observe that they are so 
placed as to have a bilaterally symmetrical environment which corresponds with the 
bilateral arrangement of their parts. Below, they rest upon the sand ; above, they 
are exposed to the water; so that the dorsal and ventral sides are very differently 
conditioned. The lateral conditions are, however, identical, since each polyp is closely 
united with a similar polyp on each side. It is impossible to avoid the conclusion 
that the bilateral environment stands in causal connexion with the bilateral structure, 
and the probabilities seem strongly in favour of the view that the bilateral structure 
—or, at least, some of its features—is a result of the environment. This view is in 
harmony with the prevailing general theories of symmetry which have been especially 
and independently developed by HAECKEL and SPENCER; namely, in SPENCER'S language, 
that the form of symmetry depends ultimately on the nature and distribution of the 
incident forces acting upon the organism. These theories are so familiar as to need 
no review here, and I will only refer to HAECKEL’s views concerning the ancestry of 
the Ccelenterata as developed in the papers upon the “ Gastrza theory” to which 
reference has been made. 
According to HaAxrcKet’s theory the Coelenterate series has been evolved from a 
primitive ancestral “ Protascus,” immediately derived from the Gastraa by the 
