DR. E. B. WILSON ON THE DEVELOPMENT OF RENILLA. 
793 
polyp- ce ll s , and a constant relation exists between the axis of the zooids and of the 
polyps on which they are placed. For the sake of convenience I shall term these 
zooids dorsal zooids to distinguish them from the marginal zooids which border the 
keel. 
Four dorsal zooids, to begin with, make their appearance on the upper side of each 
polyp-cell. They are formed successively, proceeding from the base of the polyp out¬ 
wards towards the oral extremity, as may be seen upon comparison of figs. 187, 188, 
189. In fig. 187 the polyp p 5 . has a single dorsal zooid, and p> 2 . has two. In fig. 189 
the polyp y> 3 . has three zooids and p 1 . has four. 
The bilateral arrangement of the zooids is well shown by fig. 187, in which the 
positions of the zooids are accurately represented. With two exceptions each zooid 
has its counterpart on the opposite half of the colony. The exceptions are the mar¬ 
ginal zooid zm., and the dorsal zooid zd., which appear on the right side only. 
The zooids are sometimes formed on very young sexual buds, as at yd. in fig. 187. 
This recalls the very early appearance of the power of budding in the axial polyp. 
Upon examining the axis of a dorsal zooid we find that in many cases it coincides with 
the long axis of the sexual polyp on which it is seated, and where it does not the axis 
of the zooid forms less than a right angle with that of the polyp. Moreover, the 
ventral chamber of the zooid is always placed at that end of the axis which is turned 
towards the basal part of the polyp and therefore towards the centre of the colony. 
There is a strong tendency in the zooid to assume a position on the secondary polyp 
corresponding with the position of the exhalent zooid with respect to the primary polyp 
(see p. 791); and the variations from this position caused by the greater or less obliquity 
of the axis must be considered as departures from the type. Upon the axial polyp 
only one zooid as a rule, though sometimes two, appears in front of the exhalent zooid. 
Multiplication of the zooids. 
The zooids have thus far been described as if remaining simple, as is really the 
case up to the stage shown in fig. 188. Soon after this, however, the zooids them¬ 
selves become centres of multiplication and each zooid becomes the parent of a whole 
group of secondary zooids. It is therefore necessary to distinguish primary and 
secondary zooids as we have recognised primary and secondary sexual polyps. 
The axis of the primary dorsal zooid, as we have seen, stands in a definite relation 
to that of the sexual polyp. The axes of the secondary zooid, on the contrary, show 
no direct relation to those of the sexual polyp hut to those of the primary zooid. 
Hence we must regard the latter as the real parent of the secondary zooids,though 
these appear to arise as buds on the dorsal side of the polyp-cell and not directly 
upon the primary zooid. We must, at any rate, grant that the primary zooid is a 
centre of force which controls the development of the secondary zooids, and it will be 
convenient for our purpose to consider the latter as the progeny of the former. 
The multiplication of the zooids varies exceedingly, as we might expect from their 
