794 
DR. E. B. WILSON ON THE DEVELOPMENT OF RENILLA. 
rudimentary structure and great numbers, but the variation affects only the number 
and arrangement of the zooids, leaving the relations between their axes unaltered. 
Figs. 194 to 203 illustrate the multiplication from a simple zooid (fig. 194) to a 
group of eighteen. The figures, it will be understood, are not drawn from different 
stages of an individual group but represent a number of different groups in various 
stages of development. 
In what may be regarded as the typical case a group of four zooids is first formed 
(fig. 199). The upper one ( d .), situated at the dorsal side of the primary zooid (p.), 
is usually first to appear (figs. 195-197,) but the lateral zooids (II.) may appear, singly 
or together, before the upper one (see fig. 198). In the group of four the primary 
zooid is distinguished by its greater size and by the possession of calyx-teeth on the 
ventro-lateral chambers. The ventral chambers of the zooids (v.) are turned away 
from the centre of the group. Thus the axes of the lateral zooids form an angle of 
90° with that of the primary zooid, and the axis of the upper zooid is 180° from that 
of the primary zooid. 
New zooids now make their appearance in irregular succession in the angle 
between the four already formed (figs. 200-202) so that the group then consists 
typically of eight zooids. The same relation of the axes holds good for the new zooids 
— i.e., the ventral chamber is turned outwards, or away from the centre of the group. 
The superiority in size of the primary zooid is still marked and its calyx-teeth are 
very well developed. In most, though not in all, cases the upper zooid also acquires 
a pair of calyx-teeth as shown in the figures, and sometimes one of the lateral zooids 
also (fig. 202, l.). Most of the secondary zooids remain however without calyx-teeth, 
though the ventro-lateral chambers are always larger than the others. The ventral 
chamber is always very narrow in the zooids which have calyx-teeth, but in the other 
zooids it is often scarcely narrower than the ventro-lateral chambers. The axis of the 
zooid can however be always recognised by the elongation of the mouth and the 
crowding of the six upper septa. 
Many of the subsequently formed zooids develop in the same manner as those 
already described, appearing in the angles between pre-existing zooids and having then 
ventral chambers turned away from the centre of the entire group. In some cases, 
however, the secondary zooids become in their turn centres of multiplication, thus 
forming minor groups which repeat, more or less completely, the formation of the 
primary group. This is shown in fig. 203. The primary zooid of the system is 
marked p„ and above it lies the dorsal secondary zooid ( d .) corresponding with the upper 
zooid in fig. 199. A considerable number of the lateral zooids are simple and their 
axes are related to that of the primary zooid (as may be seen from the position of 
their ventral chambers v.v.). At x., however, is a group of four zooids which are 
arranged about a centre of their own and form a secondary group quite similar to the 
primary group shown in fig. 199. The principal zooid (p 11 .) of this secondary group has 
the usual position with respect to the primary group and appears to correspond with 
