792 DR. E. B. WILSON ON THE DEVELOPMENT OF RENILLA. 
The zooid develops in quite the same manner as the young sexual polyp or the 
exhalent zooid, but never progresses beyond the stage in which two calyx-teeth (those 
of the ventro-lateral chambers) are formed. The zooid is therefore structurally as 
well as physiologically (see p. 784) identical with the young sexual polyp. In the fully- 
developed zooid the cesophagus is of an oval form, elongated slightly in the dorso- 
ventral axis, and connecting with the exterior through an oval mouth. The inner 
wall of the cesophagus is covered on its ventral side with powerful cilia, by the action 
of which water may be drawn in from the exterior in precisely the same manner as 
by the young sexual polyps. The mouth is furnished with a sphincter muscle by 
which it may be tightly closed when the cavities of the colony are sufticiently dis- 
tended with water. As already described, the sexual buds, as they increase in size, 
gradually cease to perform the function of drawing in water. The zooids, however, 
retain this function permanently and have been specialised for this purpose alone 
since they have neither tentacles, mesenterial filaments, nor reproductive organs. 
The sexual buds hand over their early function, as it were, to the zoids as they 
become themselves adapted to play another part in the economy of the organism. 
As shown in fig. 202 the two calyx-teeth of the zooids become greatly elongated 
and in some specimens, especially in the posterior parts of the disc, may attain an 
enormous development. Their walls are soft and flexible and are considerably 
thickened towards the tips where the cells assume a columnar form. It seems very 
probable that they may perform tactile functions, but I have been unable to demonstrate 
this in living specimens. 
The chambers have the usual arrangement, there being always a somewhat narrow 
ventral chamber enclosed between two wide ventro-lateral ones. The five upper 
chambers are always smaller than the ventro-lateral ones and are nearly equal in size. 
It is therefore always easy to distinguish the dorso-ventral axis of the zooid, which for 
the sake of brevity I shall call simply the azis. 
The zooids are produced in pairs like the sexual polyps, though with less regularity. 
The first pair (fig. 185,21.) make their appearance on the dorsal side of the axial polyp 
near the bases of the first pair of sexual polyps, at the time when four or five pairs of 
sexual polyps have appeared. Behind these there appear two or three pairs of zooids 
somewhat irregularly placed on the upper side of the axial polyp. They are arranged 
(cf. figs. 188, 189) on either side of a longitudinal space which remains permanently 
free from zooids and is very conspicuous in the fully-formed colony. KOLLIKER has 
termed this area the keel (Kiel), and it is of common occurrence among the Pennatulida. 
In the adult colony it extends forwards from the posterior sinus about half-way across 
the disc. The exhalent zooid is placed at its anterior end and groups of inhalent 
zooids border it on either side. The axes of these zooids are very irregularly placed, — 
but as a rule the ventral side of the zooid is turned towards the posterior part of 
the colony. 
The remaining zooids appear on the dorsal side and in the median line of the sexual 
