DR. E. B. WILSON ON THE DEVELOPMENT OF RENILLA. 
791 
filamente frtiher als die anderen auf, allein diese stehen dicht beisammen, und bilden 
sich nach allem, was wir wissen, alle acht Septa auf einmal.” 
Nevertheless, in Renilla at least, the exact reverse of what is indicated by the 
passages cited is actually the case; and the presumption is that the same holds true 
in Sarcophyton, Halisceptrum and other Alcyonaria. 
The facts presented by the bud-development of Renilla tend to show that a definite 
sequence in the appearance of symmetrically repeated parts may very readily be 
acquired or modified through the action of secondary causes which are, however, for 
the most part too obscure to be recognised. 
§ 17. Development of the zooids. 
a. The exhalent zooid. 
The exhalent zooid {ex. in all the figures) makes its appearance some time after the 
appearance of the first pair of secondary polyps and always before the second pair 
are developed. It occupies always the same position, viz. : on the median line of the 
dorsal compartment a short distance in front of the pair of buds (fig. 181). Its early 
development is in all respects identical with that of the sexual polyps, and when the 
septa are well established they are found to have the same arrangement as in the 
latter. The ventral chamber is very narrow and remains always without a calyx-tooth. 
As the zooid increases in size a short calyx-tooth appears on each of the other chambers, 
and these are developed simultaneously so far as observed (figs. 188, 189). The 
zooid is in this stage closely similar to the mouth of the cell of a sexual polyp when 
the latter is contracted {cf. fig. 189). The zooid remains in essentially the same 
condition during its whole existence, but the calyx-teeth become much more elongated 
and the ventral chamber becomes so small as almost to disappear. No tentacles are 
ever developed and I have never observed the least rudiment of them. 
We find that the dorso-ventral axis of the zooid, which may at once be determined 
by the elongation of the mouth and the position of the ventral compartment, coincides 
with the long axis of the primary polyp ; and furthermore that the ventral side is 
turned towards the posterior part of the latter. This relation of the axis is constant, 
though the axis of the zooid sometimes forms a small angle with the long axis of 
axial polyp. It sometimes happens that two exhalent zooids are formed. In this 
case one of them is usually placed in the normal position and the other lies at one side 
with its axis more or less oblique. In one case the zooid was devoid of a mouth¬ 
opening. 
b. The inhalent zooids. 
An especial interest attaches to the development of these zooids on account of the 
curious and constant relations existing between their axis and between these and the 
axis of the sexual polyps. 
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