DR. E. B. WILSON ON THE DEVELOPMENT OF RENILLA. 18 
ectoderm cells at the aboral end by which the larva attaches itself retain a high 
columnar form and very granular structure, and secrete a yellowish cement by which 
the young polyp is firmly attached. The entoderm is also much thickened at this 
point. This thickening of the layers is shown at a, in figs. 115 to 117. The cement 
substance is undoubtedly to be regarded as the first rudiment of the axis, which is 
therefore an ectodermic product. It is probable that budding takes place from the 
basal part of the young polyp so that the colony has at first the form of a flattened 
plate or encrustation covering the object to which it is attached by the cement secreted 
by the bases of the polyps. This may be inferred from the structure of the adult 
colony, but I did not succeed in observing the budding in young stages although the 
young polyps were kept for seven weeks in the aquarium. The individual shown in 
figs. 115 to 117 did not attach itself, and the thickening of the layers at the base was 
much greater than in those which became attached. Upon making a longitudinal 
section through this specimen, when seven weeks old, the dark mass, a, was found to 
contain a solid yellow horny mass composed of a substance quite like the cement by 
which other individuals were attached. The basal part of the wall of the body seems 
to have been invaginated and the cement then secreted in the cavity thus formed. 
The polyp therefore appeared to have an internal axis, but this must be regarded as 
an unusual condition which probably occurs only when the larva fails to attach itself. 
The tentacles, in both genera, appear soon after the attachment of the larva as 
conical outgrowths from the anterior ends of the radial chambers (figs. 115 and 176). 
I have not observed the least difference in the time of their appearance though I have 
observed them in every stage of development and in many different individuals. In 
this respect, as in the formation of the septa, the development of the Alcyonaria is 
more abbreviated than that of the Zoantharia; for in many representatives of the 
latter group the tentacles, like the septa, develop in regular sequence. 
The tentacles are at first quite simple, with no indication of pinne. The latter soon 
make their appearance along the sides of the tentacles, a new pair being formed, 
roughly speaking, every day. The new pinnee are formed near the base of the tentacle 
and are carried outwards by the longitudinal growth of the latter. The formation of 
pinnee ceases after about ten to twelve pairs have appeared and the growth of the 
tentacles is arrested. The pinnz are somewhat irregularly disposed and the paired 
arrangement often disappears towards the tip of the tentacle. Those in the middle of 
the tentacle are always longer than the basal or apical ones. The pinnz are formed 
as simple diverticula from the tentacle and consist accordingly of a layer of ectoderm 
and entoderm separated by the lamella and enclosing a prolongation from the cavity of 
the tentacle. The tips of the tentacles and pinne are often slightly swollen from 
the accumulation of minute thread-cells at these points. 
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