698 
MR. S. J. HICKSON ON THE CILIATED GROOVE 
tion require a stronger propulsive power than would be supplied by such a siphonoglyphe, 
certain of the polyps are arrested in their development in order that they may supply 
that additional power, and the colony becomes dimorphic. 
The dimorphic Alcyonaria invariably present a considerable amount of fleshy 
sarcosoma, or else large spaces in which a circulation of sea-water is maintained, and 
it is usually the case that the circulation is entirely maintained by individuals which 
have become specially modified for that purpose—the siphonozooids. 
Following this line of reasoning it is not difficult to understand the absence of the 
siphonoglyphe in the Gorgonidse. In these animals there is always present a hard 
axis which may be either horny or else horny and calcareous. This axis frequently 
occupies the greater bulk of the colony (e.g., Primnoa), so that the sarcosoma remains 
as a thin bark covering it. The result of this arrangement is that the canal system 
does not traverse so large an area as it does in such forms as Paragorgia, &c., in which 
there is no solid axis to the colony. The less the extent of the area supplied with 
canals the less the need of a strong propulsive arrangement, and consequently the 
siphonoglyphe is proportionately useless and disappears, the diminished circulation 
being carried on by the ordinary cilia of the stomodseum. 
In the genus Codogorgia we have an example of an Alcyonarian which resembles the 
Gorgonidse in many respects, but differs from them in the important fact that a single 
large canal occupies the position of the solid axis of the other forms. We must 
suppose that there is a constant circulation going on in this axial canal as well as in 
the ordinary canals of the colony, and corresponding with this we find a well-marked 
siphonoglyphe in the polyps for carrying on this more extensive circulation. 
In the genus Solenogorgia described by Genth (1) there is a somewhat similar 
condition, large canals being present in the axis of the colony and again in Solenocaulon 
described by Gray (4). A re-examination of these genera would probably reveal the 
fact that their polyps possess a well-marked siphonoglyphe such as we find in the genus 
Codogorgia. • 
The genus Heliopora presents us with a condition which is not so easy to 
understand. The sarcosoma of the other forms of Alcyonaria is here represented 
only by a delicate layer of tissue covering the skeleton, but at the same time, owing 
to the large amount of space occupied by the coenenchymal tubes, there must be a con¬ 
siderable amount of fluid constantly circulating throughout the colony. It is, therefore, 
somewhat surprising to find but a feebly-developed siphonoglyphe in the stomodseum 
of the polyps. It may be, however, as Professor Moseley has suggested to me, that a 
rapid circulation would be of no particular advantage to a colony which possesses but 
a small amount of living sarcosoma, or indeed by hurrying away the food particles, it 
might be positively disadvantageous to it. This may possibly 7 account for the feeble 
siphonoglyphe. 
There is however a considerable difficulty in accounting for the presence of a sipho- 
