IN THE ST0M0D2EUM OF THE ALCYONARIANS. 
701 
the colonies. In another direction arose the ancient group of which Heliopora is a 
survival. This group, which was formerly placed amongst the Tcibulata, was probably 
very rich in genera and species in palaeozoic times, but it is gradually becoming extinct. 
In another direction arose the modern genus Alcyonium and its numerous allies. 
This genus differs from the ancestral form x chiefly in the fact that the polyps are 
capable of being retracted within the sarcosoma, but in other respects is probably 
more closely related to it than any other genera. The fact that the polyps are 
capable of being retracted is not one of very great importance, for we find both in 
this family and in the Gorgonidee, that nearly allied genera differ from one another 
in this respect. The dimorphic genus, Sarcophyton, is probably closely related to 
Alcyonium. The presence of dimorphism is not sufficient to warrant the supposition 
that they sprang from different stocks, for this condition occurs in so many widely 
different genera that it is reasonable to suppose that it was introduced more than 
once in the course of the evolution of the group. 
As an example of this we find that the genus Xenia, which probably followed 
another line of evolution from the hypothetical ancestor, is not dimorphic, whereas the 
genus Heteroxenia, very similar to it in other respects, is dimorphic. 
The lines which evolution took in producing the large number of genera of 
Primnocicece, Gorgonacece, &c., are much more difficult to make out, but the following 
represents perhaps as near an approximate to them as our present knowledge permits. 
Taking Siphonogorgia, as a form intermediate between the ancestral type and the 
true Gorgonidse, we find that the chief diversion lies in the fact that the colony has 
assumed an arborescent shape, and a support for it is produced by a more copious 
development of spicules in the axial portions of the colony. The body-cavities of the 
polyps, however, remain long, as they were in the ancestral form. Most probably 
there was another stage between Siphonogorgia and this ancestral form which was 
not dimorphic. From Siphonogorgia, Paragorgia differs chiefly in the fact that the 
body-cavities of the polyps have become reduced in length, and a complicated system 
of canals occupies the position which they formerly occupied. If Corallium is 
dimorphic, as Ridley (19) and Moseley (16) consider it to be, it was derived from 
an ancestor similar to Paragorgia in which, by a fusion of the spicules, a solid rod 
occupies the axis of the colony. The rare genus Pleurocorallium differs from 
Corallium in the fact that the polyps are not retracted into the coenenchym, but 
this condition may be simply due to a more copious development of spicules in the 
walls of the polyps, thereby offering a physical difficulty to the retraction of the polyps. 
Passing by the form which I suppose at one time existed similar to Siphonogorgia 
but not dimorphic, we should obtain such forms as Briareus, in which the body- 
cavities of the polyps are short; there is no solid axis and no dimorphism, and from 
such genera, Ccelogorgia, Solenogorgia, &c., might be obtained by the development of 
large canals in the axis of the colony, or again by the development of horny or cal- 
, careous axes, we should obtain the remaining families of Gorgonacae, Primnoaceae, &c. 
MDCCCLXXXIII. 4 X 
