420 DR. A. FARRE ON THE STRUCTURE OF THE CILIOBRACHIATE POLYPI. 
lation of fluids may be observed passing up from the chamber of the body and re- 
turning to it again. They appear to answer the double purpose of prehensile tenta- 
cula and of respiratory tubes. They are not ciliated externally*, and in form have 
little resemblance to the tentacles of the other two classes. In this respect, however, 
they vary very much, being, in some forms of Actinia , arranged simply in one or 
more circles round the mouth ; in others, elevated upon semilunar lobes ; whilst, in 
other instances, as in Xenia, Gergonia, &c., these lobes may be supposed to be drawn 
out into the conical or cylindrical arms, having a dentated margin ; in which cases 
the whole arm does not correspond with a single arm of Actinia, but each of the den- 
tiform processes upon its sides. The character of distinct ovaria producing ciliated 
gemmules appears to be very prevalent through this class. 
The Anthozoa, then, are distinguished from Nudibrachiata chiefly by the separation 
of the stomach from the parietes of the body, which has a membranous character ; 
and from Ciliobrachiata by the single external opening, and the absence of cilia from 
the surface of the arms. They appear to hold a place immediately below Acalepha and 
Echinodermata, the transition between these three classes being exceedingly gradual. 
Thus, with Asterias, the affinities are easily traced. The single external opening to 
the membranous stomach is found equally in Actinia and Asterias ; but while in the 
former this organ communicates posteriorly with the main cavity of the body, in the 
latter it is closed in this position, and the immediate communication cut off. Again, 
in the conical arms of the corticiferous polypes, with their fringe of tubular processes, 
may be traced the analogue of the rays and respiratory tubes of Asterias ; both are 
distended by the fluids which circulate through them, probably for respiratory pur- 
poses. Moreover, in the position and form of the ovary the closest resemblance 
exists ; and when to these points is added the stem of the crinoid animals, the affi- 
nities between the two classes are rendered still more striking. 
Again, between Anthozoa and Acalepha analogous points of resemblance might be 
traced ; and here again the transition appears so gradual that it might be difficult to 
determine where the one ends and the other begins. 
Thus, then, it appears that under the commonly received name of Polypi there 
exist three distinct types of structure, which must be referred to the same number of 
separate classes, possessing but few points in common, and those generally of the 
most superficial kind, but which have nevertheless induced naturalists, from the want 
of a sufficient degree of attention to their intimate structure, to group together, in ac- 
cordance with such superficial resemblance, animals that have no title to be clas- 
sically associated. 
* It might be objected that as the arms of Anthozoa are not ciliated, at least externally, the term “ Nudi- 
brachiata” is equally applicable to this class. The Anthozoa, however, could never be confounded with either 
of the other two classes, to the mutual distinction of which the names that I have applied to them have re- 
ference. 
