DR. A. FARRE ON THE STRUCTURE OF THE CILIOBRACH1ATE POLYPI. 417 
With a view to a subdivision of this class, after a more natural method than has 
hitherto been followed in arranging the various forms of Polypes, it would be desirable 
to regard the varieties presented by the alimentary canal, which from the conspicuous 
position of the parts are the more easily determined. Thus the presence or absence 
of the manducatory organ are points of much importance ; and the position of the 
anus may be also worthy of consideration. The structure of the alimentary canal, 
however, will probably be found to present but few essential points of difference, and 
it may be necessary soon to revert to the form of the cell as a secondary means of 
distinction. The position of the aperture and the character of the operculum then 
becomes of consequence, especially in its mode of termination, whether by separate 
spines or by a notched or smooth margin. But if it be found that where the aperture 
of the cell is lateral the manducatory organ is sometimes present, and in other cases 
absent, as I have shown it to be when the aperture is terminal, then the character of 
the cell must yield in importance to that of the alimentary canal, and the animals be 
arranged accordingly. With the lateral position of the aperture the operculum is 
generally simple, and the cells have seldom a distinct connecting medium. But 
where the aperture is terminal the operculum is more complicated, and the cells are 
generally united by a ramified or creeping stem ; but the passage from the one form 
of arrangement to the other is shown in Halodactylus , in which the cells, though 
usually placed perpendicularly with their sides in contact, and the aperture terminal, 
are yet sometimes placed so obliquely as to resemble in arrangement an encrusting 
species, having the aperture directed laterally. The structure of the cell, however, 
is not in this case affected by its accidental position. 
It will then be of consequence to determine the degree of importance to be attached 
to those characters which have been erroneously considered primary, namely, those 
that are derived from the common mass, or polypary. These, however, are generally 
the most superficial and least important ; since a very slight alteration in the arrange- 
ment of similar parts will give a very different character to the whole, as exemplified 
in the readiness with which an encrusting species might be converted into a ramified, 
and that again into a foliaceous one. But the mode of growth of the stem might oc- 
casionally afford useful characters for generic distinctions ; thus the definite mode of 
growth in Vesicularia spinosa is contrasted with the irregular arrangement of the 
cells in Bowerbankia and Lagenella ; whilst trivial characters are readily found in 
the number of the arms, and similar points of inferior importance. 
In natural affinities Ciliobrachiata is evidently allied both to Tunicata and Rotifera. 
In Tunicata the tentacles are reduced to mere rudiments at the entrance to the re- 
spiratory sac, and the cilia are distributed over the surface of this cavity, which is in 
proportion magnified, and is analogous to the pharynx of Ciliobrachiata. The more 
immediate entrance to the alimentary canal, thence called mouth, being situated at 
the bottom of this sac, corresponds with the part that I have called cardia ; and the 
analogies between the remaining course of the alimentary canal, position of the ovary, 
