176 Mr. A. Hancock on the Anatomy of the 
c',c, uniting the bases of the tentacles. In these respects Frede- 
ricella shows an approximation to those with oral arms, or as it is 
generally termed, a crescentic disc. In these there is always a 
similar membrane, Pl. III. figs. 1, 2 & 3e, f & d, at the base of 
the tentacles, and in all of them, as well as in Fredericella, this 
membrane is attached to the external surface of the tentacles, and 
is much wider at the margin than the spaces between them, and 
consequently it bags out, giving to the upper portion a flounced 
appearance, particularly in the latter, Pl. LI. fig. 1 ce. 
The tentacles themselves in all these genera are rather stout 
and linear with the end obtuse; they have the appearance of 
being tubular, as have likewise the oral arms of Plumatella: the 
tentacles are clothed with long cilia, which vibrate upwards on one 
side and downwards on the other in the same manner as de- 
scribed in the marine species ; and as in them, when the polype 
is retracted the tentacles are drawn down in an erect position, 
having first been brought together into a compact linear bundle, 
Pl. Il. fig. 2 a, and Pl. IV. fig. 2d. They do not appear to be 
at all contractile, and in all the species are transparent and almost 
homogeneous in their structure. There can be little doubt that 
they are not merely tentacles, but that they are likewise respi- 
ratory organs: food is brought to the mouth by their ciliary 
currents, and also by the action of the tentacles themselves, one 
or more of which may frequently be seen bending suddenly in- 
wards, and securing such particles as come within their reach. 
They occasionally act in concert in the capture of animalcules by 
bringing their tips together, thus forming, in those with a cir- 
cular disc, a very elegant oval cage, within which the imprisoned 
prey may be seen for an instant or two dashing about previously 
to passing into the cesophagus or to liberation, which not unfre- 
quently happens, the captive proving distasteful to the polype. 
The tentacles then may be considered prehensile labial or oral 
appendages, notwithstanding their respiratory function, and as 
such they are a portion of the alimentary system. 
The oral orifice of Plumatella is semicircular, Pl. III. fig. 3 a, 
and protected by a strong, rounded, fleshy valve, b, which, ari- 
sing from the side of the mouth at the point on the inner margin 
of the crescent where the two arms unite, projects upwards and 
slightly overhangs the opening. ‘This valve is completely under 
the control of the animal, and can be made to act as a sort of 
operculum, closing the orifice to prevent the admittance of food ; 
or it can be used to force food into the pharynx. The mouth, 
Pl. II. fig. 1 d, of Fredericella is likewise semicircular, and is also 
provided with a similar valve, e. It is immediately behind it that 
the tentacular disc is a little flattened, proving that this point 
corresponds to the space between the arms in Plumatella ; indeed 
