ON FRESHWATER POLYZOA. 313 
the tentacle. In Cristatella, a minute closed cavity, distinct from the rest of 
the lobe, may be very distinctly seen in the extremity of each tentacle ; it 
would also seem to exist in other genera, but is less easily demonstrated in 
these. 
In all the freshwater genera, with the exception of Paludicella, the entire 
plume of tentacula is surrounded at its base by an exceedingly delicate trans- 
parent membrane in the form of a cup or calyx. This cup is adherent to the 
back of the tentacula, and its margin is in most instances prolonged more or 
less upon each tentacle, as a narrow triangular process, so as to present a sort 
of scalloped or festooned appearance; the festooning of the margin is most 
marked in F’redericella ; in some species of Plumatella it is scarcely percep- 
tible. A high power of the microscope, and carefully adjusted illumination, 
will enable us to detect in the calyciform membrane certain delicate anasto- 
mosing lines, which I at first suspected to indicate the presence of a vascular 
net-work ; further examination, however has caused me to prefer viewing 
them as the lines of contact of delicate cellules of which the membrane is 
eomposed. The appearance in Cristatedla especially confirms the latter sup- 
position. The calyciform membrane has not yet been detected in any 
marine Polyzoon. In the curious marine genus Pedicellina the tentacula are 
indeed surrounded at their base by a kind of membranous calyx, but this is 
of an entirely different import from the membrane connecting the bases of 
the tentacula in the freshwater Polyzoa. 
The perigastric space and interior of the tentacula and lophophore all 
freely communicate with one another, and are filled with a clear fluid, in 
which foat numerous particles of very irregular form and size. In this fluid 
may be observed a constant rotatory motion, rendered apparent by the fleat- 
ing corpuscles as they are whirled away under the influence of the currents. 
That the fluid thus contained in the perigastric space, and thence admitted 
into the tentacula, consists really of water which had obtained entrance 
from without, there can, I think, be little doubt, and yet I have in vain 
sought for any opening through which the external fluid can obtain ad- 
mittance to the interior. I have allowed the transparent genera Cristatella 
and Zophopus to remain many hours in carmine without being able to 
detect a single particle of this pigment in the perigastric space, though I 
have seen this space rapidly empty itself on the removal of the animal from 
the water, and again fill on restoring it to its natural element. Van Beneden* 
believed that he had detected in Alcyonella apertures, which he names 
“bouches aquiféres,” at the base of the fentacula; but this distinguished 
naturalist is certainly here in error: I shall presently point out the source 
of his mistake. Meyen asserts the existence of an aperture in the vicinity 
of the anus, through which, he tells us, he bas witnessed the escape of an 
egg in Aleyonellat ; and Siebold admits the correctness of this statement, 
end considers the aperture described by Meyen to be that through which 
the external water gains admittance to the interior{. I have, however, fully 
convinced myself that no such aperture exists, and the phenomenon de- 
scribed by Meyen must certainly be due to an accidental rupture of the 
tissues, though the high authority of Van Beneden describes the passage of 
the eggs through an aperture similarly placed in the marine genus Lagun- 
cula§. It is possible that certain apertures may exist in some of the tissnes 
of the animal so minute as to defy our attempts at detection, and yet capable 
* Quelques Observations sur les Polypes d’eau douce, Bull. de l’Acad. Roy. de Bruxelles, 
839 
{ Meyen, Naturgeschichte der Polypen, Iris, 1828. 
t Loc. cit. § 41. § Recherches sur l’Org. des Laguncula, Joe. cit. 
