440 
POPULAR SCIENCE REVIEW. 
The entire plume of the tentacula is surrounded at its base 
by an exceedingly delicate, transparent membrane in the 
form of a cup, adherent to their backs, arranged in such a 
manner as to present a scalloped appearance ; this is peculiar 
to the fresh- water genera alone, it not having been sufficiently 
clearly demonstrated to exist in the marine species. A free 
communication exists between the perigastric space and the 
interior of the tentacula and lophopore, which are filled with 
a clear fluid, containing numerous spherical bodies of various 
sizes, that are rapidly whirled about under the influence of 
the currents. Their presence in the perigastric fluid, which 
consists mainly of water that has obtained entrance from 
without, is supposed to be purely parasitical. The circulation 
through the perigastric space is the only representative in the 
polyzoa of the circulation in the higher animals ; for they 
possess neither a heart nor a vascular system. This circula- 
tion, therefore, performs the threefold function of respiration 
and the distribution of the blood and chyle. True sexual 
organs have been satisfactorily made out in some genera of 
the fresh-water polyzoa, and are doubtless present in all ; the 
minute details of the anatomy of these organs, however, are so 
difficult and technical, that we should weary our readers were 
we to enter upon them here ; we shall therefore content our- 
selves with stating the fact of their existence. The mode 
of reproduction in these animals is threefold — by ova , a 
true sexual reproduction; by gemmce, proceeding at once 
to their full development; and by a peculiar form of buds, 
called statoblasts, in which development is for a time latent, 
and which have always been erroneously regarded as the eggs 
of a polyzoon, but they must on no account be confounded with 
the genuine ova ; these two latter modes may be classed 
under the head of non-sexual reproduction. The first two 
modes of reproduction, by ova and gemmae, having only been 
accurately observed in two of the genera ; and as that by stato- 
blast is the most frequently met with and the most generally 
noticed, and consequently likely to be of the greater interest 
to our readers, we have rather preferred describing it in 
detail to dwelling slightly on the whole of them. 
The statoblasts differ in form in the various genera in 
Lophopus. They are elliptical, with a short acute point at 
each extremity of their long diameter, but usually they present 
the appearance of lenticular bodies, varying from an orbicular 
to an elongated oval figure, enclosed between two concave 
discs of a somewhat tough consistency, united by their mar- 
gins, round which a ring, differing in structure from that of 
the discs, runs, thus serving to strengthen it. In two of the 
genera the statoblast when mature is furnished wdth hooked 
spines, but in Lophopus and the rest of the genera these 
