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4 
ON FRESHWATER POLYZOA. 317 
of the endocyst near the top of the cell, two in front and two behind the 
polypide, and are inserted into the opposed surface of the tentacular sheath. 
Their wse is to check the complete evagination of the sheath in the way we 
shall presently see. 
(4.) Vaginal Sphincter.—This was also pointed out for the first time by 
Mr. Hancock. It consists of a set of fibres which run transversely round 
the invaginated tunic. I have not succeeded in dividing it into an inferior 
and superior set, as described by Mr. Hancock. Its wse is to close the inva- 
ginated endocyst after the retraction of the polypide. 
(5.) The Parietal Muscles—'Vhese are numerous, short but strong, and 
very evident fibres, which run transversely in the endocyst in small groups of 
two or three fibres each, embracing about a third or fourth of the cireum- 
ference of the cell. Their use is to compress the endocyst, and by thus di- 
minishing the cavity of the cell, effect the exsertion of the polypide. 
The description now given of the muscular system in the freshwater Po- 
lyzoa, will enable us to understand the mechanism by which the protrusion 
and retraction of the polypide are effected. 
The grand agency to which we must assign the protrusive act, is without 
doubt the contraction of the endocyst effected in Paludicella by the well-de-. 
veloped parietal muscles, and in the other freshwater genera by the action of 
the corresponding delicate fibres already alluded to, or by the general con- 
tractility of the tunic itself; and indeed it does not seem possible to refer 
the act of protrusion to any other cause than the consequent pressure of the 
perigastric fluid against the body of the polypide, and the necessary compul- 
sion of the latter to move in the direction of least resistance, or through the 
orifice of the cell; for the mere straightening of the cesophagus, to which 
Dr. A. Farre* attributes so large a share in the production of this act among 
the marine Polyzoa, can at most raise the lophophore and tentacula a very 
short distance, and can exercise no exsertile influence on the inferior portion 
of the polypide, which, indeed, it must rather tend to repel into the bottom 
of the cell; while in all the freshwater genera, with the exception of Paludi- 
cella, the cesophagus, in the retracted state of the polypide, is scarcely at all 
bent, so that here its agency in exsertion is at once out of the question. 
Let us now suppose the polypide withdrawn into the recesses of the cell, 
and that hunger or some other stimulus impresses on it a desire of protru- 
sion. The endocyst now contracts on the perigastric fluid, which, pressing 
on the polypide, forces it onwards towards the aperture; at the same time 
the vaginal sphincter relaxing, affords to the cone of tentacula a free passage 
through the tube of the inverted endocyst. 
The succeeding steps in the process take place somewhat differently in 
the two great groups. In Plumatella and the other species with bilateral 
lophophores, as the polypide continues to advance from the cell, the invagi- 
nated endoeyst is gradually carried out with it by a process of evagination, 
which proceeds up to a certain point, where it is stopped by the action of the 
inferior parieto-vaginal muscles, which, by straining upon the invaginated 
membrane, had already afforded a fixed line, on which it rolled outwards du- 
ring eversion. This line constitutes the extreme limit of eversioa, and that 
portion of the invaginated endocyst which lies between it.and the mouth of 
the cell remains permanently invaginated. In Paludicella tne process is some- 
what more complicated ; here the relaxation of the upper fibres of the supe- 
rior parieto-vaginal muscles permits the eversion of the endocyst, but only 
* Observations on the Minute Structure of some of the higher forms of Polypi. Philo- 
sophical Transactions, 1837. 
