ON FRESHWATER POLYZOA. 309 
—— 
_ the polypide. These ribs I look upon as the true homologue of the sete 
which crown the cell in Bowerbankia and other marine Polyzoa; if these 
sete were reduced in number to four, and instead of being free were at- 
tached to the sides of the cell, they would at once be converted into the ribs 
of Paludicella; the fact of their being connected to one another by a delicate 
,membrane does not in the least invalidate the view here taken, and the cir- 
cumstance of their being detached from the sides of the cell will account for 
the different mode in which they are withdrawn during retraction. 
The septa already alluded to as existing between the cells of certain fresh- 
_ water Polyzoa, are formed both by the ectocyst and endocyst. In Paludicella 
they acquire their maximum in development and constancy; they exist here 
; between every cell, and consist of an annular process which projects trans- 
__-versely from the ectocyst into the interior of the cell, with a covering of 
endocyst on its upper and under surface. The septum thus formed is ren- 
dered complete by the aperture in its centre being closed by a peculiar body, 
which projects into the cavity of the cell at each side. The structure of this 
body is remarkable ; it consists of a central mass or nucleus, surrounded by a 
distinct layer of somewhat elongated cellules placed perpendicularly to the 
surface of the nucleus. The body which thus closes up the centre of the 
annular septum has, without doubt, some office to perform besides that of 
simply completing the septum ; but upon the nature of this office I can form 
no satisfactory opinion. In the other genera the septa are by no means 
so constant ur complete as in Paludicella. In several species of Plumatella, 
especially P. coralloides, septa exist, but these generally occur only at inter- 
~ vals, leaving several cells between them, which communicate freely with one 
another; not unfrequently the septum itself is imperfect, admitting of a com- 
munication through its centre between two neighbouring cells. In Aleyonella 
fungosa, and in Fredericella sultana, imperfect septa may here and there be 
observed, while Cristatella and Lophopus would seem to be quite deprived of 
them, the cells in these genera all opening into one another.. 
B. Organs of Digestion —The digestive system is very similar in all those 
species in which the lophophore is bilateral ; these we shall therefore consider 
together; Paludicella, the only representative of the division with orbicular 
lophophore, presents some peculiarities, and should be examined by itself. 
1. Species with bilateral Lophophore.—The mouth is a simple edentulous 
orifice of a circular or slightly crescentic form, placed in the centre of the 
body of the lophophore, and consequently occupying the bottom of the ten- 
tacular crater. Its margin is slightly elevated, and is continuous posteriorly, 
with a hollow valve-like organ of very: peculiar formation. This organ arches 
over the mouth, and may be aptly enough compared in shape to the epiglottis 
of certain mammifers. The cavity in its interior communicates through an 
opening in the lophophore with the perivisceral space ; its anterior walls are 
thick, and densely clothed on their external surface with vibratile cilia, while 
the posterior walls are thin, membranous, and transparent, and destitute of 
cilia. It may be observed, when the polypide is exserted from its cell, to be 
in a constant motion, which consists in an alternate elevation and depression 
of the organ. The elevation is effected by distinct muscular fibres, which are 
visible through the transparent posterior walls, and will be afterwards more 
particularly described, while the depression is probably the result simply of 
an antagonistic elasticity. On the true function and import of this curious 
organ I am unable to throw any light; though it is here described in con- 
nection with the organs of digestion, its relation to the digestive system is 
| perhaps very remote. It may possibly be more correctly viewed as connected 
with sensation. 
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