Freshwater Bryozoa, with descriptions of new Species. 175 
exserted, there is a delicate membranous cup, Pl. IV. fig. 1 d, 
projecting upwards from the inner surface of the mouth of the 
cell. This cup is the homologue of the circle of setze surrounding 
the aperture of Bowerbankia and other marine genera. In Palu- 
dicella the tunic is sprinkled with large nucleated cells, fig. 4m, 
and at certain intervals bends abruptly inwards, figs. 1 & 2 u,7r, 
dividing the polypidom into cells at the points indicated by the 
constrictions in the horny tube. Thus each polype is isolated, is 
contained in fact within a distinct membranous cell, the end-walls 
of which abut against the end-walls of the adjoining cells. The 
divisions are therefore double, and being of living membrane and 
iu contact, it is probable that all the inhabitants of the polypidom 
are in some degree connected in vital action. The end-walls are 
considerably thickened in the centre, forming a bulb or boss pro- 
jecting into the cell. The polypes of Fredericella are not sepa- 
rated the one from the other, though a few divisions appear to 
exist ‘at distant points. Thus it would seem that groups of ani- 
mals are associated together as it were in one tube. Neither in 
Plumatella are the polypes separated. 
The polype hes in the longitudinal axis of the cell, Plates IT. 
& IV. figs. 2, 2, being provided with numerous muscles for pro- 
trusion and retraction. It is held in its place principally by a 
membranous tube—the tentacular sheath, Pl. IL. fig. 2 m,n, and 
PI. IV. fig. 2d’, d', which blends with the inverted lips of the 
tunic, Pl. II. fig. 2 7, a little below the orifice of the cell, and con- 
tinuing downwards within the cell incloses the bundle of re- 
tracted tentacles, and is attached round the tentacular disc a’. 
Digestive System.—The organs of digestion, comprising nearly 
the whole of the polype, float freely in the visceral cavity. The 
entrance to the alimentary canal is furnished with tentacles, 
Pl. IL. fig. 16; these arise from a margin surrounding the oral 
opening in two different fashions ; in the one they form a com- 
plete circle round the mouth, in the other they are arranged in 
a crescentic manner, the limbs of the crescent bemg two arms, 
Pl. ILI. figs. 1, 2 & 3c, e& c, extending from the sides of the 
mouth, fig. 3 a, having their bases confluent and with a row of 
tentacles on their inner and outer margins. Paludicella and 
Fredericella, P\. IV. fig. 1 f, and Pl. II. fig. 14, are examples of 
the first mode of arrangement ; and Plumatella, P1. ILI. figs. 1, 2 
& 3, and Alcyonella of the second. In Paludicella the tentacles 
when spread out form a very exact inverted cone, closely resem- 
bling the shape they assume in some of the marine species. The 
base or dise supporting the tentacles is not exactly circular in 
Fredericella ; in this genus it is a little flattened at the point cor- 
responding to the space between the oral arms in Plumatella ; 
and there is also a delicate transparent membrane, PI. II. fig. 1 
