370 MR. HANCOCK ON THE ANATOMY OF THE FRESHWATER 
though a few divisions appear to exist at distant points. Thus it 
would seem that groups of animals are associated together as it were 
in one tube. Neither in Plumatella are the polypes separated. 
The polype lies in the longitudinal axis of the cell, Plates ITI. 
and V. 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. III. fig. 2 m, x, 
and Pl. V. fig. 2 d’, d’, which blends with the inverted lips of the 
tunic, Pl. III. fig. 2 7, a little below the orifice of the cell, and 
continuing downwards within the cell encloses 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. III. fig. 1 6; 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 being two arms, 
Pl. IV. 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, Pl. V. fig. 1 f, and Pl. III. fig. 1 6, are examples 
of the first mode of arrangement; and Plumatella, Pl. IV. figs. 1. 
2 & 3, and Alcyonella of the second. In Paludicella the tenta . 
cles, when spread out, form a very exact inverted cone, closely 
resembling the shape they assume in some of the marine species. 
The base or disc supporting the tentacles is not exactly circular 
in Predericella,; in this genus it is a little flattened at the point 
corresponding to the space between the oral arms in Plumatella; 
and there is also a delicate transparent membrane, Pl. III fig. 1 
c’, c, uniting the bases of the tentacles. In these respects Frede- 
ricella shows an approximation to those with oral arms, or as it 
is generally termed, a crescentic disc. In these there is always 
a similar membrane, Pl. IV. figs. 1, 2&3, f&d, at the base 
of the tentacles, and in all of them, as well as in /redericella, 
this membrane is attached to the external surface of the tenta- 
cles, and is much wider at the margin than the spaces between 
