490 
POPULAR SCIENCE REVIEW. 
Diatomacece, Conferva, &c., wliicli are entangled in the viscid 
substance.* 
M. ncijas I have never met with. Ehrenberg describes it as 
having a disk divided into two lobes, each of which is slightly 
indented, so that it is four-petaled, but less distinctly than in 
M. ringens. In other respects it seems to present no pecu- 
liarity worth notice. 
M. cepfcalo siphon has a disk of much the same form, the 
sinuous indentations being even less marked. This has lately 
been discovered near London in abundance on the leaves of the 
Anacharis alsinastrum , — that recently-introduced ditch-weed, 
which is fast usurping the place of all other aquatics in our 
ponds and rivers. Mr. Slack, in his “ Marvels of Pond Life,” 
p. 149, first figured it, and has since, by supplying me with 
specimens, kindly enabled me to describe it more fnlly.t The 
species dwells in a tall but narrow case, of so dark brown a 
colour as to be quite opaque. Its texture is coarse and rough, 
but it is not made up of well-fixed symmetrical pellets, as is 
that of M. ringens. The most remarkable peculiarity of the 
species is a single antenna (each of the preceding species has a 
pair of these organs), which is of great length, forming a trans- 
parent tube, furnished at the tip with a tuft of diverging bristles. 
This organ is evidently used as a feeler ; and it is highly inter- 
esting to see, when the animal is emerging from its case, how 
this long antenna is thrust out first, and is jerked with great 
vigour and suddenness from side to side, poking here and there, 
as if to ascertain that the coast is clear for the disk to be pro- 
truded and to commence its ciliary operations with security 
against dang’er or interruption. 
M. crystallinus, if I may judge from my own observations, 
and I have seen it in great numbers, does not deserve the spe- 
cific name which Ehrenberg applied to it. He describes the 
translucency of the case to be such as to render it difficult of 
detection. Possibly he met with only youthful specimens; for I 
find, on the contrary, that it is very dark, and so nearly opaque 
that the animal can scarcely be seen through it, at least in old 
individuals. The disk forms a simple circle. Though a beau- 
tiful creature, there is little in its economy sufficiently special 
to call for further remark. 
The last species, M. ptygura, if I rightly identify it, is some- 
what peculiar, and seems to approach in its form to the follow- 
ing genus. In October, 1849, I found two specimens of a fine 
wheel -animal which I could not satisfactorily identify with any 
* I have given the history of this species in detail, with figures, in my 
“ Evenings at the Microscope,” pp. 302 — 310. 
t In the “ Intellectual Observer,” for Feb. 1862, p. 49. 
