HISTORY OP THE ROTIEERA, OR WHEEL ANIMALCULES. 491 
of Ehrenberg^s descriptions, and which I therefore named, as a 
new species, Megalotrocha velcita. I have, however, had reason 
since to think it was his Ptygura melieerta (see fig. cl). The 
animal was not contained in any case, that I could discern, nor 
was it attached to any others of its kind ; but the observations 
were insufficient to determine absolutely that these conditions 
were permanent. It laid a large egg in my possession, and 
actually under my eye, so that it was in adult age. On the 
other hand, it displayed two distinct eyes, — in general, a mark 
of youth in this family. Its general figure was somewhat 
trumpet-shaped, slightly swelling in the middle. The disk was * 
very large, forming a continuous nearly circular outline, and 
partially surrounded by a layer of granular tissue. The foot 
terminated in an adhering - sucker, whose figure resembled that 
of a glass stopper in a phial ; the dilated extremity of this was 
capable of adhering to any foreign substance. 
I come now to speak of the three species, which I propose 
to unite into the one genus, Megalotrocha. They have a disk 
remarkable (as the name signifies) for its great size ; it forms a 
very broad circular or ovate figure, notched at one point. To 
be more precise, the course of the cilia is continuous all round, 
with the exception of the middle of the ventral side, where 
there is an abrupt indentation or sinuosity. But their most 
remarkable peculiarity is their social habit : they voluntarily 
seek the society of others of their own species at a certain age, 
before which they have been free and solitary, and then adhere 
together by the mutual contact of the bases of their feet, so that 
they project in a radiating manner on all sides from a common 
centre, thus constituting compound spheres, which sometimes 
swim at large on a roving commission, and in other cases are 
permanently anchored to some aquatic plant. I suspect in all 
cases (though Ehrenberg says it is not so in M. alho-fiaviccms) 
that there is secreted a common gelatinous envelope, corre- 
sponding to the isolated cases of the Melicertce, but so aggluti- 
nated together by the proximity of the animals, and by the 
viscous nature of the substance, that a common ball of jelly 
is formed, pierced with cells in which the creatures dwell, 
each in its own. 
My own personal acquaintance with this form is confined to 
M. ( Gonochilus ) volvox, of which I found many clustered spheres, 
some years ago, in one of the tiny pools on Hampstead Heath 
(see fig. e). The appearance of these — from then- form, their 
colour, and their association, and their majestic rolling move- 
ments — was very interesting. 
The clusters are very distinctly visible to the naked eye, 
swimming slowly along, ascending or descending, by the motion 
of the powerful cilia that surround the head. Each cluster 
