INFUSORIA. 105 
can assume a trumpet-like form, the bell of which is closed by a 
convex membrane, the edge being furnished with a row of very strong 
obliquely-placed cilia, ranged in a spiral, meeting at the mouth, which 
1s placed near this edge. When they swim freely, their body admits 
of very considerable modifications of form. The Stentor Muelleri 
(Fig. 39) is to be found in ponds everywhere, often freely swimming 
in vast multitudes, or, again, attached in dense patches to some 
aquatic plant. 
Some of the animals which belong to the Vorticellina are fixed 
during one part of their existence, and become free in another stage. 
So long as they are fixed, they resemble, in their expanded state, 
a bell or funnel, with the edges reversed and ciliate. When they 
become free, they lose their crown of cilia, take a cylindrical form, 
more or less ovoid and elongated, and move themselves by means of 
a new row of a posterior circlet of cilia, which is temporarily 
developed. ‘There is no animal,” says Dujardin, “which excites 
our admiration in a higher degree than a vorticellate Infusorian, by 
its crown of cilia, and by the vortex which it produces; by its ever- 
varying form; above all, by its pedicle, which is susceptible of rapid 
spiral contraction, drawing the body backward, and again extending 
it.” This pedicle is remarkably contractile, drawing itself into a close 
coil with extraordinary rapidity, and again uncoiling itself with equal 
quickness. It is a hollow tube, containing a thread or band within 
it, to which its great contractile power is due. The genus Cothurnia 
belongs also to this group (Fig. 36). 
We cannot conclude our brief history of these curiously-organised 
beings without again recording the doubt which still exists in the 
minds of our most eminent naturalists, whether some of them are, 
as we have before menticned, animal or vegetable in their origin. 
The Desmidee, long classed among animals, are now generally 
recognised as plants; so also the group of the Diatomacee. The 
Monads and Volvocinea are still subjects of discussion, the evidence 
apparently inclining in favour of those who argue for their vegetable 
nature. Messrs. Busk, Williamson, and Cohn, have published, in 
the Microscopical Transactions, minute details of the evolutions of 
these curiously-organised Volvocinea, which seem to prove their 
vegetable nature. On the other hand, it is somewhat difficult to 
imagine so accurate an observer as Agassiz writing so positively 
as he does on a doubtful subject, unless he had a very firm con- 
viction as to the truth of his observations. Remarking on a former 
paper, in which he had shown that the embryo hatched from 
the egg of a Planarian was a true polygastric animalcule of the 
