Fig. 40. Stentor Muelleri (Enr.),
magnified 75 times.
The Bursarians are animals with an oval or oblong contractile body, provided also with vibratile cilia, especially on the surface, having also a large mouth, surrounded with cilia, forming a sort of microscopic moustache, spirally arranged.
Among the species belonging to this group may be noted the Condylostoma patens (Fig. 39), remarkable for its size and voracity. It sometimes attains the twelfth of an inch, and abounds on every shore from the Mediterranean to the Baltic. Another Bursarian, a species of Plagiostoma, lives between the intestines and the external muscular bed of the earth-worm, Lumbricus terrestris. To the group of Urceolarians belong the Stentors, which are in number the most numerous of the Infusoria; they are, for the most part, visible to the naked eye.
The Stentors are inhabitants of fresh, tranquil water, not subject to agitation, and covered with water plants. They are nearly all coloured green, blackish, or blue; their bodies covered with cilia. They are eminently contractile, and very variable in form. They can attach themselves temporarily, by means of the cils at their posterior extremities, when they 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 is placed near this edge. When they swim freely, they alternately resemble a club, a spindle, or a sphere. The Stentor Muelleri is seen in ponds in the neighbourhood of Paris and elsewhere; it has been found even in the basins of the Jardins des Plantes (Fig. 40).
The animals which constitute this genus are fixed in the first part of their existence, but free in the second. So long as they are fixed, they resemble, in their expanding 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 organ. "There is no animal," says Dujardin, "which excites our admiration in a higher degree than the Vorticellate Infusoria, by their crown of cilia, and by the vortex which it produces; by their ever-varying forms; above all, by their pedicle, which is susceptible of rapid spiral contraction, by drawing the body backward and again extending it. This pedicle is a flat membranous band, thicker upon one of its edges than the other, and containing on the thicker side a continuous channel, occupied, at least in part, by a fleshy substance, analogous to that of the interior of the body. During contraction, this thick edge is shortened more than the thin side, and hence results the precise form of the spiral of the corkscrew."
We cannot conclude our brief history of these curiously-organized beings without recording the doubt which still exists in the minds of our most eminent naturalists, whether some of those we have named are animal or vegetable in their origin. The Desmideæ, long classed among animals, are now generally recognized as plants. The group of Diatomaceæ are still considered doubtful, and the Monads and Volocina are still subjects of discussion, the evidence 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-organized globules, which seem to prove their vegetable nature. On the other hand, it is difficult to imagine so accurate an observer as Agassiz writing so positively as he does on a doubtful subject. Remarking on a former paper, in which he had shown that the embryo hatched from the egg of a Planaria was a true polygastric animalcule of the genus Paramecium, he adds, that in former writers a link was wanting, viz., tracing the young hatched from the egg of Distoma. "This deficiency," he says, "I can now fill. It is another Infusorium, a genuine Opalina. With such facts before us there is no longer any doubt left respecting the character of all these Polygastria; they are the earliest larvæ condition of worms." Amid these friendly disputes we congratulate ourselves that we have to do with the oceanic creations, both animal and vegetable.