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. [Illustration: Fig. 40. Stentor Muelleri (Enr.), magnified 75 times.] 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."