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 

 is 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 

 iu 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 mentioned, animal or vegetable in their origin. 

 The Desmidecs, long classed among animals, are now generally 

 recognised as plants ; so also the group of the Diafoffiacece. The 

 Monads and Volvocitjea 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 Trayisactions, 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 



