but the cilia had acted as a strainer, keeping all particles from the air-tubes. The swimming apparatus of the Opalina has led us far away from the little animal, who has been feeding while we have been lecturing. At the mention of feeding, you naturally look for the food that is eaten, the mouth and stomach that eat. But I hinted just now that this ethereal creature dispenses with a stomach, as too gross for its nature; and of course, by a similar refinement, dispenses with a mouth. Indeed, it has no organs whatever, except the cilia just spoken of. The same is true of several of the Infusoria; for you must know that naturalists no longer recognize the complex organization which Ehrenberg fancied he had detected in these microscopic beings. If it pains you to relinquish the piquant notion of a microscopic animalcule having a structure equal in complexity to that of the elephant, there will be ample compensation in the notion which replaces it, the notion of an ascending series of animal organisms, rising from the structureless _amœba_ to the complex frame of a mammal. On a future occasion we shall see that, great as Ehrenberg’s services have been, his _interpretations_ of what he saw have one by one been replaced by truer notions. His immense class of Infusoria has been, and is constantly being, diminished; many of his animals turn out to be plants; many of them embryos of worms; and some of them belong to the same divisions of the animal kingdom as the oyster and the shrimp: that is to say, they range with the Molluscs and Crustaceans. In these, of course, there is a complex organization; but in the Infusoria, as now understood, the organization is extremely simple. No one now believes the clear spaces visible in their substance to be stomachs, as Ehrenberg believed; and the idea of the _Polygastrica_, or many-stomached Infusoria, is abandoned. No one believes the coloured specs to be eyes; because, not to mention the difficulty of conceiving eyes where there is no nervous system, it has been found that even the spores of some plants have these coloured specs; and they are assuredly not eyes. If, then, we exclude the highly-organized _Rotifera_, or “Wheel Animalcules,” which are genuine Crustacea, we may say that all Infusoria, whether they be the young of worms or not, are of very simple organization.