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. CHAPTER V. POLYPIFERA. "Happy is he who, satisfied with his humble fortune, lives contentedly in the obscure state where God has placed him."--RACINE. Entering on the class Polypifera, we leave the domain of the infinitely small to enter the world of the visible. Beside the Infusoria, the Polypifera, which are sometimes several inches in length, are very important beings. Science has made great advances towards giving us an exact knowledge of these singular animals. Many scientific prejudices have been dissipated, many errors have been corrected. The Polyps, as they are defined in the actual state of Science, correspond not only with the _Polypes, properly so called_, of Cuvier and De Blainville, but also with the _acalephous zoophytes_ of the same authors. We now know that certain Polyps engender _medusæ_, or acalephous zoophytes, and that there exist some medusæ scarcely differing in their structure and habits of life from the ordinary Polyps.