An internal digestive cavity, the first element of all the organs subservient to individual nutrition, is observed in every class of animals and almost in every genus; and where this part has not yet been perceived, there can be little doubt, from analogy, of its existence. Its form and structure vary according to the kind of food on which the various tribes of animals are destined to subsist, and the extent of elabo ration it requires to undergo to assimilate it to the animal's body ; so that the diversities of this first part of the digestive apparatus are intimately related to all the living habits of animals, and to all the peculiarities they pre sent in their other assimilative organs and in their organs of relation.
1. Polygastrica. In the monads a digestive apparatus is distinctly seen, and in almost all the other genera of animalcules, where, indeed, the internal cavities connected with this im portant function are so numerous in almost all the known forms of these animals that this lowest class of animals has been termed poly gastrica to express their common character. From the transparency of these minute animals, their digestive sacs appear, when empty or when filled with water, like portions of the common cellular substance of the body, or like animalcules which have been swallowed, or like internal gemmules; and from not being generally recognized as alimentary cavities, many observers were led to suppose that the animalcules are nourished solely by superficial absorption like marine plants. Leuwenhoeck, however, not doubting that they possessed a sto mach, believed that they devour each other ; this was observed also byEl lis, and Spallanzani main tained that they devour each other so voraciously that they are seen to become distended with this food. Goeze saw the trichoda seizing and swallowing the animalcules which were smaller than itself. Baron Gleichen, in order to dis cover the form of their internal digestive cavi ties, placed them in infusions coloured with carmine which they soon swallowed, and in his coloured plates he has represented this red colouring matter as filling the internal stomachs of numerous trichode, vorticellre, and other animalcules. Indeed those internal globular cavities of animalcules are represented in the plates of Milner, Bruguiere, and all the older writers on this class. But Ehrenberg, by adopting the plan of Gleichen and Trembley of employing opaque colouring matter to detect the forms of these internal cavities, and by using principally carmine, sap-green, and indigo, carefully freed from all impurities which might prevent their being swallowed, has succeeded better than all his predecessors in unfolding the structure of the digestive organs of animal cules. Such coloured organic matter diffused
as fine particles mechanically suspended in the water in which animalcules are placed, is readily swallowed by them, and renders visible through their transparent bodies the form and disposition of their alimentary cavities; but however long they remain in these coloured infusions, with their stomachs distended with the colouring matter, it is not perceived to communicate the slightest tinge to the general cellular tissue of their body. In most of the animals of this class there is an alimentary canal with an oral and an anal orifice, which traverses the body and is provided with nume rous small round cwcal appendices, which open into its sides throughout its whole course, and which appear to perform the office of stomachs in receiving and preparing the food. In the simplest forms of animalcules however, (as in the monas atomus represented in fog. 4 A) there the two circles of cilia around the head at which it commenced, having numerous meal stomachs communicating with its cylindrical equal canal throughout its whole course. This circular form of intestine opening at both its extremities in the same ciliated aperture, is seen also in the carchcsium, zoocladium, episty lis, ophrydium, vaginicala, and other genera, which from this character are termed rye/of-a/a. In some of the animalcules of this group, as in the stentor polymorphus, (fig. 5 B,) the intes is but one orifice (fig. 4 A, a) to the alimen tary cavity, and the numerous ccecal appendices (fig. 4 A, b) open into this general wide orifice placed at the anterior extremity of the body. This simpler form of the digestive apparatus is found in the monads and in about forty other known genera of polygastrica, which, from this circumstance of their having no intestine passing through their body, have been grouped together as an order under the name of anen tera. In the monas termo, which is only about the two-thousandth of a line in diameter, four and even six of these round stomachs have been seen filled with the colouring matter, although they did not appear to be half the number which might be contained in its body. Each of these round stomachs was about aka of a line in diameter, and they appear to open, as in other anentcra, by a narrow neck into a wide funnel-shaped mouth surrounded with a single row of long vibratile cilia, which attract the floating organic particles or minuter invisi ble animalcules as food. This anenterous form of the digestive sacs is found both in the lori cated and in the naked kinds of animalcules belonging to the lowest genera of the class, many of which, however, have been found to be only the young of supposed higher genera.