553. Plant-like animals have a plant-like mode of nutrition. The transition from the one class to the other is so gradual as to be almost insensible. Fixed to the same spot in the ocean as the tree to the land, the nutritive surface of the poriferous animal is always in contact with the water, as the soil is with the external surface of the plant. The cellular substance of which the bag of the poriferous animal is composed is permeated in all directions by ramifying and anastomosing canals, which, beginning by minute pores placed on the external surface, terminate in larger orifices, termed vents, which are fecal openings. These internal canals are incessantly traversed by streams of water, which enter through the minute, and are discharged through the larger orifices. By these currents the nutrient matter contained in the water is conveyed to every part of the body, and the streams that issue from the fecal orifices abound with minute flocculent particles, the residue of the digested matter. No separate part of the body is appropriated to the function of digestion any more than in the plant; there is merely a general absorbent surface; the water is to this animal what the soil is to the plant; its whole surface is a root; every point of that surface is constantly in contact with its food, and every point is absorbent.
554. In the class above the porifera, the margins of the superficial pores are merely lengthened out into minute sacs, irritable and sentient, surrounded with vibratile cilia ([342]). These sacs, which are termed polypi, are so many little stomachs, which select, seize, and digest the food brought to them in the currents of water created by the action of the cilia ([344]).
1. The Hydra with its tentacula expanded. 2. The tentacula. 3. The body of the Hydra. 4. Disc for attachment. 5. The Hydra in the act of creeping. 6. The Hydra with an animalcule in its digestive cavity.
555. The fresh-water polype, the little hydra (fig. [CXLVIII]. 1), is one of these minute sacs detached and endowed with the power of locomotion (fig. [CXLVIII]. 5), a sentient, self-moving digestive bag. Capable of swallowing animals many times its own size, as the red-blooded worm, this little creature stretches its whole body like a thin elastic membrane over its prey, so as completely to alter its own shape, and the membranous substance of which it is composed becoming transparent by the distention, allows the subsequent process to be distinctly seen. The red fluid of the worm, as the process of digestion advances, is slowly diffused over every part of the internal surface of the polype. The whole internal surface of this minute self-moving bag is digestive; a true and proper stomach (fig. [CXLVIII]. 6). By dexterous manipulation, this internal surface may be rendered external, and the animal turned completely inside out. Then the external begins to perform the office of the internal surface, carrying on the function of digestion, just as well as that which was primitively formed for it; while the originally digestive becomes the generative surface, for the creature buds from this surface, now the outer one; a striking and instructive illustration of the analogy between the external covering of the animal body or the skin, and its internal lining, or the mucous surface.
Group of Monades; the dark spots in the interior of their bodies representing their digestive sacs.
556. In the monades (fig. [CXLIX].), and in all the lower animalcules, the digestive apparatus, instead of forming the entire internal surface of the body, consists of numerous sacs, which constitute so many separate stomachs, whence the name of the class, polygastrica. When empty, or when filled with water, these digestive sacs cannot be distinguished from the common cellular tissue of the body; but on feeding the animals with coloured organic matter, minutely diffused in water, the coloured particles readily enter the digestive sacs, and render apparent their form and arrangement. In the minutest animal hitherto appreciable, the monas termo, the 2000th part of a line in diameter, four rounded sacs have been seen filled with coloured particles (fig. [CXLIX].). Each of these sacs, about the 6000th part of a line in diameter, opens by a narrow neck into a funnel-shaped mouth, surrounded with a single row of long vibratite cilia, by the action of which the floating organic particles are brought within the reach of the mouth. In general, even in this class, an alimentary canal traverses the whole extent of the body, into which all the different stomachs open. Sometimes numerous branches proceed from the main trunks of the alimentary canal, bearing the nutritive matter to the different parts of the body (fig. [CL]. 2). Often, in order to extend the digestive surface, the alimentary canal is produced, forming rounded enlargements called cœcal appendages, all of which act as so many additional stomachs (fig. [CLI]. 3). In some individuals, observed under favourable circumstances, nearly 200 of these cœcal stomachs, filled with coloured matter, have been counted, and there may have been many more unseen, because empty and collapsed. In the lowest tribes of this class there is but one orifice to the alimentary canal, the oral; the food entering, and the fecal matter passing out of the system by the same aperture; but in the higher orders there is both an oral and an anal orifice, and the mouth and the anus are placed at opposite extremities of the body, as in the higher animals.