TYPES or ALIMENTARY TRACTS. The simplest may be designated the temporary type—that exhibited by Amoeba. This jelly-like, amorphous organism, when it comes upon a solid particle in the water, simply engulfs it at any point by flowing around it. The engulfed particle is sur rounded by a sphere of water. From the plasma of the Ameba an acid is secreted into the sphere of water, and this dilute acid gradually dissolves the solid particle; the solution is then absorbed by the protoplasm. In the ease of the Anneha there is no definite, permanent alimentary tract. The same is true of all the rhizopodous Protozoa, and of the parasitic ones, which do not feed on solids. Many of the ciliate and flagellate Infusoria, on the other hand, ingest solid particles through a permanent mouth and gullet into the general protoplasmic spaces. The surface around the mouth opening may he pro vided with cilia to carry food into the mouth.
The second type of alimentary tract is perma nent but diffuse. This is the type exhibited by sponges. There is not one digestive region, but hundreds of them, as ninny, indeed, as there are pores and canals passing through the body wall. The solid food passes into these canals; the canals are lined by curiously modified "col lared" cells. These pick up the particles and engulf them, as a flagellate infusorian does. The whole sponge, indeed, behaves like a colony of Protozoa, specialized in different directions in different regions of the sponge body.
The third type of alimentary tract is perma nent, and concentrated in one cavity, and that cavity is a sac, having only one external open ing. This type is characteristic of all the Cnidaria, and is found in certain flatworms. It is typically illustrated by Hydra. Hydra con sists of a body wall surrounding, a central cavity that has one opening at the upper pole. sur rounded by a circlet of tentacles. The opening serves both as mouth and anus. The body wall is two-layered : the outer layer is the sensory one; the inner layer is digestive. The origin of this type is uncertain; it seems quite likely that it has not developed from the sponge type, but that it represents an altogether new line of evolution, in which the body is not to be considered as a colony of infusoria-like cells, but as a greatly enlarged protozoan, with many nuclei and hence with many cells. On this last hypothesis the digestion cavity of Hydra would be homologous with that of an infusorian. In the sea anemones the digestive sae is more com plicated than in Hydrozoa, in that it is divided into a number of alcoves opening into one cen tral chamber. The alcoves arise in consequence
of a series of radial partitions (called mesen teries) arranged in a plan of four and its mul tiples or six and its multiples, that pass from the outer body wall toward the -entre. In the sea anemones the entrance to the digestive sae is an elongated slit that serves both as mouth and anus. According to one theory, the separate mouth and anus of higher forms arise from opposite extremities of this slit, while in the middle part of the slit the lips are fused together. In the lower flatworms, the planarians and trematodes, the body is elongated, and the digestive sac is elongated likewise; hut it is still a sac with a single opening. The cestodes, being abject parasites living in the digestive juices of the host, need no digestive tract and have none. In the higher flatworms, nemathel minths, Nemertinea, Bryozoa, and Brachiopoda, as well as in mollusks, mouth and anus have become distinct, and the digest ire sac has become a digestive tube or canal, as in higher groups. With the formation of a digestive tube three portions may be distinguished, namely: fore gut, mid gut, and hind gut. The first and last are usually of ectodermal origin. The mid gut is usually lined by entoderm. These three parts of the alimentary tract undergo special modifications. The beginning of the fore gut, or mouth, becomes fitted with grasping and sensory organs; and lower down in the (esopha gus there is frequently found a crushing organ, the gizzard. The mid gut is very glandular. In many species the glands have enlarged to per form their work better, and a pp,ar as appendages of the mid gut: e.g., the pancreas or hepatopan creas. The hind gut is the rectum. These con ditions are shown in their simplest form in the annelids. The sandworm of the sea coast has great jaws in the (esophagus, which is protru sible. Behind, a pair of digestive glands open into the food canal. In the earthworm, the (esophagus leads into a crop, and this in turn into a muscular gizzard. In the intestine two dorsal grooves add to the glandular surface. Passing to arthropods, we find the mid gut occasionally coiled, and frequently hearing diges tive glands, that gain a great size in the Deea poda. A gastric mill is present in the Main costraca. In both annelids and arthropods the month is on the same side of the body as the great nerve cord, and the anus is placed in the last metamere of the body.