Page:The New International Encyclopædia 1st ed. v. 01.djvu/417

ALIMENTARY SYSTEM. 50 feet. It begins below the base of the skull, and passes through the thorax, abdomen, and pelvis, and eonsists, in brief, of the mouth, phar- ynx, oesophagus, stomach, small intestine and large intestine. The above description refers to the alimentary canal in human anatomy; its parts are variously modified in dilferent animals, as will be found in the articles on its several subdivisions. The process of carrying the digest- ed food to the tissues of the body is discussed under Cir.crL.Tio.. ALIMENTARY SYSTEM, Evolution of THE. An alimentary system as defined above is almost Avanting among plants, which, practically without exception, use only fluid or gaseous food, or else render solid substances fluid before ingestion. This difference between animals and plants is one of the best distinguishing charac- ters.

Types of Alimentary Tracts. The simplest may be designated the temporary type — that exhibited by Amoeba. This jelly-like, amorphous organism, when it conies upon a solid particle in the water, simply engulfs it at any point by flo'O'ing around it. The engulfed particle is sur- rounded by a sphere of water. From the plasma of the Amoeba 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 case of the Amipba there is no definite, permanent alimentai-y 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 bo pro- vided with cilia to carry food into the mouth. Tlie second t^'pe of alimentary tract is perma- nent but diffuse. This is the type exhilnted by sponges. There is not one digestive region, but hundreds of them, as many, 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. T!ie 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 sac 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 sac 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 ; but 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- niinths, Nemertinea, Bryozoa, and Brachiopoda, as well as in mollusks, mouth and anus have become distinct, and the digestive 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 oesopha- 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 appear 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 cesophagus. which is protru- sible. Behind, a pair of digestive glands open into the food canal. In the earthworm, the oesophagus 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 bearing diges- tive glands, that gain a great size in the Deca- poda. A gastric mill is present in the Mala- eostraea. In both annelids and arthropods the mouth 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.

In the Chordata the alimentary tract has very different relations from those found in the Annelida and Arthropoda. In the latter groups the alimentary tract lies dorsal to tbe main nerve and ventral to the heart; in the .chorda tes the tract is dorsal to the heart and ventral to the spinal cord. The question how the vertebrate condition is derived from the invertebrate condition is a diflicult one to answer. It has even led some to deny that vertebrates are related to Annelida or Arthropoda, as it is impossible to think of an animal adapted to traveling on one surface turning over and traveling on its back and tran.smitting this tendency to its descendants. It is more likely that the intermediate form was one that, like many of the lowest Chordata — the tunicates — was sessile in a ventral position at some time of life, and consequently had neither dorsal nor ventral surface.

Kmbryologicl History, The history of the