CENTA), and this region is the cloaca into which the aliment ary, urinary and generative ca nals or ducts all open, but later two lateral folds appear which, by their union, divide the cloaca into a ventral and a dorsal part, the former being genito urinary and the latter alimentary or intestinal, In this way the rectum or dorsal compartment is shut off from the genitourinary. Later an ectodermal invagination at the hind end of the embryo develops and forms the anal canal; this is the proctodaeum, and for some time it is separated from the hind (caudal) end of the rectal part of the mesodaeum (or part of the intestinal canal formed from the mesoderm) by a membrane called the anal membrane. This is eventually absorbed and the digestive tract now communicates with the surface by the anus.
The oesophagus is essentially merely a passage, as straight as may be, from the pharynx to the stomach, varying in length with the length of the neck and thoracic regions in different animals, and in calibre with the nature of the food. It is almost invariably lined with a many-layered epithelium, forming a tough coating, readily repaired and not easily damaged by hard food masses. There are few exceptions to this structural and functional sim plicity. In fishes (see ICHTHYOLOGY, Anatomy) the swim-bladder is developed as a dorsal outgrowth of the oesophagus and may remain in open connection with it. In many birds part of the oesophagus may be temporarily dilated, forming a "crop," as for instance in birds of prey and humming birds. In the flamingo, many ducks, storks and the cormorant the crop is a permanent although not a highly specialized enlargement. Finally, in the vast majority of seed-eating birds, in gallinaceous birds, pigeons, sandgrouse, parrots and many Passeres, particularly the finches, the crop is a permanent globular dilatation, in which the food is retained for a considerable time, mixed with a slight mucous se cretion, and softened and partly macerated by the heat of the body. Many birds feed their young from the soft contents of the crop, and in pigeons, at the breeding season, the cells lining the crop proliferate rapidly and are discharged as a soft cheesy mass into the cavity, forming the substance known as pigeon's milk.
Stomach.—Wherethe oesophagus passes into the stomach, the lining wall of the alimentary tract changes to a mucous epithelium, consisting of a single layer of endodermal cells, frequently thrown into pits or projecting as processes; from being chiefly protective, it has become secretory and absorbing, and maintains this character nearly to the anus. The fundamental form of the stomach is a sac-like enlargement of the canal, the whole form ing an enlarged bent tube. At the distal end of the tube the in testinal tract proper begins, and the two regions are separated by a muscular constriction. In fishes the stomach may be a simple bent tube, or ari expanded, globular or elongated sac. In Batra chia and Reptilia it is in most cases a simple sac, marked off from the oesophagus only by increased calibre. In the Crocodilia, how ever, the anterior portion of the stomach is much enlarged and very highly muscular, the muscles radiating from a central tendinous area on each of the flattened sides. The cavity is lined by a hardened secretion and contains pebbles and gravel for me chanical trituration of the food, so that the resemblance to the gizzard of birds is well marked. This muscular chamber leads by a small aperture into a distal, smaller and more glandular cham ber. In birds the stomach exhibits two regions, an anterior glandu lar region, the proventriculus, the walls of which are relatively soft and contain enlarged digestive glands. The distal region (gizzard) is larger and is lined in most cases by a more or less permanent membrane which is thick and tough in birds with a muscular gizzard, very slight in the others.
In mammals, the primitive form of the stomach consists of a more or less globular or elongated expansion of the oesophageal region, forming the cardiac portion, and a forwardly curved, narrower pyloric portion, from which the duodenum arises. The whole wall is muscular, and the lining membrane is richly glandu lar. In many mammals one, two or three protrusions of the cardiac region occur, whilst in the manatee and in some rodents the cardiac region is constricted off from the pyloric portion. In the Artiodactyla the stomach is always complex, the complexity reaching a maximum in ruminating forms. In the chevrotains, which in many other respects show conditions intermediate between non-ruminant artiodactyles and true ruminants, the oesophagus opens into a wide cardiac portion, incompletely di vided into four chambers. Three of these, towards the cardiac extremity, are lined with villi and correspond to the rumen or paunch ; the fourth, which lies between the opening of the oesophagus and the pyloric portion of the stomach, is the ruminant reticulum and its wall is lined with very shallow "cells." The fourth or true pyloric chamber is an elongated sac with smooth glandular walls and is the abomasum, or rennet sac. In the camel the rumen forms an enormous globular paunch with villous walls and internally showing a trace of division into two regions. It is well marked off from the reticulum, the "cells" of which are extremely deep, forming the well-known water-cham bers. In the true ruminants, the rumen forms a capacious, villous reservoir, nearly always partly sacculated, into which the food is passed rapidly as the animal grazes. The food is subjected to a rotary movement in the paunch, and is thus repeatedly subjected to moistening with the fluids secreted by the reticulum, as it is passed over the aperture of that cavity, and is formed into a rounded bolus. The f ood bolus, when the animal is lying down after grazing, is passed into the oesophagus and reaches the mouth by antiperistaltic contractions of the oesophagus. After pro longed mastication and mixing with saliva, it is again swallowed, but is now passed into the psalterium, which, in true ruminants, is a small chamber with conspicuous longitudinal f olds. Finally it reaches the large abomasum where the last stages of gastric digestion occur.
In the Cetacea the stomach is different from that found in any other group of mammals. The oesophagus opens directly into a very large cardiac sac the distal extremity of which forms a long caecal pouch. At nearly the first third of its length this com municates by a narrow aperture with the elongated, relatively narrow pyloric portion. The latter is convoluted and constricted into a series of chambers that differ in different groups of Cetacea. In most of the Marsupialia the stomach is relatively simple; in the kangaroos, on the other hand, the stomach is divided into a relatively small, caecal cardiac portion and an enormously long sacculated and convoluted pyloric region, the general arrange ment of which closely recalls the large caecum of many mammals.
Fishes.—Inthe Cyclostomata, Holocephali and a few Teleostei the course of the gut is practically straight from the pyloric end of the stomach to the exterior, and there is no marked differ entiation into regions. In the Dipnoi, a contracted sigmoid curve between the stomach and the dilated intestine is a simple be ginning of the complexity found in other groups. In very many of the more specialized teleosteans the gut is much convoluted, exhibiting a series of watchspring-like coils. In a number of different groups, increased surface for absorption is given, not by increase in length of the whole gut, but by the development of an internal fold known as the spiral valve. A set of organs peculiar to fish, known as the pyloric caeca, is present in num bers ranging from one to nearly 200 in the vast inajority of fish. These are outgrowths of the intestinal tract near the pyloric ex tremity of the stomach, and their function is partly glandular, partly absorbing.
In the Batrachia the course of the intestinal tract is nearly straight from the pyloric end of the stomach to the cloaca, in the case of the perennibranchiates there being no more than a few simple loops between the expanded "rectum" and the straight portion that leaves the stomach.
In fishes, batrachians and reptiles the intestinal tract is swung from the dorsal wall of the abdominal cavity by a mesentery which is incomplete on account of secondary absorption in places. There are also traces, more abundant in the lower forms, of the still more primitive ventral mesentery.