In the Sauropsida and Mammalia the cloaca appears as a dilatation of the mesenteron, which receives the opening of the allantois almost as soon as the posterior part of the mesenteron is established.
The eventual changes which it undergoes have been already dealt with in connection with the urinogenital organs.
Intestine. The region in front of the cloaca forms the intestine. In certain Vertebrata it nearly retains its primitive character as a straight tube; and in these types its anterior part is characterised by the presence of a peculiar fold, which in a highly specialised condition is known as the spiral valve. This structure appears in its simplest form in Ammocœtes. It there consists of a fold in the wall of the intestine, giving to the lumen of this canal a semilunar form in section, and taking a half spiral.
In Elasmobranchii a similar fold to that in Ammocœtes first makes its appearance in the embryo. This fold is from the first not quite straight, but winds in a long spiral round the intestine. In the course of development it becomes converted into a strong ridge projecting into the lumen of the intestine ([fig. 388], l). The spiral it makes becomes much closer, and it thus acquires the form of the adult spiral valve. A spiral valve is also found in Chimæra and Ganoids. No rudiment of such an organ is found in the Teleostei, the Amphibia, or the higher Vertebrata.
The presence of this peculiar organ appears to be a very primitive Vertebrate character. The intestine of Ascidians exhibits exactly the same peculiarity as that of Ammocœtes, and we may probably conclude from embryology that the ancestral Chordata were provided with a straight intestine having a fold projecting into its lumen, to increase the area of the intestinal epithelium.
In all forms in which there is not a spiral valve, with the exception of a few Teleostei, the intestine becomes considerably longer than the cavity which contains it, and therefore necessarily more or less convoluted.
The posterior part usually becomes considerably enlarged to form the rectum or in Mammalia the large intestine.
In Elasmobranchii there is a peculiar gland opening into the dorsal side of the rectum, and in many other forms there is a cæcum at the commencement of the rectum or of the large intestine.
In Teleostei, the Sturgeon and Lepidosteus there opens into the front end of the intestine a number of cæcal pouches known as the pancreatic cæca. In the adult Sturgeon these pouches unite to form a compact gland, but in the embryo they arise as a series of isolated outgrowths of the duodenum.
Connected with the anterior portion of the middle region of the alimentary canal, which may be called the duodenum, are two very important and constant glandular organs, the liver and the pancreas.