be found at least traces of more cusps. Now in some of them we may be dealing with instances of a complete tooth change; the suppression, save for one tooth, which is found in Marsupials, was probably not developed in at least some of these early mammals. The simplicity may therefore have been preceded by complexity, and may have been merely an adaptation to an insectivorous diet.
Alimentary Canal.—The mouth of the Mammalia is remarkable for the fact that with a few exceptions, such as the Whales, there are thick and fleshy lips. The office of these is to seize the food. The roof of the mouth is formed by the "hard palate" in front, which covers over the maxillary and palatine regions. This region is often covered with raised ridges, which have a symmetrical disposition, and are particularly strong in Ruminant animals. They are much reduced in the Rodents, where the anterior part of the palate is ill-defined owing to the way in which its sides fade into the lateral surface of the face. It has been shown that these ridges, in the Cat at least, develop as separate papilliform outgrowths, and it has been suggested that these papillae, which later become united to form the ridges, are the last remnant of palatine teeth such as occur in lower vertebrates.
Fig. 40.—Palatal folds of the Raccoon (Procyon lotor). p.p, Papilla palatina; r.p, palatal folds. (From Wiedersheim's Structure of Man.)
The tongue is a well-developed organ, usually playing a double part. It acts as an organ of prehension, especially in such animals as the Giraffe and the Anteater, where it is long and protrusible beyond the mouth for a considerable distance. It also carries gustatory organs, which serve for the discrimination of the nature of the food. Beneath the tongue there may be a hardish plate, known as the sublingua. This is especially prominent in the Lemurs, where it projects as a horny structure below the tongue, and has an independent and free tip. It is supported in some of these animals by a cartilaginous
structure. It is held by Gegenbaur that this organ is the equivalent of the reptilian tongue, and that in the skeletal vestiges which it contains are to be found the equivalents of the hyoid skeletal cartilages which support the tongue in lizards. In this case the tongue of mammals is a subsequently added structure.
The oesophagus leads from the mouth cavity to the stomach. The latter organ has commonly a distinctive shape in mammals. This is well shown in Man. The orifices of the oesophagus and intestine are somewhat approximated; and this causes a bulging of the lower border of the organ, usually spoken of as the greater curvature. A stomach of this typical form is found in many orders of mammals, and is unlike the stomach in any of the groups of lower vertebrates in shape. Sometimes the shape of the organ is greatly altered: it may be drawn out, sacculated, or divided, as in the Ruminants and Whales, into a series of differentiated chambers, each of which plays some special part in the phenomena of digestion.
The intestine of mammals is always long and much coiled, though the length and consequent degree of coiling naturally varies. On the whole it is perhaps safe to say that it is shorter in carnivorous than in vegetable-feeding beasts. Thus the Paca has an intestine of 39 inches total length, while the Cat, an animal of about the same size, has an intestine which is only 36 inches long. A fish diet, however, to judge from the Seals, is associated with a long intestinal tract. The intestine is divisible in the vast majority of mammals into a small and a large intestine. The two are separated by a valvular constriction save in certain Carnivores; and in the majority of cases the distinction is also emphasised by the presence at the junction of a blindly-ending diverticulum, the caecum. This latter organ varies greatly in length, being very short in the Cat-tribe and exceedingly long in Rodents. Its size is, to some extent, dependent upon the flesh-eating or grass-eating propensities of the animal in which it occurs. One of the longest caeca is possessed by the Vulpine Phalanger, in which the organ is one-fifth of the length of the small intestine; while the opposite extremity is reached by Felis macroscelis, which has a small intestine one hundred times the length of the caecum.
