al. C.

connects the mouth with the rest of the alimentary canal, which gives out many side branches or diverticula, which are themselves branched, so that the alimentary sac or stomach is a system of ramifying tubes extending from a central main tube to all parts of the body of the worm. There is no anal opening. In the round or thread worms, of which the deadly Trichina is an example, the alimentary canal is a simple straight tube with both anterior or mouth opening and posterior or anal opening. In the seaurchins and sea-cucumbers (Fig. 33) the alimentary canal is a simple tube with two openings, but it is longer than the body between mouth and anus, and so is more or less bent or coiled. In the earthworm the alimentary canal (Fig. 34), although a simple straight tube running through the body, plainly shows a differentiation into particular regions. Behind the mouth opening the alimentary tube is large and thick - walled and is called the pharynx; behind the pharynx it is narrower and is called. the oesophagus. Behind the œsophagus it expands to form a rounded, thin-walled chamber called the crop, and just behind this there is another

rounded but very thick-walled cham- FIG. 34.-Earthworm dissected ber called the gizzard. From the gizzard back the alimentary canal is about uniform in size, being rather wide and having thick, soft walls. This portion of it is called the intestine. The

posterior part of the intestine, called the rectum, leads to the anal opening. There is some differentiation of the inner surface of the canal. In the great group of mollusks, of which the common fresh-water clam or mussel is an example, the alimentary canal (Fig. 35) shows much variation. The microscopic plants, which are the food of the mussel, are taken in through the mouth and pass into a short œsophagus, thence into a wide stomach and there digested. Behind the stomach is a long, much-folded, narrow intestine which winds about through the fleshy "foot" and finally reaches the surface of the body, and has an anal opening at a point opposite the position of the mouth.

Among the insects there is a great range in degree of complexity of the alimentary canal. The digestive organs are, however, in most insects in a condition of high specialization. The mouth opening is provided with well-developed

and CORI.

biting and masticating or piercing and sucking mouth parts; pharynx, œsophagus, stomach, and intestine are always differentiated and sometimes greatly modified. In the common cockroach, for example (Fig. 36), the mouth has a complicated food-getting apparatus, and the canal, which

is much longer than the body of the insect, and hence much bent and coiled, consists of a pharynx, oesophagus, fore-stomach or proventriculus,

true digesting stomach or ventriculus, intestine, and rectum which opens at the posterior tip of the body. The inner lining of the canal shows much differentiation in the different parts of the canal, and there are numerous accessory glands connected with various parts of the canal.

Finally, among the highest animals, the vertebrates, we find still more elaborate specialization of the alimentary canal. As an example the alimentary canal of a cow has already been described in detail.

43. Stable and variable char

al c

FIG. 36.-Cockroach dissected to show alimentary canal, al. c.-After HATSCHEK and CORI.

/acteristics of an organ. In spite of all this variation in the structure and general character of the alimentary canal, there are certain characteristics which are features of all alimentary canals. In the examination of an organ we must ever distinguish between its so-called constant or stable characteristics and its inconstant or variable characteristics. The constant characteristics are the fundamentally essential ones of the organ; the variable ones are the special characteristics which adapt the organ for the pecul iar habits of the animal possessing it-habits which may differ very much from those of some other animal of similar size, similar distribution, similar abundance.

44. Stable and variable characteristics of the alimentary canal.-A tiger or a lion has an alimentary canal not more

than three or four times the length of its body, while a sheep has an alimentary canal twenty-eight times as long as its body. The tiger is carnivorous; the sheep herbivorous. Associated with the different food habits of the two animals is a striking difference in the alimentary canals. Animals like the horse or cat, which chew their food before swallowing it, have a slender oesophagus; animals like snakes which swallow their food whole have a wide œsophagus. Birds, that have no teeth and hence can not masticate or grind their food in their mouths, usually have a special grinding stomach, the gizzard, for this purpose. And so we might cite innumerable examples of these inconstant or variable characteristics of the alimentary canal. On the other hand, the alimentary canals of all the many-celled animals except the lowest agree in certain important characteristics. Each alimentary canal has two openings, one for the ingress of food and one for the exit of the indigestible portions of the matter taken in, and the canal itself stretches through the body from mouth to anus as a tube, now narrow, now wide, now suddenly expanding into a sac or giving off lateral diverticula, but always simply a lumen or hollow inclosed by a flexible muscular wall. The inner lining of the wall is provided with secreting and absorbing structures. Indeed, we can reduce the essential characters of the alimentary canal to even more simple features. The organ of digestion or assimilation of all the many-celled animals is merely a surface with which food is brought into contact, and which has the power of digesting this food by means of digestive secretions, and of absorbing the food when digested. This surface is small or great in extent, depending upon the amount of food necessary to the life of the animal and the difficulty or readiness with which the food can be digested. This surface might just as well be on the outside of the animal's body as on the inside, if it were convenient. In fact, it is on the outside of some animals. Among the Protozoa the

digesting surface is simply the external surface of the body. And not alone among the one-celled animals. Many of the parasitic worms which live in the bodies of other animals, and the larvæ or " grubs" of many insects which lie in the tissues of plants bathed by the sap, have no inner alimentary canal, but take food through the outer surface of the body. But in these cases the food is ready for immediate absorption, so that no special treatment of it is necessary, hence no complex structures are required.

Even were no such special treatment of the food necessary in the case of the larger animals, it would still be im

10

FIG. 37.-Diagram illustrating increase of volume and surface with increase of diameter of sphere.

possible for the simple external surface of the body to serve for food absorption, because of the well-known relation. between the surface and the mass of a solid body. When a solid body in the form of a sphere increases in size, its mass or volume increases as the cube of the diameter, while the surface increases only as the square of the diameter (Fig. 37). The external surface of minute animals a few millimeters in diameter can take up enough food to supply the whole body mass. But among large animals this foodgetting surface is increased as the square of the diameter of