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The locomotive is an inorganic machine; the animal is an organic machine. There is a great and real difference between an organism, a living animal, and a locomotive, an inorganic structure. But for a good understanding of the relation between function and structure, and of the composition of the body of the complex animals, the comparison of the animal and locomotive is very instructive.

41. The specialization of organs.-The organ for the performance of some definite function in one of the higher animals may be very complex. The corresponding organ in one of the lower animals for the performance of the same function may be comparatively simple. For example, the organ for the digestion of food is, in the case of the polyp, a simple cylindrical cavity in the body into which. food enters through a large opening at the apical or free end of the body. The digestive organ of a cow is a long coiled tube, comprising many regions of distinct structural and physiological character and altogether extremely complicated. An organ in simple or primitive condition is said to be generalized; in complex or highly modified condition it is said to be specialized. That is, an organ may be modified and complexly developed to perform its function in a special way, in a way differing in many particulars from the way the corresponding organ in some other animal performs the same general function. The specialization of organs, or their modification to perform their functions in special ways, is what makes animal bodies complex, for specialization is almost always in the line of complexity. Later we shall see more clearly how specialization is brought about. For the present we may study one of the more important organs of the animal body for the sake of having concrete examples of some of the general statements made in this discussion of function and structure.

42. The alimentary canal.-The organ which has to do with the taking and digesting of food is called the ali

mentary canal. In some of the higher animals this is a very complex organ. In the cow, one of the cud-chewing mammals or ruminants, it consists of several distinct portions, which differ among themselves very much (Fig. 30). First, there is the mouth, or opening for the entrance of the food. The mouth is sup

plied with teeth for tearing
off and chewing the food,
with a tongue for manipu-
lating it, and with taste pa-
pillæ situated on the tongue
and palate for determining ...
the desirability of the food.
Into the mouth a peculiar
fluid (the saliva) is poured
by certain glands, organs ac-
cessory to the alimentary
canal. The herbage bitten
off, mixed with saliva, and
rolled by the tongue into a
ball, passes back through a
narrow tube, the œsophagus,
and into a sac called the ru-
men, or paunch. Here it

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i

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FIG. 30.-Alimentary canal of the ox (after COLIN and MÜLLER). a, rumen (left hemsiphere); b, rumen (right hemisphere); c, insertion of œsophagus; d, reticulum; e, omasum; f, abomasum; g, duodenum; h and i, jejunum and ileum; j, cæcum; k, colon, with its various convolutions; 7, rectum.

lies until the cow ceases for the while to take in food, when it passes back again through the oesophagus and into the mouth for mastication. After being masticated it again passes downward through the oesophagus, and enters this time another sac called the reticulum, lying next to the rumen. From here

it passes into another sac-like portion of the alimentary canal called the omasum, where it is strained through numerous leaf-like folds which line the walls of this part of the canal. From here the food passes into a fourth

sac-like part of the canal, called the abomasum. Here the process of digestion goes on. The four sacs-rumen, reticulum, omasum, and abomasum-are called stomachs, or they may be considered to be four chambers forming one large stomach. In the abomasum, or digesting stomach, digestive fluids are poured from glands lining its walls, and the food becomes converted into a liquid called chyle. The chyle passes from the stomach into a long, narrow, tubular portion of the canal called the intestine. The intestine is very long, and lies coiled in a large mass in the body of the cow. The intestine is divided into distinct regions, which vary in size and in the character of the inner wall. These parts of the intestine have names, as duodenum, jejunum, ileum, cæcum, colon, etc. Part of the intestine is lined inside with fine papillæ, which take up the chyle (the digested food) and pass it through the walls of the intestine to other special organs, which pass it on to the blood, with which it becomes mixed and carried by an elaborate system of tubes to all parts of the body. Part of the grass taken into the alimentary canal by the cow can not be digested, and must be got rid of. This passes on into a final posterior part of the intestine called the rectum, and leaves the body through the anus or posterior opening of the alimentary canal. The whole canal is more than twenty times as long as the body of the cow; it is composed of parts of different shape; its walls are supplied with muscles and blood-vessels; the inner lining is covered with folds, papillæ, and gland cells. It is altogether a highly specialized organ, a structurally complex and elaborately functioning organ.

Let us now examine the alimentary canal, or organ of digestion, in some of the simpler animals.

The Protozoa, or simplest animals, have no special organ at all. When the surface of the body of an Amaba comes into contact with an organic particle which will serve as food, the surface becomes bent in at the point of its con

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tact with the food particle, and the body substance simply incloses the food (Fig. 3). Food is taken in by the surface. The whole outer surface of the body is the foodtaking organ. In the simplest many-celled animals, the sponges, there is no special food-taking and digestive organ. Each of the cells of the body takes in and assimilates food for itself. The sponge is like a great group of Amaba holding fast to each other, but each looking out for its own necessities. Among the polyps, however, there is a definite organ of digestion that is, food is only taken and digested by certain parts of the body. The simple polyp's body (Fig. al. S.-. 31) is a cylinder or vase closed at one end and open at the other end, and attached by the closed end to a rock. The opening is usually of less diameter than the diameter of the body, and it is surrounded by a number of tentacles, whose function it is to seize the food and convey it to the mouth opening. There are, of course, no teeth, no tongue, none of the various parts which are in or are part of the mouth of the higher animals. The polyp's mouth is simply a hole or opening into the inside of the body. This body cavity, or simplest of all stomachs, is simply the cylindrical or vase-shaped hollow space inclosed by the body wall. This space extends also into the tentacles. other opening, no posterior or anal opening.

FIG. 31.-Obelia sp., a simple polyp; vertical section, highly magnified. m, mouth opening; al. s., alimentary sac.- After PARKER and HASWELL.

There is no

We can not

speak of an œsophagus or intestine in connection with this most primitive of alimentary sacs. The cells which line the sacs show some differentiation; some are gland cells and secrete digestive fluids; some are amoeboid and are provided with pseudopods or flagella for seizing bits of food. The food caught by the tentacles comes into the alimentary sac through the opening or primitive mouth, and

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what of it is digestible is, by the aid of the gland cells and the amoeboid cells, taken up and assimilated, while the rest of it is carried out by water currents again through the single opening.

In the flatworms (Fig. 32) like Planaria (small, thin, flattened worms to be found in the mud at the bottom of fresh-water ponds) the mouth opens into a short, narrow tube which may be called an oesophagus. The oesophagus

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