=Dorsal and Ventral Surfaces.=—The earthworm always crawls with the same surface to the ground; this is called the _ventral_ surface, the opposite surface is the _dorsal_ surface. This is the first animal studied to which these terms are applicable. What are the ventral and dorsal surfaces of a fish, a frog, a bird, a horse, a man? [Illustration: FIG. 72.—FOOD TUBE of earthworm. (Top view.) ] =The name “worm”= is often carelessly applied to various crawling things in general. It is properly applied, however, only to _segmented animals without jointed appendages_. Although a caterpillar crawls, it is not a worm for several reasons. It has six jointed legs, and it is not a developed animal, but only an early stage in the life of a moth or a butterfly. A “grubworm” also has jointed legs (Fig. 167). It does not remain a grub, but in the adult stage is a beetle. A worm never develops into another animal in the latter part of its life; its setæ are not jointed. [Illustration: FIG. 73.—FOOD TUBE AND BLOOD VESSELS of earthworm showing the ringlike hearts. (Side view.) ] =The Food Tube.=—The earthworm has no teeth, and the food tube, as might be inferred from the form of the body, is simple and straight. Its parts, recognizable because of slight differences in size and structure, are named the pharynx (muscular), gullet, crop, gizzard (muscular), and stomach-intestine. The last extends through three fourths of the length of the body (Fig. 72). The functions of the parts of the food tube are indicated by their names. [Illustration: FIG. 74. ] =Circulation.=—There is a _large dorsal_ blood vessel above the food tube (Fig. 73). From the front portion of this tube arise several large tubular rings or “hearts” which are contractile and serve to keep the blood circulating. They lead to a _ventral vessel_ below the food tube (Fig. 74). The blood is red, but the _colouring matter_ is in the liquid, not in the blood cells. [Illustration: FIG. 75.—GANGLIA NEAR MOUTH and part of nerve chain of earthworm. ] =Nervous System.=—Between the ventral blood vessels is a _nerve cord_ composed of two strands (see Fig. 75). There is a slight swelling, or _ganglion_, on each strand, in each segment (Fig. 75). The strands separate near the front end of the worm, and a branch goes up each side of the gullet and enters the two pear-shaped _cerebral ganglia_, or “brain” (Fig. 75). =Food.=—The earthworm eats earth containing organic matter, the inorganic part passing through the vent in the form of circular casts; these are found in the morning at the top of the earthworm’s burrow. The earthworm has no teeth. It excretes through the mouth an _alkaline fluid_ which softens and partly digests the food before it is eaten. When this fluid is poured out upon a green leaf, the leaf at once turns brown. The starch in the leaf is also acted upon. The snout aids in pushing the food into the mouth.