A magnifying glass will show on most species tiny bristle-like projections called _setæ_. How are the setæ arranged? (_d_, Fig. 70.) How many on one ring of the worm? How do they point? Does the worm feel smoother when it is pulled forward or backward between the fingers? Why? Are setæ on the lower surface? Upper surface? The sides? What is the use of the setæ? Are they useful below ground? Does the worm move at a uniform rate? What change in form occurs as the front part of the body is pushed forward? As the hinder part is pulled onward? How far does it go at each movement? At certain seasons a broad band, or ring, appears, covering several segments and making them seem enlarged (Fig. 71). This is the _clitellum_, or _reproductive girdle_. Is this girdle nearer the mouth or the tail? =Draw= the exterior of an earthworm. =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? =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 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. 72.--FOOD TUBE of earthworm. (Top view.)] [Illustration: FIG. 73.--FOOD TUBE AND BLOOD VESSELS of earthworm showing the ring-like 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. On account of slight variation in size and structure, its parts are named the pharynx (muscular), gullet, crop, gizzard (muscular), and the long intestine extending through the last three fourths of its 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 _coloring matter_ is in the liquid, not in the blood cells.