Fig. 71.—Earthworm, mouth end above.
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?
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.
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.
