[CHAPTER XIX]

BRANCH VERMES:[8] THE WORMS

THE EARTHWORM (Lumbricus sp.).

Technical Note.—Obtain live earthworms of large size, killing some in 30% alcohol and hardening and preserving them in 80% alcohol, and bringing others alive to the laboratory. The worms may be found during the daytime by digging, or at night by searching with a lantern. They often come above ground in the daytime after a heavy rain. Live specimens may be kept in the laboratory in flower-pots filled with soil. "They may be fed on bits of raw meat, preferably fat, bits of onion, celery, cabbage, etc., thrown on the soil."

External structure (fig. [25]).—Examine the external structure of live and dead specimens. Which is the ventral and which the dorsal surface? Which the anterior and which the posterior end? Note the segmented condition of the body; the number of segments or somites, and their relative size and shape. Note absence of appendages such as limbs and the presence of locomotor setæ (short bristles). How many setæ are there on each segment and what is their disposition? The mouth is covered by a dorsal projection called the prostomium. The anal opening is situated in the posterior segment of the body. The broad thickened ring or girdle including several segments near the anterior end of the body is the clitellum, a glandular structure which secretes the cases in which the eggs are laid. On the ventral surface of the fourteenth and fifteenth segments (in most species) are two pairs of small pores; two other pairs of small openings (usually difficult to find), one between segments 9 and 10, and one between segments 10 and 11, are present. All these are the external openings of the reproductive organs.

Fig. 25.—Dissection of the earthworm, Lumbricus sp.

Make drawings showing the external structure of the earthworm.