Fig. 235.—Slug, p.a., respiratory pore. (× 1.)
Slugs ([Fig. 235]) are of very similar structure, but in them the shell has almost disappeared, even the trace which remains being concealed by the mantle fold.
Among the commonest of fresh-water snails are various species of Limnaea ([Fig. 234], A). They may be found abundantly in ponds, and are often kept in aquaria, where they perform a useful service by devouring the minute plants which are apt to accumulate to an undesirable extent and form green scum on the sides. These snails, like the garden snail, breathe air, and often come to the surface to take a fresh supply into their lung chambers. Some other water snails, however, breathe dissolved oxygen by means of gills beneath the shell.
67. EARTHWORMS.
1. External characters.—Dig up several earthworms from the soil and examine them. What is the length of the largest and of the smallest specimens? What is the thickness of the body? Watch the worms crawling about, and describe the method of locomotion. Has the animal any legs? Do the length and thickness of any one worm vary during its movements? Is the variation connected with locomotion? How? Can you distinguish between the fore (anterior) and hind (posterior) ends, and between the upper (dorsal) and lower (ventral) surfaces? How do they differ?
Kill a large worm by immersion in methylated spirit and examine it more closely. Notice the segmented character of the body, and estimate the number of segments. Which part of the body has the largest segments? Observe the swollen appearance of segments 32 to 37; this region is called the clitellum. Notice the mouth (overhung by a short lobe) in the first segment, and the vent in the last segment. On the ventral surface of segment 14 try to see a pair of small pores; these pores are the openings through which the eggs are discharged from the body. Pull the worm gently between your fingers, and notice the bristly feel; in which direction of motion is this most apparent? Examine the ventral surface with a strong lens in a good light, and try to see four double rows of very small bristles.
2. Habits.—Examine the surface of the ground of a garden or lawn at night by help of a lantern or lamp, being careful to tread lightly, and observe the actions of any worms you see. Are more worms to be seen at night than during the day? Do the worms seem disturbed by the light of the lantern? Try to grasp one; is it easily caught? Why not? Where does the worm retreat? If you can grasp a worm before it has time to withdraw completely into its burrow, observe the difficulty of drawing it out without tearing it. Try if the worms are disturbed by a loud shout (be careful not to blow upon them when shouting), or by a heavy stamp of your foot.
Carefully lay open several burrows and notice:
(a) Whether the mouths of the burrows are plugged in any way, and, if leaves are used for this purpose, whether the leaves have been dragged into the hole (i) by the broad ends, (ii) by the narrow ends, or (iii) by the sides; do you find any signs of intelligence in the method adopted?