“That shell was placed among the snails by Linnæus,” said Mrs. Merton; “but it is now called Planorbis, or the coil-shell. Look what a horny, almost transparent, substance it has; indeed, I believe it is sometimes called the Horny Snail. It does not live in the sea; but it is found in ditches, or any stagnant water that is nearly dry in summer. When attacked, it emits a dark reddish liquid, to hide itself from its enemies, by rendering the water so dark that it cannot be seen.” “How clever!” “Instinct teaches many molluscous animals to do the same. The violet snail emits a beautiful lilac fluid; and the cuttle-fish a liquid as black as ink. But this is not all that I have to tell you about the Planorbis: it lays its eggs upon a leaf, where they look like those of the spider, or of some kind of insect.” “Look mamma! Here is another shell, quite different from the Planorbis.” [Illustration: _Fig. 18._ THE MARSH-SNAIL (_Lymnea communis_). ] “It is different in shape, but it is nearly allied in other respects, for that is the Marsh-snail, or Lymnea. Some of the species of this genus crawl with their backs downwards along the under surface of the water, if I may so describe it, just as you have seen a snail crawl on a glass; and the species of the genus Physa, which is another little black fresh-water-snail, not only creep in the manner I have described, but let themselves down by a thread in the water, just as you may have seen some kinds of caterpillars do on land.” As they strolled along the beach they noticed several immense plants of Sea-rocket, which grew close to the cliffs, and some of the fleshy leaves of which Agnes gathered and ate. “I know I am safe in eating this,” said she; “because I see by the four opposite petals of the flower that it is one of the Cruciferæ, or cabbage-tribe, and I know the plants of that tribe are wholesome.” “Take care, however, lest you should some day find that though the Cruciferous plants are eatable they are not always agreeable; for, remember, Horseradish, and some other pungent plants, belong to that tribe: but I am glad to find that you have remembered what I told you about the shape of the flowers, which are called cruciferous, or cross-bearing, from their four petals being arranged in the form of a Greek cross.”