[Illustration: FIG. 11.--SEA-URCHIN (Echinus), WITH AND WITHOUT SPINES.] [Illustration: FIG. 12.--SPINE OF ECHINUS (A, natural size, B, a section magnified).] The Star-fishes are inhabitants of the sea, and have generally five arms covered beneath with tentaculæ having small suckers at the point, it is by means of these that they move from place to place; the mouth is in the centre. They live upon small marine animals, and have the power of reproducing any of the rays which happen to be broken off; and if cut into two or more pieces, each will produce a fresh animal. There is a species of Star-fish, so brittle, that it is quite impossible to touch it without breaking it, or rather it breaks itself, for it seems to have the power of casting off its arms at pleasure. These forms complete the Radiate series of animals, they have nearly all a somewhat regular radiated form, and some of them are very beautiful, such as Sertularia, which grow in communities and form many beautiful branched objects, much resembling sea-weeds and flowers, hence the name Zoophyte, which means animal flowers. [Illustration: FIG. 13.--EARTH WORM (Lumbricus terrestris), leech (Hirudo medicinale).] The Articulata form the second division of the Invertebrata. It has five classes, the first of which is the Annellata; in this order are included the Worms and Leeches (fig. 13), these have no hard covering but are soft throughout, they are made up of a number of soft rings which encircle their bodies from one end to the other, and are capable of being shortened or elongated by the muscles which draw together the rings (these are placed longitudinally), or by those which contract and widen them, placed circularly; this enables them to move along, and in so peculiar a manner that it has received the name of "vermicular," or worm-like. The most familiar example of this class is the Earth-worm (Lumbricus terrestris), this lowly creature lives in soft mud and clay and bores its way into the ground, in doing so it is assisted by a slimy secretion which covers it, its food consists of earth containing organic matter, this it swallows constantly and extracts what is fit for its nourishment ejecting the remainder apparently unaltered; the curious little heaps of soft dirt which are so often seen in wet weather on the garden-paths, &c., are formed in this way. The Leech is another familiar example of the Annellata or Annelidæ, this little useful creature lives in ponds and stagnant waters, and feeds by sucking the fluids of other animals (chiefly frogs and fish) with which it will gorge itself till it is double its original size, it will then remain (often for months) without food till it is all appropriated, the process going on very slowly; a full-grown Leech is perhaps twenty years old, they grow so slowly, (hence it is, that Leeches may be kept for months after being gorged with blood without any food, and they will live and be healthy).