eight legs, and a pair of claws like a lobster, the body is lengthened into a sort of tail, at the end of which is a claw or sting, like the poison fang of a serpent, with a poison bag at its root; its effects are often fatal to man, and always so to the insects and other small animals upon which the Scorpion feeds, its usual mode of killing which, is to catch them with its claws and then to sting them to death. The Centipede is a scarcely less frightful creature; in South America and the West Indies it is sometimes found to grow to eight or ten inches in length, it is of a long jointed form, with generally twenty-one pairs of legs, although some have double this number; its bite is much to be dreaded, as it is poisonous, especially the "Scolopendræ," which have a poison gland, like the snake; in this country they are very small indeed, and are generally found in moist places. The Centipedes and Millipedes are, by some, placed in the class Arachnida, but by others in a separate class called "Myriopoda," divided into two orders, the Chilognatha, which have rounded cylindrical bodies, and generally more than one pair of legs to each segment of the body, and the Chilopoda, which have a distinct flattened head and but one pair of legs to each segment. [Illustration: FIG. 33.--MOLLUSCA. 1, Nautilus (Argonauta); 2, Clio Borealis; 3, Mussel (Mytilus edule).] The last of the three great divisions of the Invertebrata is formed by the Mollusca; it is divided into six classes, (see page 218). The Mollusca are characterised by having no internal skeleton, nor external horny case, as in the Articulata; they are rather soft, and either void of solid covering, or possessed of "shells" composed chiefly of earthy matter (chalk), and in one or two pieces, called valves, hence the names, uni-valve and bi-valve; all shells of this description belong to the Mollusca, some of which have no means of locomotion, as Oysters; others have a "foot" covered with a muscular expansion, called the "mantle," by which they glide onwards, as the Snail. 1. The Tunicata have no shell or hard covering, but are of a pretty firm consistence; they are either fixed to rocks and sea-weeds, or float about freely, and are either solitary, social, or compound. The "Ascidians" are united in groups, and are all connected by a common stalk or "Stolon," from which they grow by buds.