_Vermes_ (worms). Animals having head and tail composed of segments. The digestive organ is tubular, and the nervous system a double chain of ganglia[4] on the ventral[5] surface. There are six classes of vermes. The animals differ greatly in appearance. CLASS I.—_Flat worms_ are best known as the parasites that infest animals, such as the liver fluke of the sheep, and the tapeworm. The flat worms pass through a very peculiar metamorphosis, some varieties taking as many as seven different forms. CLASS II.—_Round or Thread Worms_ are represented by the pin worm and _Trichina_. The latter is the dangerous worm which finds its way into the human system from pork flesh, in which it is imbedded. CLASS III.—_Wheel Animalculæ_, or _Rotifera_. A most interesting microscopic worm, abounding in fresh water and in the ocean. They will remain dried up for years, and then recover life. Their shapes are very peculiar. CLASS IV.—_Moss Animals_, or _Polyzoa_, are the animals which form a coral-like shell. They are abundant on the seashore, and are called sea mosses. CLASS V.—_Lamp Shells_ (_Brachiopoda_). These worms are marine, and form a bivalve shell, the valves being on either side of the body. The body has long arms on one side of the mouth, which bear fringes; the motion of the fringes draws food into the mouth. They are also used in respiration. But few species of the Brachiopods are now living, though they were once very plenty. CLASS VI.—_Annelidæ._ This last class includes the leeches, a flat worm, whose body is divided into segments; the earth or angleworm, a familiar worm of many segments, and the marine worms. Each segment of the latter bears clusters of bristles, used in swimming. SUB-KINGDOM VI. _Mollusca_ (soft). General characteristics—Mollusks, a numerous branch of the animal kingdom, are so called from the softness of their bodies, which usually have no internal skeleton or framework to support them. They are covered with a tough, muscular skin, and generally protected by a shell. They have a gangliated nervous system, in some cases well developed, the medullary[6] mass not enclosed in a cranium or spinal column—of which they are mostly destitute—but distributed more or less irregularly through the body. They have hearts, and an imperfect circulative system, the blood being pale or blueish. Some breathe in air, some in fresh, more in sea water. Some—both of those on land and those in water—are naked, others have a calcareous covering or shell. The larger marine mollusks are often guarded by very strong, heavy shells. Some are viviparous, others oviparous, and multiply rapidly. [Illustration: SNAIL.] Three general, and many subdivisions are recognized. The total number of living species is said to exceed twenty thousand, of which only a few can be mentioned. The classes under this division are _Lamellibranchiata_, _Gasteropoda_, and _Cephalopoda_. The chief representatives of the first class are all ordinary bivalves.