The third class (Hirudinidæ) of Annulata is that of the leeches, those ugly, but useful, worms of land and sea. In spite of their sluglike appearance the leeches are segmented worms, although the wrinkles on their gray, mottled skins do not indicate the position of the segments beneath. The mouth on the under side of the head is armed with jaws and sharp teeth that make three or more cuts through the skin, whence the blood is sucked; there is also a holding sucker near the tail. Their attacks cause little pain, and that fact has led physicians to put them into use when bleeding is required. The eggs of leeches are laid in moist earth in little packets, and hatch in five or six weeks. The growth to maturity is slow, and continues during a long life. Many species abound in ponds and stagnant waters. Asia has terrestrial leeches, swarming in moist vegetation; and in Ceylon the minute leeches are a terrible plague in certain regions. Many also are wholly marine. Some of the larger forms attack fishes directly, and quickly kill them by sucking their blood away; others are true parasites. On the other hand the leeches of our lakes are fed on by the whitefish and similar fishes. They are a great pest to our fresh-water turtles. CHAPTER VIII BUILDERS OF THE PEARLY SHELLS The mollusks, or "shellfish" (phylum Mollusca) are a homogeneous group of soft-bodied, unsegmented, typically bilateral, elaborately organized animals, mainly aquatic and marine, whose origin--probably as a derivative from a wormlike stock--is lost in the mists of geologic prehistory. In most cases the mollusks secrete from a larval gland an external shell which serves as skeleton and defensive armor; are bisexual and produce eggs, or if monoecious are never self-fertilizing. They possess a heart, and blood circulation (usually colorless); breathe in the water by means of gills, or, in the air, by a primitive kind of lung; have a nervous system and senses in some cases of a high order; the organs are normally paired, and protected by a general covering integument called the "mantle"; and the creeping species move by a muscular, elastic, ventral organ styled the "foot," while the swimmers are provided with a variety of swimming organs. Mollusks vary in size from all but microscopic minuteness to a bivalve weighing 500 pounds or a squid half as big as a right whale. They occur in all seas at all depths, abound in fresh waters both swift and stagnant, and are scattered over the earth wherever vegetation flourishes. The phylum Mollusca is divided into five classes, as follows, and it will be noticed that four of the names refer to the locomotive organ or "foot" (Greek _pous_, "foot"):