With regard to the internal organs we note the constant presence of three pairs of ganglia, viz., the brain, lying above the pharynx, which sends nerves to the feelers, eyes, and auditory organs; the pedal ganglion, which sends nerves to the foot, and the visceral ganglion, which sends nerves to the viscera. This is a condition of the nervous system characteristic of all molluscs. The heart is a well-developed pulsating sac in the upper part of the body composed of either two or three chambers, and there is a well-defined closed system of arteries and veins, specially complete in the cuttlefishes and octopi. This highly developed condition of the circulatory system also distinguishes the molluscs from the other invertebrates. =Development.=--Reproduction among the molluscs is always sexual. Multiplication by budding or by the parthenogenetic production of eggs is not known to occur. The eggs are usually laid in a mass held together by a gelatinous substance. In most species the young mollusc on hatching from the egg does not resemble its parent, but is a free-swimming larva called a _veliger_. It is provided with cilia for organs of locomotion. It must undergo a radical change in order to reach the adult stage. Thus metamorphosis occurs in this branch as well as among the Arthropods and Echinoderms. In the development of some molluscs, however, there is little or no metamorphosis, the young being hatched in a condition much resembling, except in size, the parent. Some of the special characteristics of structure, life-history, and habits of the molluscs will be noted in our consideration of the various kinds. =Classification.=--The branch Mollusca is divided into five classes, three of which include the more familiar kinds. These three classes are the Pelecypoda, including the mussels, cockles, clams, scallops, oysters, etc., molluscs with a shell composed of two pieces, one on each side of the body and hinged together; the Gastropoda, including the snails, slugs, periwinkles, whelks, and a host of other univalved shell-fish, that is, molluscs which have a shell composed of a single piece; and the Cephalopoda, including the squids, cuttlefishes, octopi, and the pearly nautilus. =Clams, scallops, and oysters (Pelecypoda).=--TECHNICAL NOTE.--Shells of scallops, oysters, and sea-mussels should be had for examination; also specimens of _Teredo_ or _Pholas_ in alcohol or formalin, and pieces of pile bored by _Teredo_. Make drawings of various bivalve shells, and of _Teredo_. [Illustration: FIG. 103.--Dissection of fresh-water mussel, _Unio_ sp.]