SUB-KINGDOM MOLLUSCA (Lat. monis, soft). Soft-bodied animals, usually having a shell or exoskeleton, and commonly known as shell-fish. The blood-circulating system is placed near the back, the nervous system near the ventral surface, the alimentary canal lying mostly between the two. When well. developed, the .nervous system con sists of three principal nervous masses or ganglia. There is usually, but not always, a heart, or blood-propelling organ. The digestive system consists of a mouth, cesoplia gus or gullet, stomach and intestine and excretory orifice, though in some the latter organ is absent. The mouth in some is furnished with ciliated tentacles, as in polyzon; in others with two ciliated arms, as in brachiopoda. In the bivalves, or lamellibra raid Oa, the mouth has four membranous palpi; sometimes it has a complicated system of teeth, as in gasteropods and pteropoda. • Generally there are salivary glands, and tie
liver is well developed, pouring the bile into the stomach or commencement of the gut. In the mollus.ca proper, kidneys have also been found. Blood colorless or very slightly tinged. In polyzoa the circulation is effected by the motion of cilia, In tunicata the is tube-like, and propels the blood periodically in either direction. In the higher orders there is always a distinct heart, which is systemic, consisting of an auricle and a ventricle. See MonJusca. This sub-kingdom includes two great divisions called mol luscoida and mollusca proper, both comprising seven principal classes.
DtvisioN A. MoLuiscoma. Nervous system consists of only one ganglion, cr pair with accessory ganglia; heart imperfect or absent. Divided into three classes: Polyzoa, tunicata, and brachiopoda.