GASTEROPOD A

the name given to one of the three large cla es into which molluscs are divided and, as the name indicates, all its genera are characterized by the ventral position of the feet. Gasteropods are subdivided into I so pleura, symmetrical, and An i so pleura, unsymmetrical forms. The former contain the simplest and most primitive molluscs, such as Chiton; they are elongated in form, the mouth being at one end, and the anus at the other end of the body, the pedal and visceral nerve cords run parallel to one another the whole length of the body; and the gills, kidneys, genital ducts, and circulatory organs are bilaterally symmetrical; Neomenia and Prone me on i a are small flat forms whose shell consists only of minute plates and spines in the skin; Chaetoderma are more elongated and cylindrical, but their shells are also rudimentary; Chiton has eight dorsal sheli plates, fitted, one behind the other. In the An i so pleura the head and foot are bilateral, but the visceral nerve-cord is twisted, bringing the gills, kidneys, and anus, to the right side; the reproductive organ and genital duct are single. The free-swimming forms known as Hetero peds sometimes acquire a superficial symmetry. 'The Strep ton eur a, or loop-nerved An i so pleura, include Patella, pur a, the dog-whelk, etc.; the Euthyneura, or straight-nerved, include the Colis, etc., and the Pulmonata, Heliz, the snail, Arion, the black slug, etc. Gasteropods are voracious animals, being furnished with powerful -rasping organs which enable them to prey on other marine molluscs, while the terrestrial forms, such as snails, work havoc among flowering plants and vegetables; many of them, whelks, etc., are used for human consumption and as bait. Fo il gasteropods occur in the Cam brian rocks, and many modern types have their origin in Cretaceous times.