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24 HOW THEY BREATHE

Breathing by most aquatic marine snails takes place through the gills where oxygen is obtained from the sea water and where the waste gases are dissolved. The numerous gill leaflets are usually located on the inner side of the mantle. Except in the primitive snails with a pair of gills, water is brought into the mantle cavity through the siphonal canal or through the region to the left of the head. It then bathes the gills and passes out on the right side of the body. The current of water is maintained by thousands of microscopic, lashing, hair-like cilia mostly on the gill leaflets.

Like the bivalves, the snails display a wide variety of types of gills. The most primitive groups, such as some of the Keyhole Limpets, Slit-shells, Pleurotomarias and abalones have two pairs of gills. They are of equal size in the Keyhole Limpets, but in some others the right one is considerably smaller. In the higher groups of snails, the left gill is the only one remaining. In the Cerithidea snails, the gills are reduced to mere stumps, and respiration takes place in the mantle skin itself. The sea slugs have lost their ctenidia but have evolved very complicated and beautiful gill-like organs on the sides and back of their bodies. Many of these gills have taken on the shape of miniature shrubs and trees.

HOW THEY REPRODUCE

The subject of reproduction among the gastropods is a fascinating study of many important phases of biology. Our final concepts of the formation of species, our understanding of zoogeography, distributional methods and the basis of sex determination are dependent on a fuller knowledge of reproduction. The manner of assuring fertilization of eggs, the various methods of egg-laying and brooding of young and the interesting types of larval development are horizons of research that are now being expanded.

The gastropods exhibit nearly every possible modification of sexuality. Two of the three orders of snails, the opisthobranchs containing the sea slugs and the land snail pulmonates, combine a complete set of male and female organs in the same individual. The gonad produces both sperm and eggs, but there are separate ducts for the products of each sex. Despite the dual sex life, all mature individuals experience the mating instincts of both sexes, and during copulation there is a mutual exchange of sperm. In some sea slugs, the tectibranchs, several individuals may form rows or a ring of copulating snails. In some fresh-water pulmonates, self-fertilization is sometimes practiced, and some experimenters have bred over ninety generations, extending over twenty years, without cross-fertilization between individuals.

The marine gastropods contain representatives of several categories of