download pdf

24 American Seashells
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 reproduc-
tion. 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 copu-
lating snails. In some fresh-water pulmonates, self-fertilization is sometimes
practiced, and some experimenters liave bred over ninety generations, extend-
ing over twenty years, without cross-fertilization between individuals.
The marine gastropods contain representatives of several categories of