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 devel- 

 opment 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 



