GEOGRAPHICAL DISTRIBUTION. I 35 



The mode of existence of the Glossopliora is compactly summarized as follows 

 by Zittel (translation from " Handbuch der Paljeontologie," vol. 11. p. 161) : The 

 greater number of Glossophora are aquatic animals, and the majority marine. 

 The Pteropods, Placophors, Heteropods, Opisthobranchs, live exclusively in 

 the sea. The great order of Prosobranchs comprise also a majority of marine 

 forms. There are a certain number which live in brackish water near the dis- 

 charge of lakes (Potamides, Neritinas, Rissoas, Hydrobias), and others ia 

 fresh water (Paludinid.ne, Melaniida;, Valvatidae). The Pulmonate genera, fur- 

 nished with gills, are adapted to a terrestrial life (Cyclostomidae, Helicinida;). 

 The Pteropods and Heteropods are pelagic animals, free swimmers, inhabiting 

 the open sea ; the great part of the other Glossophora are coast animals, crawl- 

 ing upon plants, rocks, and shore debris. Some Prosobranchs are amphibious 

 (Littorina, Truncatella, Patella, Nerita), and are able to live a long time dry, 

 without water ; they then retire within their shell, close the operculum, and 

 breathe the water which they have retained with them. The Ampullarians 

 have the advantage of two different kinds of respiratory organs, and can live 

 alike on land and in water. Some Prosobranchs bore in the sand and mud like 

 Lamellibranchs (Oliva, Mitra, Natica, Buccinum) ; others inhabit coral reefs, 

 or live as parasites in other animals (Entoconcha, Stylifer). The shells of fresh- 

 water Gastropods are generally covered with a greenish-olive or brown epidermis, 

 their apices are often broken or absorbed ; their shell is thin and horny (Lim- 

 naeus). Many Gastropods subsist on fresh flesh or carrion ; there are some 

 which perforate shells of other mollusks with their tongue, and devour them 

 through the little hole thus perforated (Natica, Murex, Buccinum); the majority 

 of Glossophora (almost all the Pulmonates, and the holostomate Prosobranchs) 

 live upon vegetable food. 



Their geographical distribution is little known except for the littoral, fluvia- 

 tile, and terrestrial species ; it is known, however, that the Pteropods and 

 Heteropods, being pelagic, have a very extended distribution ; the Scaphopods 

 and the Placophors are equally found in all seas and all latitudes. There are 

 only a few pelagic forms among the Opisthobranchs and the Prosobranchs. 

 The geographical distribution of the marine Glossophora, besides the influence 

 of centres of origin, is determined greatly by the character of the bottom, the 

 form of the coast, the flux and refiux of tides, the currents, and the saltness and 

 depth of the water. The sandy shores are little favorable to Gastropods ; they 

 prefer rocky shores, where algae flourish. The shores much cut up furnish great 

 variety of conditions of habitat, and accordingly have a richer fauna than great 

 estuaries. The movements of the tide produce changes and bring in food, and 

 thus favor life. There are currents, also, which greatly affect geographical dis- 

 tribution. Most of the marine Glossophora die as soon as they are transported 

 into fresh water. There are, however, some which have the faculty of adapting 

 themselves to change of medium. Such notably are certain species of the genera 

 Patella, Rissoa, Trochus, Purpura, Littorina, and Cerithium. Some fresh-water 

 species, conversely, are able to live in salt water (Limnaeus, Planorbis, 

 Melania, Melanopsis, Physa, Neritina). It is probable that all the actual ter- 

 restrial or fluviatile species are traceable to a common origin, and that they de- 

 scended from marine types of the geological epochs, modified by adaptation. 



The temperature has a great influence upon the development of the Glos- 

 sophora : heat is favorable to them, and they are much more abundant in the 

 seas and lands of tropical regions than in temperate or polar regions. The 

 marine bathymetric zones, as the hypsometric zones on land, exercise their influ- 



