424 ANNUAL REPORT SMITHSONIAN INSTITUTION, 193 



sea and in fresh waters; clams, oysters, periwinkles, cuttle fishes, 

 squids are familiar marine mollusks. But there is a group of mollusks 

 that has a history quite as striking as that of the cave fish. This group 

 lives on the land, and sometimes, in very drj'^ situations, even semi- 

 deserts. In this situation the mollusks breathe by lungs instead of 

 gills. How have the land mollusks, the pulmonates, including the 

 snails and slugs, come to live on the land? One explanation that has 

 been offered is that some marine ancestors gradually moved into 

 streams and higher up into ponds which dry up periodically and 

 there became gradually modified to breathe air. The matter is not 

 quite so simple. The river mussels live in streams and ponds that 

 occasionally go dry; they perish under these circumstances by the 

 million; and yet they have never become adapted to land life — the 

 appropriate mutations have never been afforder. Our land snails are 

 the end result of a long series of mutations that have permitted life 



on the land. The first mutations in this 

 direction occurred in certain marine snails 

 with a gill chamber whose opening is so 

 small that it can readily be closed to pre- 

 vent the ingress or egress of water. Any 

 mutation in this direction would enable its 

 possessor to enter into the between-tides 

 zone. 



Actualh', there live on our shores shore- 

 snails, belonging to the family of Lit- 

 torinidae, inhabiting a level where they 

 are exposed to air for 12 to 20 hours of 

 the day. Indeed, among the Littorinas 



FiGUKE 3. — Lrttorina Utorcn n i • ^ i • «• 



one finds species that diner greatly in 

 their emancipation from the sea. During the recession of the tides 

 the Littorina keeps the opening to its mantel chamber closed ; so its 

 gills are held in a medium saturated air. (Fig. 3.) 



Now, it is out of this general group to which the Littorinas belong 

 that the land snails have arisen. Any continued mutation in the 

 direction of gill reduction or enclosure of the mantle cavit}^ would 

 have been in the direction that would have permitted the possessor 

 to pass to dryer parts of the shore line and, incidentally, to escape 

 from its enemies. The point is that not just any group of marine 

 snails was able to adjust itself to land life, but onl}^ a group in which 

 favorable mutations arose. The land crabs and the lung fishes rep- 

 resent the end stages of a similar evolutionary history to that of the 

 land snails. 



While it appears from these considerations probable that adjust- 

 ment to extreme conditions of life has been rendered possible by 



