"THE COMING OF A BACKBONE 21 



they will meet in shores. He has apparently conformed to 

 a vast, favorable, unchanging, everlasting environment. 



Our sketch of the clam has given us but a partial and one- 

 sided view of molluscan history. The clam represents only 

 one of three great subdivisions or classes of the whole sub- 

 kingdom. The second class of mollusks, the gasteropods, in- 

 cludes the recent forms having a single spiral or conical shell, 

 or having the foot in the form of a creeping disk. They gen- 

 erally crawl on the sea-bottom. They have spread into 

 brackish and even into fresh water. Some of them have used 

 the sack or chamber which contained the gills as a sort of lung, 

 have breathed air and emerged on land. Our snails are 

 descended from such forms. Some have taken possession of 

 cracks and fissures in the rocks, or have worked their way 

 deep into the mass of fragments under limestone cliffs. 

 Slugs are snails which have given up the cumbrous shell as 

 no longer needed for protection. 



All these forms are most at home in damp or wet localities. 

 They still betray their origin from aquatic ancestors. It 

 need not surprise us that some of the air-breathing mollusks 

 returned to the water again, and became pond-snails. 



Some of the marine gasteropods have lost the shell, so the 

 sea-slugs, nudibranchs. But even among the more typical 

 gasteropods some have returned from the crawling life of 

 primitive mollusks to the swimming habit of still more prim- 

 itive ancestors. The most noteworthy of these are the ptero- 

 pods. They have either greatly lightened or completely lost 

 the shell, have changed the broad creeping disk into a pair of 

 winglike fins by means of which some are said to dart through 

 the water much as butterflies glide through the air. One 

 group of these pteropods has retained a light shell. Its mem- 

 bers feed on the small plants and animals which swarm in the 

 ocean. They also attained an exceedingly wide and stable en- 

 vironment and have held their place since the beginning of 

 palaeozoic time. 



The second division has given up the shell, and its mem- 

 bers lead a carniverous hawklike life feeding on their shelled 



