350 MARINE INVERTEBRATES 



bled to " breathe water " through their skin. They are wonder- 

 fully beautiful creatures, and the collector must not fail to secure 

 some specimens, put them into a jar of sea-water, and watch 

 them expand. 



There are other more superficial differences between the proso- 

 branchs and the opisthobranchs, which will enable even a beginner 

 to distinguish them at a glance. When the latter are possessed 

 of an external shell, it is bulbous, generally glassy, and with a 

 simple lip, the aperture extending the entire length of the shell. 

 Again, the mantle or the propodium of the foot is greatly ex- 

 tended and usually covers the shell almost wholly. 



With the exception of the nudibranchs, which are common all 

 along the Atlantic shore, especially north of Cape Cod, there are 

 very few opisthobranchs to be found in American waters. Their 

 shells are not very abundantly found anywhere, though in Florida 

 there are two or three species which an untrained collector might 

 discover. 



Practically all the gasteropod or univalve shells that will be 

 taken by the ordinarily expert collector along the shore are proso- 

 branchs. This order includes the great majority of marine gastero- 

 pods, and is entitled to the distinction of claiming, in the great 

 number of its genera and species, the most startling eccentricities 

 of form and color to be found among the Mollusca, if not among 

 all marine invertebrates. 



Only those species which are actually abundant on the Atlantic 

 and Pacific shores of the United States, and which are likely to be 

 found by the untrained collector, are selected from the long list 

 of forms which belong to the three faunal regions involved. 



ORDER OPISTHOBRANCHIATA 

 SUBORDER TECTIBRANCHIATA 



To avoid confusion, it is well to keep the systematic table in 

 view : 



Class Orders Suborders 



( OPISTHOBRANCHIATA S TECTIBRANCHIATA 



GASTEROPODA ] PROSOBBANCHIATA I N UDIBRANCHIATA 



PULMONATA 



