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 ; TECTIBRANCHIATA 
GASTEROPODA PROSOBRANCHIATA NUDIBRANCHIATA 
PULMONATA 
