GASTEROPODS 349 
rapid. To judge also from the very large number of extinct fam- 
ilies among the cephalopods, which once flourished in astonishing 
abundance, it would appear that they are a dying race. 
The classification of the Gasteropoda is primarily a division 
into three great orders: the Opisthobranchiata, the Prosobran- 
chiata, and the Pulmonata. The Pulmonata, which include the 
land and fresh-water gasteropod shells, are characterized by the 
presence of a breathing-sac, or lung, instead of gills. They are, 
therefore, essentially air-breathers, and would perish if kept too 
long under water. The Opisthobranchiata and Prosobranchiata 
are marine, and, like all animals which breathe by means of gills, 
soon die when removed from the water. We have already noted, 
however, how some genera of marine gasteropods that find their 
station about high-tide mark are able to live for considerable 
periods out of the water, notwithstanding the fact that they are 
fully equipped with gills and not possessed of lungs. Indeed, 
there is one large family of prosobranchs (the Cyclostomatide) 
that has become entirely terrestrial in habit, its members having 
lost their gills and acquired lungs, but their organization other- 
wise is so essentially that of the prosobranchs that they have 
never been considered as pulmonates. 
The main difference between these two orders of marine gas- 
teropods is that in the prosobranchs the breathing-organs (the 
gills) are placed in a position forward of the heart, and the de- 
gree of torsion (page 330) characteristic of this molluscan class has 
been continued until the auricle of the heart is in front of the 
ventricle. There is always a shell, usually spiral, and, with few 
exceptions, an operculum. In the opisthobranchs, on the other 
hand, the relative position of the heart and gills is reversed, and 
they further differ from the prosobranchs in that the sexes are 
always united in each individual. The opisthobranchs are not al- 
ways provided with a shell. One division of the order, known as 
the “nudibranchs,” are entirely naked. These are commonly called 
the “sea-slugs,” and are to be found crawling about the marine 
vegetation in shallow water, in tide-pools, and on the piling of old 
docks. Curiously enough, the sea-slugs (see page 354) have no gills 
at all, but, having lost these apparently essential organs, are ena- 
