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 Puhnonata, 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 Opixtliolranchiata and Prosdbranchinia 

 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 CyclostomatidcB) 

 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 " nudibraiiehs," 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- 



