= gts 
THE GASTROPODS 
Of the above six classes of mollusks, only members of the Gastropoda are 
of concern for purposes ofthis publication and are presented in added detail. 
Further discussion of the other classes is omitted. 
The three subclasses of gastropods have been named in reference to the 
position or character of the respiratory apparatus. In the Prosobranchiata 
the breathing organ or gill is situated in front of the heart; in the Opistho- 
branchiata it is behind the heart. The Pulmonata have replaced the gill by 
a vascular lung which may breathe either air or water depending on the 
habits of the particular species. Some prosobranchs and opisthobranchs 
have also lost their gills, but because of other details of their anatomy they 
are retained in their respective groups. The opisthobranchs are all marine, 
but both the prosobranchs and pulmonates have representatives in fresh and 
salt water, and on land. The pulmonates have very few marine or brackish- 
water species; the prosobranchs have relatively few land species. 
In addition to respiratory and other anatomical differences, the pulmonates 
and prosobranchs usually can be distinguished by the presence or absence 
of an operculum or cover used to close the aperture when the snails are 
withdrawn into their shells. Most-prosobranchs have an operculum, but 
only the marine genus Amphibola of the pulmonates has such a protective 
structure, 
The prosobranch snails discussed in this publication represent two orders. 
The operculate land snail Helicina zephyrina belongs to the primitive 
Archeogastropoda and the fresh-water operculates (Parafossarulus, Poma- 
tiopsis, Melanoides, Tarebia, and Semisulcospira) to the large, extremely 
diverse, and widespread order Mesogastropoda. 
The pulmonate snails are divided into three large groups or orders - the 
Stylommatophora, the Systellommatophora, and the Basommatophora, 
Since each is represented by species of economic importance, they will be 
considered briefly. The largest order is that of the land snails and slugs, 
the Stylommatophora, or Geophila, This group is characterized by animals 
with eyes at the tip of the upper pair of tentacles, which are retractile, that 
is, can be inverted like a rubber glove when the fingers are withdrawn. The 
Systellommatophora, or Gymnophila, comprise a small group of tropical 
slugs (for example, Veronicella) with contractile (not inversible) tentacles 
with eyes at the tips of the upper pair. The fresh water pulmonate snails 
(such as, Fossaria, Bulinus, Physopsis, Biomphalaria, Planorbina, and 
Armigerus) also have contractile tentacles, but the eyes are situated at 
their bases, These snails are placed in the Order Basommatophora, or 
Limnophila. 
