250 ZOOLOGY. 
Marine shells in time date back to the lowest Silurian 
period ; such are Maclurea, Holopea, Murchisonia, Pleuro- 
tomeria, etc., which occur fossil in rocks of the Potsdam 
period. The Paleozoic Gastropods are few in number com- 
pared with those occurring in Cretaceous and especially Ter- 
tiary formations. 
The earliest land-snails occurred in the Coal Period; the 
living species are exceedingly numerous, and often much re- 
stricted in range, especially in the tropics; the arctic forms 
are very scarce, but four or five species occurring in Green- 
land. There are over 22,000 species of Cephalophora known, 
of which 7000 are fossil. There are 6500 species of Pulmo- 
nata. 
Subclass 4. Heteropoda.—The Heteropods form a distinct 
subclass, the systematic position of which was for a long 
time unsettled ; but they are now classed among the Gas- 
tropods, being in fact related to the Opisthobranchiata. 
Their most striking peculiarity is the form of their foot, 
the anterior and middle portions of which are expanded to 
form a leaf-like fin, which often bears a sucker; the pos- 
terior part of the foot is much elongated, and, reaching far 
backwards, appears to form a tail-like continuation of the 
body. The Heteropods are more or less transparent, and 
are found swimming upon the surface of the ocean, upon 
their backs with their foot upwards. The shell may or may 
not be developed ; when present it may be either simple or 
coiled. The nervous system resembles closely that of the 
true Gastropods, but is more highly developed; the brain 
consists of several supracesophageal ganglia forming part of 
an esophageal ring. From the brain arise the optic and 
auditory nerves. The two large eyes lie in special capsules 
near the feelers, and are movable by several muscles. The 
otocysts are also large, and contain a large spherical otolith. 
The otocysts are lined by an epithelium with bundles of 
long vibratile hairs, and with a cluster of sensorv cells, form- 
ing a macula acustica. Organs of touch have also been 
described. The sensory apparatus of the Heteropods are 
highly specialized, and have been studied bv Claus, Boll, 
Flemming, and others. The odontophore 1s well developed ; 
