270 ZOOLOGY. 



Marine shells in time date back to the lowest Silurian 

 period ; such are Maclurea, Holopea, Murchisonia, Pleuro- 

 tomaria, etc., which occur fossil in rocks of the Potsdam 

 period. The Palaeozoic 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 Cephalopliora known, 

 of which 7000 are fossil. There are 6500 species of Pulmo- 

 nafa. 



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 Opistliobrancliiata. 

 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 supraoesophageal ganglia forming part of 

 an oesophageal 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 sensory 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 by Glaus, Boll, 

 Flemming, and others. The odontophore is well developed ; 



