LECTURES ON MOLLUSCA. 23 



cerning the class of a cephalopod if he saw it alive : for though star- 

 fish and polypes, as well asBryozoa, have a central mouth surrounded 

 by arms or feelers, the great eyes and funnel, as well as the soft but 

 muscular body, would at once assign its position. It is not so with 

 the Gasteropods. To say nothing of the different shapes of the shell, 

 as e. g. in Chiton, Dentalium, Patella, Trochus, Vermetus, Cypra>a, 

 Murex, and Carinaria, the shapes of the animals are so very unlike 

 that even now naturalists are not agreed as to the limits of the 

 class ; still less on the arrangement of its fundamental divisions ; least 

 of all, on the position of particular families and genera. This should 

 by no means discourage the student ; but on the contrary fill him 

 with zeal to prosecute a study in which so many unworked materials 

 are within his own reach ; and in which, therefore, instead of merely 

 following at a remote distance in the steps of the learned, he may, 

 without neglecting the main duties of his life, add materially to the 

 stores of human knowledge, and even throw important light on the 

 dark places of our planet's ancient history. 



CLASS GASTEROPODA; 



that is, belly-footed animals, or crawlers: comprising snails, periivin- 

 Ides , ivhelks , limpets, and " univalve shell-fish' ; generally. 



These creati^es form three-fourths of the whole number of mollusks. 

 They inhabit sea-shores, and the sea-bottoms, down to the lowest 

 depths of ordinary animal life : they are found swimming in the open 

 seas, or accompanying the floating gulf weed : or they live in fresh 

 waters, crawling on stones or aquatic plants. Lastly, they are found 

 on dry land, in all kinds of situations where lime exists ; either in 

 damp and marshy places, or in rocky deserts ; either burrowing in 

 earth or crevices, or creeping on the vegetation of forests, herbage, or 

 lichen-covered stones, One cannot live anywhere, therefore, where 

 crawling mollusks are not within our reach. The following classifi- 

 cation may aid us in understanding these many-shaped creatures : 



Class. Stib-classes. Orders. Examples. 



f PECTINIBRANCHS... Whelks, Cones, Strombs, Cowries, Peri- 



PROSOBRANCHS . . . <j g CCTIBRAHCHg Limpets, Chitons, Sea-ears, Topsbells. 



I CIRROBRANCHS Tooth Shells. 



PULMONATES Snails. 



OPISTHOBRANCHS \ TECTIBRANCHS. ..Bullas, Sea Hares, Umbrellas. 



^ NUDIBRANCHS. . . . . DoriSj Eolis, &c* 

 NUCLEOBRANCHS Carinaria , Janthina. 



In the PROSOBRANCHS, the breathing cavity is at the back of the 

 head, in advance of the heart. There is always a distinct shell, which 

 generally covers the animal. They form two principal groups, (1) 

 the Pectinibranchs, in which the gill is comb-shaped,, and the animal 

 unisexual : and (2) the Scutibranchs, in which the gills are in plates, 

 like the bivalves, and the animal has the sexes united. The Cirro- 

 branchs are a small and very aberrant group. 



In the OPISTHOBRANCHS, the gills are behind the heart, and very 

 variable in position and structure. There is no shell, except in a few 

 families of the Tectibranchs, in which the gills are covered by the 



GASTEROPODS. 



