CLASS OF GASTEROPODA. 33 



LESSON III. 



Class of Gasteropoda. — General Conformation — Classifi^ 



cation. 

 Order of Gasteropoda Pulmonea. — Organization — Division 



into two families. 

 Family of Terrestrial Pulmonea. — Limax — Vaginulus — 



Helix — Bidimiis — Pupa — Chondrus — Clausilia — Achatina. 

 Family of Aquatic Pulmonea. — Onchidium — Planorbis — 



LimncBus — Physa — Auricula. 

 Order of Gasteropoda Pectinibranchiata. — Organization 



— Classification. 

 Family of Trochoides. — Trochus — Turbo— Delphinula — 



— Turritella — Cyclostoma — Valvafa — Palvdina — Littorina 



— Monodon — Phasianella — Ampullaria — Janthina — Nerita, 



CZ^ASS OF GASTEROPODA. 



1. Mollusks that are provided with a head, and crawl upon the 

 belly, or swim by means of a fin formed of this part of the body, 

 constitute a very numerous class, the type of which is the slug 

 and snail. The body is elongated, and terminated anteriorly by 

 a more or less developed head, which ordinarily has tentacula 

 inserted above the mouth ; the back is furnished with a mantle, 

 which extends more or less, and the belly is covered by a 

 fleshy mass, generally in form of a broad disk (fig. 18), and 

 serves the animal for crawling on the ground, but which is some- 

 times very much 

 compressed, and 

 then constitutes a 

 vertical fin {fig. 

 78). Many of these 

 animals are en- 

 tirely naked; 

 others have an in- 

 ternal shell con- 

 cealed in the thick- 

 ness of the mantle; 

 but most of them 

 have an external Fig. 18. — limn^ — fresh-water snail. 



shell, and this 



calcareous envelope is almost always sufficiently large to contain 

 them entirely, and aflbrd them protection. 



1. What is the general character of the class of ga'steropods ? Have they- 

 any shell ? 



