CLASS OF GASTEROPODA. 33 



LESSON III. 



CLASS or GASTEROPODA. General Conformation Classifi- 

 cation. 



ORDER OF GASTEROPODA PULMONEA. Organization Division 

 into two families. 



FAMILY OF TERRESTRIAL PULMONEA. Limax Vaginvlus 

 Helix Bulimus Pupa Chondrus Clausilia Achatina. 



FAMILY OF AQUATIC PULMONEA. Onchidium Planorbis 

 Limncevs Physa Auricula. 



ORDER OF GASTEROPODA PECTINIBRANCHIATA. Organization 

 Classification. 



FAMILY OF TROCHOIDES. Trochus Turbo Dclphinula 

 Turritella Cyclostoma Valvata Paludinn Liftorina 

 Monodon Phasianella Ampullaria Janthina Nerita. 



CLASS 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^E FRESH- WATER SNAIL. 



shell, and this 



calcareous envelope is almost always sufficiently large to contain 

 them entirely, and afford them protection. 



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

 any shell? 



