MOLLUSCA: GASTEROPODA. 415 



Pond-snails, &c., and are usually provided with a well-developed 

 shell, though this may be rudimentary (as in the Slugs), or 

 even wanting. Though formed to breathe air directly, many 

 of the members of this sub- class are capable of inhabiting 

 fresh water. The common Pond-snails are good examples of 

 these last. The condition of the shell varies greatly. Some, 

 such as the common Land-snails, have a well-developed shell, 

 within which the animal can withdraw itself completely. 

 Others, such as the common Slugs (fig. 221) have a rudimen- 



Fig. 221. Limax Smverlyi, one of the Slugs. (After Woodward.) 



tary shell, which is completely concealed within the mantle. 

 Others are entirely destitute of a shell. They are divided into 

 two sections as follows : 



Section I. Inoperculata. Animal not provided with an oper- 

 culum to dose the shell. In this section are included the 

 families Helidda (Land-snails), Limaadce (Slugs), Oncidiada, 

 LimnceidcE (Pond-snails), and Auriculidce. 



Section II. Operculata. Shell closed by an operculum. In this 

 section are included the families Cydostomidce and Adculidce. 



DISTRIBUTION OF THE GASTEROPODA IN SPACE. As a class 

 the Gasteropoda have a world-wide range, some forms being 

 exclusively marine, others inhabiting fresh waters, while others, 

 again, live upon the land. Amongst the Prosobranchiates, the 

 entire order of the Siphonostomata, and the majority of the 

 Holostomata, are marine; but, amongst the latter, the Mela- 

 niadcR and Paludinidce are confined to fresh waters, and the 

 CerithiadcE and Neritida include a number of fresh or brackish 

 water forms. The Opisthobranchiates are exclusively marine, 

 mostly littoral in their habits, but occasionally oceanic. The 

 Heteropoda are exclusively marine and pelagic. Lastly, amongst 

 the Pulmonates many forms (such as the Snails and Slugs) are 

 strictly terrestrial, whilst others (Limncea, Planorbis, Ancylus, 

 &c.) are found in fresh or brackish waters. 



DISTRIBUTION OF THE GASTEROPODA IN TIME. The 

 Gasteropoda are represented in past time from the Lower 

 Silurian rocks up to the present day. Of the Branchifera 



