434 TEXT-BOOK OF ZOOLOGY 



In the Pond Snails (Limncea) the shells are pointed, while in Plan- 

 orbis they are flat like a dinner-plate. In Germany these snails are 

 popularly known as " post-horns." 



ORDER II.: GILL-BREATHING GASTROPODS 

 (PROSOBRANCHIATA). 



The gill-breathing gasteropods are throughout aquatic in their habits. 



Among the few freshwater species, the commonest is the River Snail 

 (Paludina). Externally it bears a considerable amount of resemblance 

 to the aquatic species of Pulmonate Gastropoda (which see), but the 

 animal is capable of closing its shell with a lid, or operculum, developed 

 on the posterior portion of the foot. Even inhabitants of inland districts 

 are familiar with the rich variety of marine snails, whose many-coloured 

 shells (freed from their superficial skin and polished) serve as ornaments 

 or are turned into various articles of use, etc. The strength of the shell 

 is invariably a sign that its former inhabitant lived on the surf or on 

 shoals, where it was exposed to the full force of the waves. 



This is seen in the case of the Periwinkle (Litorina), which is 



common everywhere on the shores of the North Sea and Baltic, and 



still more so in the case of the large foreign species, especially the 



gigantic Wing-Shell (Strombus gigas). This huge creature inhabits 



the coral reefs of the West Indian Islands. Its 



shell is much used as an ornamental border for 



flower-beds. 



Hard as marble, again, are the well-known, 

 variously coloured shells of the Cowries 

 {Cyprcea), the shell of one of which, C. moneta, 

 Shell of Cowry. (Slightly i s > as i g weu known, used as money in the in- 

 less than natural size.) terior of Africa. 



° n oT ^^S^Antt:; From secretion of several Mediterranean 



view. species, which is white, but turns red when 



exposed to the air, a much-esteemed purple dye 



used to be prepared in ancient times. It was more particularly the 



Spiny Murex {Murex brandaris) which furnished this valuable substance. 



ORDER III.: WINGED GASTROPODS (PTEROPODA). 



These small molluscs are inhabitants of the open ocean, and, like 

 many other free-swimming marine animals (see jelly-fish), are possessed 

 of a jelly-like soft body of glassy transparency. In accordance with 

 their habitat, the foot of these animals has been modified to a swimming 

 organ. It consists of two lateral lobes which are moved up and down 



