GASTEOPODA. 



13 



and the external shelly coat is generally brightly coloured and 

 highly ornamented. Several very pretty species are found on our 

 own shores. The opercula of Turbo petholatus (Fig. 6), from the 

 Indian and Pacific Oceans, are frequently mounted in gold and 

 silver as scarf-pins, ear-rings, &c. 



The ISTerites are mostly found in tropical countries, and, like the Cases 

 Winkles, are very strongly made, to resist the force of the breaking i^~20. 

 waves. The Neritinas are found both in the sea, and in fresh 

 water, and are less solid shells. The third section of Neritidm, 

 the Septarke, are shaped very much like Limpets, except that the 

 apex is at one end instead of central. They are, however, very 

 different animals, and furnished with a shelly operculum imbedded 

 in the foot. 



Order 2. — Pectinibkanchia. 



In most cases the molluscs of this order have an attached Cases 

 monopectinate gill and a single osphradium. A few are fresh or " " 

 brackish water forms, but the majority are marine. 



The "River-Snails" (Vwiparidce) might be termed freshwater Case 22. 

 Periwinkles, as the animals of both are very similar. The true 



Fig. 7. 



The Common British Eiver-Snail (Vivipara vivipara). 

 a, head ; h, tentacles ; c, eyes ; d, foot ; e, operculum. 



Viviparce are viviparous. They are rather sluggish, and found at 

 the bottom of ponds and rivers feeding on decaying animal and 

 vegetable matter. 



The Gydophoridce are land-shells, which, however, cannot properly Cases 

 be considered true lung-breathers like ordinary snails. They have 

 not the closed lung-chamber of the Pulmonates, their eyes are 



23-25. 



