THE MOLLUSCA 101 



the progress of the mollusc. The plate is usually of a horny sub- 

 stance, but may be stony or shelly ; many are very beautiful 

 objects, and are sometimes fashioned into trinkets. 



The Gastropoda are divided into four classes, i. The Proso- 

 branchiata includes all the univalve shells which breathe by means 

 of pectinated or plume-like gills contained in a vaulted chamber, 

 formed by the mantle, at the back of the animal's head, in front 

 of the heart. 



2. The Amphineura are symmetrical Molluscs. The shell is 

 never spiral, but, when present, is composed of eight plates which 

 overlap one another like tiles on the roof of a house. The gills 

 are arranged in pairs, and are similar on both sides of the body. 

 This class includes the Chitons and certain worm-like Molluscs 

 which have no shell, but have the upper surface of the body 

 studded with minute spicules or spines. 



3. The Opisthobranchiata, often called Sea-slugs, have usually 

 no shell ; when present it is either small or rudimentary and 

 more or less concealed by the mantle. The gills are placed behind 

 the heart. 



4. The Pulmonata. Air-breathing Molluscs provided with a 

 lung-cavity in place of the ordinary gill. To this class belong 

 the Land Snails and Slugs and the Fresh-water Snails. 



Far the greater number of the marine Gastropods are dwellers 

 in the littoral zone or the shallow waters just beyond, and with 

 the exception of the extreme Arctic and Antarctic regions there is 

 hardly a part of the coast all over the world where some of these 

 strange and interesting creatures are not to be found. The warmer 

 parts of the globe are richest in number and variety of species and 

 in the large size and brilliant colouring of the shells. The beauti- 

 fully marked and tinted Cones, Olives, and Volutes, beloved of 

 shell collectors, the brilliant Orange and Tiger Cowries, the huge 

 Wing and Fountain Shells, the Helmet Shells, Mitras Shells, and 

 the curious Scorpion Shells are all dwellers in tropical or sub- 

 tropical seas. 



The Wing Shells (Strombidce) are the largest Gastropods possess- 

 ing a snout, or proboscis, which is non-retractile. These molluscs 

 do not crawl with the usually slow, gliding movement, but pro- 

 gress through the shallow water by a series of curious jerks. They 

 are very active and great scavengers, feeding on dead fish and 



