GASTEROPODA 83 



The helmet-shells {Cassid'idce) are composed of layers of different colored 

 material and are used for carving cameos. 



The tritons or sea conchs {T riton' idee) have handsome shells, frequently 

 more than a foot in length. The shells of one species is used by the South Sea 

 Islanders as a trumpet. The Triiott'idae have a proboscis, a well-developed 

 siphon, and a short foot. 



" The long, nearly cylindric shells of the CavolinidEe make up much of 

 the ■ pteropod ooze ' of the deep seas." 



The common periwinkle (Liitori' na) (see Fig. 57, p. 78) abounds on the 

 coast of New England and southward, where it is used as food. It is 

 a native of Europe. It is a vegetable feeder, and is valuable in cleaning 

 up the seaweeds from oyster-beds. 



The oyster drill ( Urosal'pinx ciiier'ea) (see Fig. 57, p. 78) bores a hole 

 through the shell of the oyster and feeds upon its soft parts. 



Natica is another drilling sea-snail common on our Eastern coast. 

 It burrows in the sand for clams and bores a hole with its radula, rotating 

 its own body in the action. 



The Nudibranchs. — In the Nudibranchs the shell is entirely absent in 

 the adult. True ctenidia are replaced as breathing organs by a number of 

 secondary branchiae, sometimes simple, sometimes branched processes or 

 leaf-like tufts, which may be distributed over the dorsal surface (as in 

 E'oHs), or placed in a row on each side beneath the mantle-flap (as in 

 Pleurophylli'dia) . These soft naked sea-slugs live in shallow water near 

 the shore, crawling about and feeding upon the seaweeds. Their protect- 

 ive resemblance is very great on account of both color and form. They 

 move very slowly. This also aids them in escaping the notice of their 

 enemies. 



The land snails and slugs (Pulmona'ia) are air breathing. The air 

 enters the mantle cavity through a small opening which is near the right 

 side in the dexiral forms (that is, the spiral of the shell turns like the hands 

 of a clock from left to right), and on the left side in the left-handed {sinis- 

 tral) forms. 



Land snails ( Helic'idw) are common in moist woods. They come out at 

 night or in cloudy weather to feed on succulent vegetation. When they 

 are numerous they do much damage. They, in common with the pond 

 snails, have thin spiral shells. They have two pairs of tentacles. The 

 upp :>r and larger pair bears the eyes at their tips, and the shorter pair is t he 

 organ of touch. (See Fig. 61, p. 81.) 



The land snail ( Helix) has no operculum, and when frost comes it with- 

 draws into its shell, fitting the opening to some smooth object, and secretes 

 a layer of mucus. This hardens upon drying and forms a tough membrane, 

 the epiphragm, which closes the opening. In at least one species of Helix 

 a small hole is found just below the lung aperture, through which an ex- 

 change of gases may take place. 



As a rule, snails lay their eggs in strings or masses, but the land snails 

 bury their eggs singly or deposit them thus in moist places. Snails are 

 used as food, being even shipped to the United States from Europe. 



Land slugs (Limac'ida) are naked. The shell is vestigial and con- 

 cealed by the mantle. They have a rasping tongue like the snail's. The 

 giant yellow slug of California reaches a length of 12 inches. 



The Pulmonata are hermaphroditic. The garden snail hibernates by 

 coiling up in its underground burrow in winter. 



Pond Snails. — The common pond snails have but one pair of tentacles, 

 and the eyes are situated at the bases of these. They breathe by means 



