THE INVERTEBRATE LEGIONS 121 



in length, but the Pacific Cryptochiton reaches almost a foot. There are many 

 species off the Pacific Coast of North America. Chitons are less numerous in 

 the Atlantic. 



Snails: Class Gastropoda — Figure 40 



This is the largest class of molluscs by far. The body is short and is usually 

 covered by a single, spiral shell. Locomotion is accomplished by means of the 

 flat, muscular foot upon which the animal glides. Some conchs use the foot 

 to make short jumps rather than as a glider, and the small, planktonic pteropods 

 have 2 winglike feet for swimming at the surface of the sea. 



All snails have tentacles bearing chemoreceptors and eyes. None of them 

 move rapidly. The mouth has a tongue with a radula. The beautiful shells of 

 snails have caught the attention of collectors. 



The most primitive of gastropods are the flattened limpets, keyhole limpets, 

 and abalones. These are all animals of tidal or subtidal rocky shores. All scrape 

 algae from rocks with their many-toothed radula or eat dead organic matter, 

 and all are more active by night than by day. The limpets and keyhole limpets 

 are the smallest, never reaching over 4 inches and averaging much smaller. 

 The abalones, however, reach a foot in length and are delicious as food. They 

 are found only in the Pacific and are treated as game animals protected by law 

 on the Pacific Coast of the United States. The limpet, in spite of the primitive 

 nature of its nervous system, shows a remarkable homing behavior. Each 

 limpet has a definite place on a rock to which it adheres when it is uncovered 

 at low tide. It wears a depression in the rock which conforms to the shape 

 of its shell. When covered by water at high tide, the limpet moves away as 

 much as three feet from its rock, "scar" home to browse, returning again at 

 low tide. It finds its way back by remembering the topography of its home 

 rock and always fits itself into the scar. 



The rachiglossans are snails that have only three large teeth in the radula. 

 These are used to bore through the shells of clams and oysters or other molluscs 

 or perhaps other animals. All of them are carnivorous. The well-known whelk 

 (scungill), Biisycon, is a large species which lays eggs that look like a string 

 of flattened beads. The oyster drills, Urosalpinx, are pests in oyster beds. Murex 

 CColor Plate 10^ is a widespread tropical genus which has a shell that is dis- 

 tinctly laminated and often very spiny at the places where the lamellae overlap. 

 Nassa is a small species that eats other molluscs but also scavenges; it is very 

 common on mud bottom. 



The taenioglossans make up the great majority of snails. They include 

 carnivorous species such as the moon shell, Natica, which feeds on shellfish 

 and lays eggs in sand collars, common objects cast up on beaches. The slipper 

 shells, Crefidula, and periwinkles, Littorina, are common tidal or subtidal 

 vegetarians of rocky shores. The queen conch, Strombus gigas QColor Plate 10^, 

 forms a home for the little conchfish and is among the largest of snails, often 

 weighing over 5 pounds. It progresses by jumping and is a scavenger. The 

 cowries, Cyffaea CColor Plate JO), are among the most beautiful and most 

 sought after of shells. They are most common in the tropics and not very 

 abundant in North America. The helmet shells. Cassis, include the king conch 



