APPENDIX B 



645 



a shell composed of a longitudinal row of eight curved plates, the joints between 

 which allow the animal to roll up like an armadillo. Chitons are all marine, living 

 mostly on rocky coasts. There are several hundred existing and about 100 known 

 fossil species. 



Class 2. The Snails and Shigs (Gastropoda). In this class, there is a well-defined 

 head, and the bilateral symmetry is usually obscured by a one-piece spiral shell, 

 which is sometimes absent (Fig. 32.3). The foot has a flat, crawling sole. Most 

 snails are marine, and many of the most attractive sea shells of our beaches are 

 made by members of this group, some of which live among rocks and others in 

 sand or mud. Some of the marine snails have become pelagic and swim at the 



Fig. B.14. Paleozoic cephalopod mollusks. From straight-shelled flat-partitioned types 

 (lower right) there developed various curved (lower left), coiled (upper left) and ornate 

 (upper right) types with flat or simply waved internal partitions in the shell. Eventually 

 from some of these came the Mesozoic ammonites with their complexly frilled and wrinkled 

 partitions. (From a restoration of Devonian marine life in New York, courtesy Rochester 

 Museum.) 



surface by flapping their two-winged foot. Most are herbivorous, but many live 

 on decomposing animal matter, and some are predacious. A relatively small 

 number of kinds inhabits fresh water, and the land snails (Pulmonata) have a sort 

 of lung developed from the mantle cavity. 



Class 3. The Tooth Shells (Scaphopoda). A small group of marine mollusks with 

 a conical shell, straight or curved, and open at both ends; marine, burrowing 

 in sand and mud. 



Class 4. The Bivalve Mollusks (Pelecypoda, or Lamellibranchiata) . A very large 

 group of mollusks, which lack a head, have bilateral symmetry, a shell composed 

 of two lateral valves, and a mantle of two lobes (Fig. 32.5). Largely marine, but 

 some occur in fresh water. 



Class 5. The Cephalopods (Cephalopoda). Squids, octopods, nautiloids, am- 

 monites, etc. These mollusks have a well-developed head with prominent eyes, the 



