XV111 INTRODUCTION. 



c. Those having a chambered shell with simple par- 

 titions (Nautilus). 



The GASTEROPODS contain three orders : 



a. The land snails which breathe air (Pidmonates). 



b. The aquatic snails which breathe water (Branch' 



i/ers), fig. 88. 



c. Those which have wing-like appendages about the 



head, for swimming (Pteropods). 



The class of ACEPHALS contains three orders : 



a. Those having shells of two valves (bivalves), like 



the clam (Laniellibrancliiates). 

 I. Those having two unequal valves, and furnished 



with peculiar arms (Brachiopods). 

 c. Mollusks living in chains or clusters, like the Salpa, 



or upon plant-like stems, like Flustra (Bryozod), 



fig. 135. 



IV. The department of RADIATES is divided into three 

 classes : 



1. Sea-urchins, bearing spines upon the surface (Echi- 



noderms), figs. 12, 26, 31. 



2. Jelly-fishes (Acaleylis), fig. 31. 



3. Polyps, fixed like plants, and with a series of flexi- 



ble arms around the mouth, figs. 48, 77, 143. 



The ECHINODERMS are divided into four orders : 



a. Sea-slugs, like biche-le-mar (Holothurians). 



b. Sea-urchins (Echini) fig. 26. 



c. Free star-fishes (Asteridce), fig. 17. 



d. Star-fishes mostly attached by a stem (Crinoids), 



fig. 150, 151. 



The ACALEPHS include the following orders : 



a. The Medusae, or common jelly fishes (Discophori'), 

 figs. 31, 142. 



