INTRODUCTION. 23 



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

 135; or upon plant-like stems, like Flustra, (Bryo- 

 zoa.) 



IV. The department of RADIATES is divided into three 

 classes : 



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



oderms\) figs. 12, 26. 



2. Jelly-fishes, (Acalephs,} fig. 31. 



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



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,) 



figs. 150, 151. 



The ACALEPHS include the following orders : 



a. Those furnished with vibrating hairs, by which they 



move, (Ctenophorcz.) 



b. The Medusae, or common jelly-fishes, (Discophora,} 



figs. 31, 142. 



c. Those provided with aerial vesicles, (Siplionophorce.*) 



The class of POLYPS includes two orders. 



a. The so-called fresh-water polyps, and similar marine 



forms, with lobed tentacles, (Hydroids,} fig. 143. 



b. Common polyps, like the sea-anemone and coral- 



polyp, (Actinoids,} fig. 48. 



In addition to these, there are numberless kinds of micro- 



