ESTEODUCTION. XXH1 



The GASTEEOPODA contains three orders : 



a. The land-snails which breathe air (Pulmonatd). 



b. The aquatic snails which breathe water (Branchifera). 



c. Those which have wing-like appendages about the 



head, for swimming (Pteropoda). 



The class of ACEPHALA contains three orders : 



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



clam (Lamellibranchiatd). 



b. Those having two unequal valves, and furnished with 



peculiar arms (Brachiopodd). 



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



upon plant-like stems, like the Flustra. Bryozoa. 



IV. The department of RADIATA is divided into three 

 classes : 



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



nodermata). 



2. Jelly-fishes (Acalepha). 



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



arms around the mouth. 



The ECHIXODEEMS are divided into four orders : 



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



b. Sea-urchins (Echini), fig. 71. 



c. Free star-fishes (Asteriadce), fig. 36. 



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



figs. 69, 70. 



The ACALEPHA includes the following orders : 



a. The Medusse, or common jelly-fishes (Discophori), 



fig. 173. 



b. Those provided with aerial vesicles (Sipkonopkori) . 



c. Those furnished with vibrating hairs, by which they 



move (Ctenophori). 



The class of POLYPS includes three orders : 



a. Fresh-water polyps, and similar marine forms (Hy- 



droids), fig. 170. 



b. Marine polyps, like the sea-anemone and coral-polyp 



(Actinoids) . 



c. A still lower form, allied to the mollusca by their 



shell (Rhizopods). 



