Classification of the Animal Kingdom. 31 



c. Those having a chambered shell with simple partitions {Navr- 

 tilus). 



The Gasteropoda contains three orders : 



a. 1'he land-snails which breathe air {Pulmonaia). 



b. Tlie aquatic which breathe water {Branchifera). 



c. Those which have wing-like appendages about the head, for 



swimming [Pteropoda). 



The class of Acepiiala contains three orders : 



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



{Lamellibranchiata). 



b. Those having two unequal valves, and furnished with peculiar 



arms {Brachiopoda). 



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



like stems, like the Flustra. — Bryozoa. 



lY. The department of Radiata is divided into three classes : 



1. Sea-urchins, bearing spines upon the surface {Echinodermata). 



2. Jelly-fishes [Acalepha). 



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



around the mouth. 



The EcniNODERMS are divided into four orders : 



a. Sea-slugs, like the bich&-le-mar [Holothurians^.] 



b. Sea-urchins [Echini). 



c. Free star-fishes [Asteriadce]. 



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



The Acalepha includes the following orders : 



a. Medusae, or common jelly-fishes [Discophori) . 



b. Those provided with aerial vesicles (Siphonophori) . 



c. Those furnished with vibrating hairs, by which they move 



[Clenophori). 



The class of Polyps includes three orders : 



a. Fresh-water polyps, and similar marine forms [Hydroids). 



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



c. A still lower form, allied to the mollusca by their shell [Rhizo- 



pods) . 



In addition to these, there are mmiberless kinds of microscopic animal- 

 cules, commonly called infusory animals [Infusoria,] from their being found 

 specially abundant in water infused with vegetable matter. Indeed, a great 

 many that were formerly supposed to be animals ai'e now known to be vege- 

 tables. Others are ascertained to be crabs, mollusks, worms, &c., in their 

 earliest stages of development. In general, however, they are exceedingly 

 minute, exhibiting the simplest forms of animal life, and are now grouped 

 together, under the title of Protozoa. But, as they are still very imperfectly 

 imderstood, notwithstanding the beautiful researches already published on 

 this subject, and as most of them are likely to be finally distributed among 

 vegetables and various classes of the annual kingdom, we have not assigned 

 any special place to them. 



