RADIATES. 337 



of muscle, there is a very remarkable structure which 

 seems in some respects to take its place. This structure, 

 though found to a considerable extent in other animals, 

 is present to an extraordinary degree in the Radiates. 

 It is the ciliary structure alluded to in 566. Cilia are 

 fine hair-like filaments which cover the surface of many 

 membranes, and fringe their edges. They are quite reg= 

 ularly arranged, sometimes in straight rows, and some- 

 times spirally or in circles. They have a motion which, 

 in some cases, is obedient to the will of the animal, but 

 in others is independent of the will. When in motion 

 each filament bends from the root to its point, straight- 

 ening out again, like a stalk of grain acted upon by the 

 wind ; and we have, therefore, when many of them are in 

 motion, an appearance like the successive waves in a field 

 of grain as the wind blows over it. This motion can be 

 seen only by the aid of the microscope. It is beautifully 

 displayed in the gills of the Oyster. The object of this 

 movement is to produce currents in the fluid in contact 

 with the membrane. These currents serve various pur- 

 poses, as, for example, to bring food within the reach of 

 the tentacles, and to carry fresh portions of water through 

 the respiratory apparatus. For this latter purpose cilia 

 cover the membranes lining the chambers in the Actiniae 

 (577). Cilia are needed in those animals which are most 

 stationary, and in them, therefore, they are most manifest. 



583. We divide this sub-kingdom into three classes; 

 1. Echino-dermata (exivog, echinos, a sea-urchin; c^tyia, 

 derma, skin), prickle-skinned animals. 2. Acalephs (a/ca- 

 X.j/07/, akaleplie, a nettle), Sea-nettles, or Jelly-fishes. 3. 

 Phytozoa (0{/rov, phyton, a plant ; faov, zoon, an animal), 

 commonly called Polyps. These are fixed, like plants, 

 and have flexible arms about the mouth, as seen in the 

 Sea Anemone, Fig. 266. 



584. One of the Echinoderms, the Starfish, I noticed 

 in the first chapter ( 17). It merits here, however, a 

 more particular description. It is only the upper side of 



