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 Actiniz 
($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 (exivoc, echinos, a sea-urchin ; depua, 
derma, skin), prickle-skinned animals. 2. Acalephs (axa- . 
Anon, akalephé, a nettle), Sea-nettles, or Jelly-fishes. 3. 
Phytozoa (gitrov, phyton, a plant; Edov, 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 
