346 DIVISION IV. RADIATA.— CLASS I. ECHINODERMATA. 
ORDER I. THE PEDICELLATA. 
The characteristics of the order are numerous tentacula, furnished with 
suckers, which issue from small holes pierced in the skin, and which answer 
the purpose of feet, by which they move or adhere to rocks. They com- 
pose three great genera, 
Asrerias. — The Star-fish. These animals have a body generally in the 
form of a five-pointed star, whence their name. Some, however, have a 
pentagon body, and others are with concave sides. “The framework of the 
body is composed of horny pieces, variously arranged.” Some of the spe- 
cies are very common, and specimens may be picked up on our shores at 
any time. 
The Common Star-fish has the back thickly set with tubercles, and of an 
orange color, and the under surface pale. It has rows of feet, or suckers, 
which serve as means of locomotion, and as instruments for procuring food. 
It is interesting, when one of these creatures is placed on its back in a 
plate filled with sea-water, to observe the activity which those sucking feet 
display. At first the Star-fish is motionless; for, offended by the rough 
handling it has undergone, the feet have all shrunk into the body; but soon 
they are seen to emerge, like so many little worms, from their holes, and to 
grope backwards and forwards through the water, evidently seeking the 
nearest ground to lay hold of. Those that reach it first immediately. affix 
their suckers, and, by contracting, draw a portion of the body after them, 
so as to enable others to attach themselves, until, pulley being added to 
pulley, their united power is sufficient to restore the Star-fish to its natural 
position. 
This act of volition is surely remarkable enough in so simple an animal, 
which scarcely possesses the rudiments of a nervous system, but the simple 
mechanism by which the suckers are put in motion is still more wonderful. 
Each of these little organs is tubular, and connected with a globular vesicle 
filled with an aqueous fluid, and contained within the body of the Star-fish, 
immediately beneath the hole from which the sucker issues. When the ani- 
mal wishes to protrude its feet, each vesicle forcibly contracts, and, propel- 
ling the fluid into the corresponding sucker, causes its extension ; and, when 
it desires to withdraw them, a contraction of the suckers drives back the fluid 
into the expanding vesicles. All these little bladder-like cavities are con- 
nected with vessels which communicate with a vascular circle surrounding 
the mouth. 
The internal walls of the suckers and their vessels are furnished with 
vibratory cilia, and by this simple means a continual circulation of the fluid 
they contain goes on within them. 
