330 THE INHABITANTS OF THE SEA. 
issues. When the animal wishes to protrude its feet, each 
vesicle forcibly contracts, and, propelling the fluid into the cor- 
responding sucker, causes its extension; and, when it desires 
to withdraw them, a contraction of the suckers drives back the 
fluid into the expanding vesicles. 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. 
Numerous species of star-fishes are so very common in our 
waters, that in many places the sea-bottom is literally paved 
with them. They likewise abounded in the primeval ocean, for 
deep beds of carboniferous limestone and vast strata of the 
triassic muschelkalk are often formed by the 
accumulation of little else than the skele- 
tons of Encrinites and Pentacrinites, which, 
unlike the sea-stars which every storm drifts 
upon our shores, did not move about freely, 
but were affixed to a slender flexible stalk, 
composed of numerous calcareous joints con- 
Lily-Encrinite. nected together by a tleshy coat. The 
feathered bifurcated arms of the Crinoids 
are unprovided with suckers, which would have been perfectly 
useless to creatures not destined to pursue their game to any 
distance, but passively to receive the nutriment 
which the current of sea-water set in motion 
by their richly-ciliated pinnules conveys to the 
mouth. These beautiful creatures were for- 
merly supposed to be nearly extinct, for up to 
within the last few years only two living 
stalked crinoids were known in the ocean of 
the present period, but the dredge has latterly 
brought up new and remarkably fine species 
: from depths of more than 2000 fathoms, and 
Portion ofthe Peo there is every reason to believe that these 
Ce animals still form an important element in 
the abyssal fauna.* 
Of freely-swimming Crinoids but one single representative is 
known in the northern seas, the Rosy Feather-star (Comatula 
rosacea), whose long and delicately fringed rays and deep rose 
* See page 420. 
