358 DIVISION IV. RADIATA.—CLASS IV. POLYPI. 
of this view, it has been observed that certain lagoons are filled with a cal- 
careous mud, evidently due to the detritus of polypvids, and that, so soon 
as this is dried, it exactly resembles the chalk of our ancient mountains. 
To the action of the waves, the chief agent in transforming polypoids and 
bivalves into calcareous strata, there is joined another, much less energetic 
it is true, but extremely curious. An observing man of genius, Mr. Dar- 
win, relates that all around the Madrepore Islands the transparency of the 
water allows one to see shoals of fish, principally of the genus Sparus, which 
feed on the tips of the branched polypoids, exactly as flocks of sheep browse 
on the pasturage of our meadows. In order to nourish themselves with the 
workman, they devour along with him certain parts of his edifice; and, as 
these are absolutely indigestible, the result is, according to the English 
savant, that a part of the chalky substance which encumbers the bottom of 
sea in the vicinity of the Madrepore Reefs, comes from the defecations. 
When the Spari are dissected, their alimentary canal is seen filled with pure 
chalk. 
The Madrepore Islands generally lie on an upheaval of the bed of the 
sea. Volcanic action begins the work, and the Polypi finish it: they bring 
the construction up to the level of the waves. These islands always display 
a peculiar configuration: they are nearly all circular, and there is a crater- 
like depression in the middle. This peculiarity seems to be owing to the 
fact, that the little workmen ean support their vital energies better where the 
water, being agitated, brings them a more ample supply of nourishment. 
The animals in the centre, placed under different conditions, wasted and 
languishing, can only raise their living rampart more slowly. In the Pacific 
Ocean, where a tolerably large number of these islands are seen, the polyp- 
idoms reach the level of low tides, and after that the great surges raise the 
middle part, by casting back upon it incessantly the fragments which they 
tear away from the circumference. When in the lapse of years the island 
rises above water, the detritus of marine plants raises it still more, and the 
virgin soil is speedily fecundated by seeds which the winds, birds, and cur- 
rents carry thither. Soon after man comes to crown the handiwork of 
nature by raising dwellings on the ruins of those of myriads of unseen 
beings. Then a king arrives, who sits proudly upon his throne, amidst this 
mass of skeletons of Polypi abandoned by the sea. 
This order is divided into three families and a large number of genera. 
In the third family are found the Pennatula, or Sea-pens, and the Spongia, 
or Sponges. 
The Pennatula, and other related species, are capable of locomotion by the 
contractions and dilatations of the fleshy parts. These are compound polypes, 
and are attached to a stem which sticks loosely in the sand. The Sea-pens 
