ON POLYPI. 585 
considerable depths, they elevate more or less their foliaceous 
expansions. We are totally ignorant of their mode of growth, 
multiplication, and death. We merely know that the poly- 
parium, which is entirely calcareous, is of a closer tissue, 
near its base, and that, on the contrary, the extremities of 
the ramifications are always more porous. The inferior 
cellules are always more effaced, (the reverse is the case with 
the upper) and the extremity of the branches is often ter- 
minated by an infundibuliform excavation tolerably deep. 
It is said that the formation of the numerous reefs in the 
South Seas, the Indian Ocean, and the Red Sea, is owing to 
the very rapid growth of the madrepores, and particularly of 
that species called muricata. It is certain that most of the 
islands in those seas rest on a calcareous soil, entirely com- 
posed of stony polyparia, and that their highest mountains 
are thus composed. But it may be difficult to prove that the 
madrepores are the species which are found there in the 
greatest number. On this subject we are deficient in positive 
observations. 
PENNATULA is a genus of true zoophytes, established by 
Linneus for a set of animals extremely singular, whose form 
in the most common species resembles that of a quill, from 
which its name is derived. These animals are composed of 
a common part, or stem, most generally containing in its ex- 
terior a long {calcareous stick, and a certain number of polypi 
disposed in rather a fixed manner upon a part of the stem, or 
on some appendages which are added to it, and which con- 
stitute the barbs of the quill. To these species modern 
zoologists have reserved the name of pennatula. 
Without following naturalists in a very detailed description 
of these animals, we may say in brief, that a pennatula is a 
body of a determinate form, binary, symmetrical, composed 
of a muscular contractile tissue, most frequently supported by 
a solid calcareous part, produced by a particular membrane, 
