POLYPIFERA.
"Happy is he who, satisfied with his humble fortune, lives
contentedly in the obscure state where God has placed
him."--RACINE.
Entering on the class Polypifera, we leave the domain of the infinitely
small to enter the world of the visible. Beside the Infusoria, the
Polypifera, which are sometimes several inches in length, are very
important beings. Science has made great advances towards giving us an
exact knowledge of these singular animals. Many scientific prejudices
have been dissipated, many errors have been corrected. The Polyps, as
they are defined in the actual state of Science, correspond not only
with the _Polypes, properly so called_, of Cuvier and De Blainville, but
also with the _acalephous zoophytes_ of the same authors. We now know
that certain Polyps engender _medusæ_, or acalephous zoophytes, and that
there exist some medusæ scarcely differing in their structure and habits
of life from the ordinary Polyps.
Thus regarded, the Polyps comprehend a great variety of animals, the
bodies of which are generally soft or gelatinous substances. The
principal and smaller divisions, to the number of more than two, are
arranged round an imaginary axis, represented by the central part of the
body. These divisions of the body have in their ensemble the appearance
of a regular cylinder, of a truncated cone, or of a disk. They are
invested with a skin or envelope of calcareous or siliceous corpuscles,
and even a portion of the deepest-lying tissues may be invaded by a
calcareous deposit, the mass of which belongs sometimes to an
individual; sometimes it is common to many, constituting what Dr.
Johnston calls the Polypidom, of which Professor Grant says, "there is
but one life and one plan of development in the whole mass, and this
depends, not on the Polypi, which are but secondary, and often deciduous
parts, but on the general fleshy substance of the body;"[5] "the
ramifications," says Dr. Johnston, "being disposed in a variety of
elegant plant-like forms. The stem and branches are alike in texture:
slender, horny, fistular, and almost always jointed at short and regular
intervals, the joint being a mere break in the continuity of the sheath,
without any character of a proper hinge, and formed by regular
periodical interruptions in the growth of the polypidoms. Along the
sides of these, or at their extremities, we find the denticles or
cup-like cells of the polypi arranged in a determinate order, either
sessile or elevated on a stalk." Near the base of each of these there is
a partition or diaphragm, on which the body of the polyps rests, with a
plain or tubulous perforation in the centre, through which the
connection between the individual polyps and the common medullary pulp
is retained. Besides the cells, there are found at certain seasons a
larger sort of vesicle, readily distinguished from the others by their
size, and the irregularity of their distribution, which are destined to
contain and maturate the ovules.
With these animals the digestive tube is very simple, and presents only
one distinct orifice; the same opening serving at once for receiving the
food and the expulsion of the residuum of digestion. This is one of
Nature's economies, which it is not for us to dispute: we must record it
without further remark.
In nearly all the Polyps the sexes are separate; the generation is
sometimes sexual; but these beings multiply also by what the zoologists
call gemmation, or buds. They are provided with organs of the senses;
nearly all of them have eyes--an immense progress in organization as
compared with the animals which have hitherto engaged our attention.
Their respiration is effected by the skin--another instance of the
economy of Nature. The apparatus of their circulation is indistinct, but
they have a nourishing fluid analogous to the blood in vertebrated
animals. Vibratile cilia and stinging hairs often cover the entire
surface of the Polyps.
These general remarks may appear obscure and insufficient to the larger
number of our readers. They are necessarily so; they are generalities
upon animals very little known, even to naturalists. We quit this
difficult ground, trusting to make the special study of the several
types we shall have to describe more interesting. The group of Polyps
divide themselves into many classes, namely, the _Alcyonidæ_, the
_Zoanthina_, the _Discophora_, and _Ctenophora_. It will be our task to
describe in succession the habits and characters of each of these
classes, dwelling on such species as appear to us to offer to the reader
most real interest.
FOOTNOTES:
[Footnote 5: "Outlines of Comparative Anatomy."]