This small species of Shrimp, although it has much the same outward
appearance as the common shrimp, except that it is considerably
smaller, is, when duly examined, one of the most singularly-formed
creatures of the class to which it belongs. It is found in tolerable
abundance along the British coasts, but the northern seas literally
swarm with them; there, in spite of their small size, they are destined
to become the food of the stupendous whale, whose enormous mouth
encloses myriads at a time.
[Illustration: _Mysis chamæleon._]
The Opossum Shrimp belongs to a group of crustaceous animals which have
been called fissipeds, (_split-feet_,) on account of each of their
feet being divided nearly throughout its whole length; the inner limb
being constructed for progression and the seizing of their prey, and
the outer for swimming and giving that motion to the water which is
essential to the organs of breathing, which are, as it were, wrapped
round the base of each limb, and fully exposed to the action of the
element. In the other Crustacea, which they most nearly approach, such
as shrimps, prawns, &c., there is a single row of five feet on each
side: but the genus we are now describing possesses as many as four
rows of feet, each containing eight, so that in all, the number of feet
amounts to as many as thirty-two,—sixteen adapted for swimming, and
sixteen for seizing their prey. In consequence of this organization,
the Shrimps seek their food in the sands at the bottom, while the
present genus frequent the surface.
The most singular portion of their formation, and that to which they
are indebted for a name, is a kind of pouch which the female possesses,
fixed beneath the body, and formed of two concave pieces of shell; this
pouch, which is very capacious, considering the size of the animal, is
destined to receive the eggs, which are deposited in it, enveloped in
a kind of jelly-like substance, most probably forming the food of the
young when first hatched. As fast as the young assume the lengthened
form of the perfect animals, they are found to arrange themselves in
this pouch closely and regularly side by side, with their heads towards
the breast of the mother. After this manner they lie closely compacted
together, and present a perfectly symmetrical arrangement, easily
observed from the translucency of the valves of the pouch, and the
large size and blackness of their eyes. The males of the Opossum Shrimp
are not so numerous as the females, and are without the singular pouch
we have described.
We have already noticed the fact of these Shrimps being the food of the
Greenland whale, in the northern seas, but in these climates they serve
as food for herrings.
It is in looking closely into the structure of these little animals
that we see the perfection of the Divine Artist. Nature’s greater
productions appear coarse indeed to these elaborate and highly-finished
master-pieces, and in using more and more powerful magnifiers we still
continue to bring new parts and touches into view. If, for instance,
after observing one of their members with the naked eye, which has
informed us that the part we have been examining is composed but of one
piece, we employ a magnifying glass with a low power, the same part
appears jointed, or composed of several pieces articulated together.
Employing a higher magnifier, it appears fringed with long hairs,
which, on further scrutiny, seem to be themselves fringed with hairs
still more minute; many of these minute parts also, are evidently
jointed, and perform sensible motions. But what idea can we form of the
various muscles which put these parts in movement, of the nerves which
actuate them, and the vessels which supply them with the nourishment
necessary for growth and support, and which we know, from comparison
with other creatures, they must possess!
The Opossum Shrimps, we have seen, are the prey of the larger
inhabitants of the deep; but they, in their turn, destroy others that
are smaller than themselves,—seizing upon every animal substance they
are able to manage that comes within their reach, and, if placed in a
vessel of sea-water by themselves, devouring each other.
The species represented in the engraving has been called the _Mysis
chamæleon_, from its colour varying according to the substances on
which it feeds, through all the gradations of gray, black, brown, and
pink.
_ORDER CRUSTACEA HETEROBRANCHIÆ._
(_Crustacea with Organs of Breathing variously placed, never
concealed._)
The Crustacea which form this order differ much more from each other
than those which are arranged in the last order, and consequently we
find among them some very singular in their outward formation. Few of
the species appear to be used as food by mankind, but they constitute a
great portion of the nourishment of fishes and other inhabitants of the
water. They are at times used by fishermen as bait. Many of them are
very minute, and form most excellent objects for the microscope. They
have been divided into several sections, according to their outward
form; but as so little is known of their habits, we shall confine
ourselves to a description of some of the best known.