the hundreds of kinds of creatures which are connected with the
jelly-fish in classification, by their having many structures in
common. The creatures thus classified with the jelly-fish are
called Hydrozoa, or Water Animals, and they pass two lives, one of
which is commonly observed on the seashore everywhere. One life is
a fixed one, and the other is a free swimming one; in one stage it
is eating and drinking, and in the other, these functions are not
always or often carried out. The egg-laying and perpetuation of the
animal are the duty of the greater part of the beautiful branched
and hair-like things which are arranged by visitors to the seaside
in patterns, and retained as memorials of pleasant days, and which
are popularly called sea-weeds. They are not such things, but are the
delicate stems, branches, and bud-like homes of the parents of tiny
jelly-fish, which are only to be caught in the open sea. These horny
stems and branches end in creatures with tentacles or feelers, and
they live between tides, in rock pools, and at low water.
Sometimes these fragile, plant-looking things are fixed on to stones
or sea-weed, and some get a ride by growing on shells, inhabited,
since the death of their original possessor, by the Hermit Crab.
One very numerous tribe of these feathery-looking weeds, but which
are truly animals, is common everywhere on the shore, between
tide-marks into deep water. Their stems are branching, and the
little bud-like things on them, when watched in still water, may
be seen to put forth pretty colored bodies with tentacles around a
centre; and as there may be hundreds of them on the branches and
stem, the creature resembles a bunch of rayed flowers. Hence the name
Sertularia, from sertula, a small garland.
These Sertularians were noticed and drawn many years ago, and were
called Corallines. This is a great mistake, because the Coralline is
a plant covered with carbonate of lime. The correct name would be
the Garland polypes. When we get a bunch of these creatures from the
seaside they are dead, and the hard and preservable outside parts
alone are left, all the beauty of color and the wonderful inside
structure are gone. But even then it can be noticed that the stems
of the creature arise from a network of tubes fixed on to stones,
sea-weeds, and shells. This resembles a root, but it does not absorb
nourishment like the root of a plant. The stems, often not bigger
than hairs, are hollow, and the branches also. The surface of the
branches is covered on one, or often on both sides, by minute cups.
These give almost a saw-edge look to it, in some of the creatures;
and among these cups, which are open at their free end, are some
larger ones, which are closed where free, and often ribbed and
ornamented on their outside. It is noticed that the hollow of the
stem and branches is continued into the cups, but not into the closed
ones, and that these are shut off from it by a very delicate layer of
tissue. This is the minute structure of these things when dead; but
when alive, the inside of the stem and branches is filled with a soft
substance, which reaches up to the part where each of the cups is
attached to the outside. The cups, hundreds in number, on each stem
and branch, contain a most beautiful flower-like polype. Variously
colored, according to the kind, it has a bell shape, and has a spot
for the mouth, which leads to a stomach, whose floor is connected
with the pith of the whole branch. Around the mouth are numerous
tentacles excessively irritable, capable of stinging and paralyzing
prey; and the spectacle of hosts of these, all working for the common
good of the branching animal, is very beautiful. Sometimes there
is much red in the color, at other times green, yellow, or a dull
tint; and in order to relieve the sameness of outline presented by
thousands of cups, their horny margin is spined or toothed. In full
vigor during the summer, and living through the winter and probably
for several years, these cups with their tentacled polypes contribute
to the growth and nutrition of the whole. They all work for a common
end, and that is for the persistence of the life of the colony. They
live on microscopic things in the water, such as minute ova, or
the young of things like unto themselves, and on animalcules, and
probably on the moving spores of sea-weeds. As the spring advances,
the larger and closed cups begin to grow by budding from the stem,
and they become filled with a gummy substance with a few granules in
it, and somewhat resembling the pith of the stem. As the closed cup
grows, it often becomes marked on the outside with rings or belts,
and it becomes a very prominent object on the horny-looking Garland
polype. These closed cups are concerned in the reproduction of the
creature, for after a few weeks the substance inside them is seen to
collect into bunches of round or oval bodies, often yellow in color,
or of other tints. By and by the cup bursts, or a sort of trap-door
opens at the top, and these things escape. They are small, covered
with cilia, or moving hairs, and resemble the planulas of the
jelly-fish in shape, and they swim freely, and finally settle down on
a stone or weed. They become environed by a horny coat, the stomach
having been formed first by simple bending in of the outside of the
creature, and tentacles grow. The planula thus founds a new colony.
But there are other phases in this curious life-story; for instead of
the little round balls in the closed cups getting free as planulas,
some in certain kinds grow there, and resemble excessively minute
jelly-fish stuck fast by the back, and exposing the mouth, feet, and
the fringed umbrella. These Medusæ, as they may be called, die on the
parent, and never wander.
There is one kind of these Garland polypes called Lafœa in which the
round bodies produced in the closed cups burst forth, and not in the
shape of planulas. They come forth like tiny bells, furnished with
minute eye-spots on the edge, and they have an umbrella shape, and
long tentacles arising from the edge near the eye-spots. They are
jelly-fish, to all intents and purposes, and swim freely about, and
in time produce planulas which develop into a parent Garland polype
again.
There is a very interesting kind of these allies of the sea-jellies
which, from its feathery shape, has been called Plumularia. Specimens
are common, and there is a stem, and it has branches, and the cups
are in one row on the branchlets. Moreover, there are openings in
the horny envelope of the soft pith of the branches, which do not
give exit to a polype-looking thing with its mouth surrounded with
tentacles, but to a simple pith with many threads or stinging cells
in it. It is more or less like a long tongue, and is placed among the
cups with polypes, and it is evidently an instrument for capturing
minute creatures. The Sickle-beard is one of them.
But one of the most singular of these delicate creatures lives in
very numerous colonies, on such an unexpected place as the outside of
the shell tenanted within by a wandering hermit-crab. It is called
Hydractinia. The Hydractinia has a kind of crust for its formation,
with tubes in it, and out from them comes a white film, which, under
a magnifying-glass, presents a number of stems and polypes. Each one
has at its free part a club-shaped end, which has several rows of
tentacles around it. These, often twenty-five in number, are, like
the stem, very irritable, flexible, and are covered with a sticky
matter. The club is hollow within, for the stomach, and food gets in
by the top, where the horny skin appears to be absent.
The tentacles are rather sharp, and when they are half-contracted
they often appear to have a knob at the end, otherwise they are stuck
out and are like so many thick threads. The stalk which supports
the conical-shaped head has a thin and wrinkled skin with sharp,
dot-like points on it, and probably they secrete a sticky substance.
The polypes thus formed cluster together like so much moss on the
shell, and it would appear, if not in the English species, certainly
in one which frequents the American coasts, that some groups are male
and others female. But there is no doubt, from the researches of Mr.
Moseley, F.R.S., of the _Challenger_, that some of the polypes act
as catchers of food, and feed those which pay especial attention
to increasing and multiplying. These last have minute little knobs
growing on the stalk beneath the head, and which develop round
bodies inside. After a while motion is noticed in them, and there is
every reason to believe that they burst forth and swim off as small
jelly-fish. Some, however, give out little planulas, which settle
down and grow like the parent.
It will have been noticed from this description that these polypes
are different in shape from the Garland polypes and that instead
of being bell-shaped, with tentacles around the mouth, they are
club-shaped, and have more than one row of tentacles half way down.
This last shape gives a characteristic to many little polypes which
lead, like the others, two lives--one fixed on the seashore on plants
and stones, and the other, freely swimming, as a pretty little
jelly-fish.
Another kind lives fixed on sea-weeds in shallow water, and its stem
consists of a single tube, which is creeping and threadlike. The cups
for the polype are on long stalks, with markings like rings on them,
and are bell-shaped with a toothed edge. The mouth is surrounded
by tentacles, as in the Garland polype, and, in fact, there is not
much outside distinction between them and this Campanularia, or Bell
polype.
There is a stomach in the polype bud which leads to a canal that
goes down the stalk, and even into the creeping stem or root. The
buds, on stalks of their own, are very remarkable. They contain,
soon after they are formed, numerous little jelly-fish attached to
stalks. There may be twenty or thirty of these in each bud, and the
day will come when they will burst forth, be cast loose from their
stems, and swim off. They resemble hand-bells of a very flat kind,
and the mouth is prolonged into a four-parted projection, which
protrudes through a structure not seen in the great jelly-fish, but
peculiar to these smaller ones, which fills up the disk underneath,
from the mouth to the edge, with a layer of muscular fibres. They
have four long tentacles, and when they have lived for some time they
begin to play some very curious tricks. Thus a foreign naturalist,
M. Van Beneden, was examining some creatures in his aquarium, and
found hundreds of these very small jelly-fish, and he caught some
in order to examine them with his microscope. He began to draw some
of them carefully in order to write a description of them. About an
hour afterward, on again looking at his specimen, he was amazed to
find its shape changed, and the animal apparently turned inside out.
The tentacles on the edge seemed to be reversed in their position;
the umbrella-like dome, from being convex was the reverse, and
the curled lip-like proboscis seemed converted into the stem of a
solitary polype. One of these Campanularians, called erroneously the
Wrinkled-thread Coralline, grows on sea-weeds near low-water mark,
and especially on the great Riband Tangle. It is a small thing, about
an inch in height, and its stem is of a pink or rose-red color; it is
sparingly branched in a zigzag manner, and its stem is ringed. The
buds develop jelly-fish, but they never escape, and hang on by stalks
for the rest of their lives.
FISHES
--ANDREW WILSON
This division every one must know as that of the Vertebrata, a
word which may be used in a popular sense, as corresponding to the
expression “backboned” animals. At the head of this group man and
quadrupeds are found, while the fishes form the lowest class in the
division.
There are few groups of the animal world more interesting to the
ordinary observer than that of the fishes. To survey the various
forms and shapes presented by these animals as displayed in a great
museum should prove a sufficient incentive to gain a more intimate
acquaintance with the class; and when, even in a popular sense, we
investigate the structure and habits of fishes, the study increases
in its fascination and interest. While if we reflect that on a
knowledge of the habits of fishes, of their distribution in our
oceans and seas, and of the special products which many of them offer
for our use and luxury, the commercial success of our fisheries
depends, it can need no further argument to convince us that, after
all, there is something of great practical benefit to be derived from
the study of zoological science.
It is not our intention at present to say anything regarding the
commercial or economic aspects of fishes, and even their general
habits must be very briefly touched upon. We rather aim at giving
some account of the structure of the fishes, and at noting such
peculiarities in their habits and life as may prove most interesting
to our readers. Primarily, then, we find that fishes may be
recognized by having the body usually, but not always, covered with
_scales_, of various forms and kinds. Then, secondly, we have the
limbs represented by certain _fins_; and, thirdly, we find almost
all fishes to breathe by _gills_ during the whole of life. These
three points are, in the main, sufficient to distinguish fishes from
their higher as well as their lower neighbors. The scales which cover
the bodies of fishes present great diversities in shape, size, and
appearance. Some fishes thus exhibit an utter want of scales; while
others, like knights of old, are incased in a veritable suit of scaly
armor. The lampreys, and their curious neighbors the hag-fishes, are
destitute of scales; and in our familiar eels, the scales are very
small and insignificant. Such fishes, however, are amply compensated
for the want of scales by the power they possess of throwing out from
the skin a vast quantity of glutinous or oily matter, technically
named _mucus_. The presence of this secretion, which has given origin
to the phrase “as slippery as an eel,” serves to protect the surface
of the body, and no doubt also assists in the easy progress of these
fishes through the water. So large is the quantity of this oily
matter which the hag-fishes can emit from their body, that one form
has received the specific name of glutinosa; the fish being able in
this way to literally convert the water of the vessel in which it is
contained into a jelly-like mass. The familiar blennies, found in
rock-pools after the tide has receded, are also able to emit a large
amount of this glutinous fluid.
Illustrating the opposite extreme of the development of scales,
we find such fishes as the bony pikes of North American lakes and
rivers, the bodies of which are covered with an armor of closely
fitting and overlapping scales or plates, named ganoid, from their
shining appearance (Greek _ganos_, splendor). The scales of this
fish are said to be employed in the manufacture of the little
“mother-of-pearl” buttons, so commonly used. Many fossil fishes were
also abundantly provided with these hard, bony plates; and our living
sturgeons possess scales of similar nature, although in the latter
fishes they do not completely cover the body. The bright silvery
scales of the herring and its neighbors are thin structures, and
are very easily detached from the skin; and a curious form of scale
is seen in the perches; the hinder edge of each scale in the latter
case being cut into comb-like teeth. In the sharks, skates, and rays,
the scales are small and horny, and are often provided with little
spines. If we draw our hand along the back of a dog-fish from tail to
head, as when we stroke a cat’s back the wrong way, we feel numerous
small projecting points, borne on the scales. The rough skin surface
thus produced is frequently used under the name of “shagreen” in
the manufacture of spectacle-cases and like articles, and is also
employed for polishing the surface of wood.
In their general shape the bodies of fishes exhibit a great
compression from side to side, a rounding of the sides, and
a pointing of either extremity, adapting the animals for easy
progression through the water. Some fishes, such as the soles,
flounders, plaice, etc., are named “flat fishes” from the great
flattening exhibited by their bodies; although, at the same time, it
is important to observe that these fishes are simply more compressed
from side to side than their neighbors. Most persons, on looking at
a sole or flounder, are apt to think that one of the flat surfaces
must represent the back, and the other the under surface of the
body. This idea is strengthened by the fact that the so-called back
surface is dark, and the apparent under surface light in color, and
because both eyes exist on the dark-colored surface. That, however,
the flat surfaces are really the _sides_ of the fish may be seen by
noting that on each surface a breast-fin is developed; these fins
being placed invariably one on each side of the body. And while the
eyes in early life are disposed one on each side of the head, in the
position in which eyes are naturally situated, they are gradually
brought round to one side by the bones of the head becoming curiously
twisted in the course of development. Thus these fishes lie and swim
on one side--that which is light-colored--and present a most singular
combination of curious and abnormal features.
The fins of fishes constitute interesting features in their
structure. Almost all fishes have two sets of fins--those which
exist in pairs and those which are unpaired, and which are developed
in the middle line of the body. To the former class belong the two
pectoral or “breast-fins” and the two ventral or “belly-fins.” The
“breast-fins” correspond to the forelegs of other animals or to the
arms of man; while the ventral fins correspond to the hindlegs or to
man’s lower limbs; and these latter fins may be placed, as hind-limbs
should be, to the rear of the body (as in sturgeons); or they may be
found (as in the cod) placed beneath the breast-fins on the throat.
It may be asked, How do we know that these two pairs of fins
represent the limbs of other animals? We reply, because when we
investigate their structure we find them to be supported by a bony
skeleton, the various portions of which correspond to those existing
in the skeleton of the limbs of man or other vertebrates. And it is
only through this important principle of tracing out what are known
as the homologies or resemblances between parts, and by looking at
and comparing their structure, that we are enabled to find out the
real nature of many organs in animals; similar organs frequently
existing under very different and varied guises.
The other fins of fishes do not exist in pairs, but are placed in
the middle line of the body. Hence they are named the median or
unpaired fins. Thus we find the back or dorsal fins to represent the
unpaired fins, as also do the tail and anal fins; the latter being
placed on the lower surface of the body. These unpaired fins, if
they correspond to any other structures in the fishes, are simply to
be regarded as special developments of the skin, and therefore bear
no true relationship to the limbs of other animals. We may find one
or more dorsal and one or more anal fins; but the tail-fin, by the
action of which, as every one knows, the fish chiefly swims, is
always single, but may be divided into halves. Most of our common
fishes have the halves of the tail-fin of equal size; others, such as
the sharks, sturgeons, etc., having the upper half greatly exceeding
the lower half of the tail-fin in size. In one species of shark,
named the Thresher or Fox-shark, the upper half of the tail-fin
appears enormously developed as compared with the lower half; and the
names of this species have been derived from the use the fish makes
of its tail in lashing the water, and from the long-tailed appearance
suggesting a resemblance to the familiar Reynard of the land. In
fishes the tail-fin is always placed vertically, or in the same line
as the body, and moves from side to side; while in the whales--which
are not fishes, but Mammalia or quadrupeds possessing fish-like
bodies--the tail-fin is placed across the body. Some fishes may
want arms or legs--that is, the pectoral or ventral fins; the eels,
for example, possessing no ventral fins. The flying-fishes, on the
contrary, possess a very large development of the pectoral or breast
fins, and support themselves temporarily in the air by their aid.
Fishes are usually very well provided in the matter of teeth. What
would be thought of a quadruped which had teeth not only in its jaws,
but had its tongue, its palate, the sides and floor of its mouth,
and other parts, also bearing rows of these structures? Yet such
is the case with many fishes. Then, also, where the teeth of one
set in fishes are lost, or destroyed through the natural wear and
tear to which they are subjected, new teeth are developed to supply
the place of the lost members. Any one may gain a good idea of the
formidable array of teeth in fishes, and of the manner in which one
set succeeds another, by inspecting the jaws of a shark in a museum.
In fishes the teeth are not implanted in sockets, but are fastened by
ligaments to the surface of the bones which bear them. Sometimes one
tooth only is developed in fishes. This is the case in the curious,
eel-like hag-fishes already mentioned; these fishes possessing but
a single large tooth, borne on the palate; and by means of this
formidable weapon, which possesses saw-like edges, they bore their
way into the bodies of other fishes, and there take up their abode
as unwelcome guests. A cod or large haddock may sometimes be found
with five or six hags contained in its interior. The parrot-fishes,
or Scari, of tropical seas, are so named from their possessing jaws
shaped like the beaks of those familiar birds, and these jaws are
rendered all the more extraordinary from their being covered or
incrusted by numerous small teeth, which are as closely packed on the
jaw as paving-stones are in a street, and which serve these fishes
as useful instruments when they feed upon the living parts of the
hard and limy coral-animals. In the jaws and floor of the mouth of
the Port Jackson shark, or in the Eagle rays, or skates, the teeth
may be seen to be flat and broad. Such teeth form a regular pavement
arranged like a mosaic pattern, and are admirably adapted for
crushing whatever substances enter the mouth.
Fishes are well provided in the way of digestive apparatus. A
throat or gullet, stomach, intestines, liver, and other glands,
serve for the digestion of the food, and a heart and blood-vessels
exist for the circulation of the blood thus manufactured from the
food. The blood is purified in the gills. Each gill--consisting
in common fishes of a supporting “arch” bearing a great number of
delicate filaments arranged like the teeth of a comb--may be viewed
as simply a network of blood-vessels. The blood, pumped into this
network by the heart, is purified by the action of the oxygen gas
contained in the pure water which the fish is constantly taking into
its gill-chamber by its mouth; while the pure blood is recirculated
through the body, and the water used in breathing is got rid of by
being ejected behind the “gill-cover” at the neck, so as to allow a
fresh inflow to be drawn in by the mouth. The gills of some fishes
may be very differently constructed from those of the common members
of the class. Thus the lampreys breathe by pouch-like gills which
open each by a separate aperture. Seven gill-apertures may be seen
on each side of the neck of the common lamprey; and the sharks,
skates, and their neighbors also breathe by sac-like gills. Certain
curious facts regarding the breathing of fishes will be afterward
alluded to. Fishes illustrate plainly what is meant by aquatic or
water-breathing. They possess gills or organs, adapted for separating
the atmospheric air which is entangled or contained in the water;
land animals breathing the same air directly from the atmosphere.