CRUSTACEANS.--PRAWN, SHRIMP, MYSIS, CRABS;
AMPHIPODS; ISOPODS
The next group of Arthropod animals with which we have to deal is that
of the Crusta´cea. Some or other of the members of this class are
well known to everybody, if only in the shape of toothsome food--the
Prawn, the Shrimp, the Lobster, the Crayfish, and the Crab. The great
characteristic of this class of the Arthropod phylum is the so-called
‘shell,’ which differs greatly from true shell in being composed of
chitine, hardened with salts of lime. Most of the species live in the
water and breathe by means of gills or through the skin. In dealing
with these creatures, some long words must be employed, if our present
work on them is to be a stepping-stone to something more advanced. The
difficulty is more apparent than real, and if boldly faced will soon be
overcome.
Our first division, or sub-class, of the Crusta´cea is that of the
Malacos´traca, or animals with soft shells--a name originally adopted,
as Mr. Stebbing tells us[42], ‘to distinguish such creatures as crabs
and crawfish and prawns from such others as oysters and clams; not
because of the absolute, but because of the comparative softness
of their shells.’ Under this sub-class are grouped two Orders--the
Stalked-eyed and the Sessile-eyed Crustaceans, the technical names for
which are the Podophthal´ma and the Edriophthal´ma.
To the Stalked-eyed Crustaceans belong the Prawn, the Shrimp, Mysis,
or the Opossum Shrimp, and the Crabs, to mention only those forms with
which we are dealing here. The reason for scientific and popular names
will be evident if living or spirit specimens are examined, for it will
be seen that the eyes are elevated on stalks. Mr. Stebbing[43] relates
an amusing story of a very intelligent student, who, on being told that
the eyes (of the shrimp) were stalked, candidly confessed to having
always thought that this appearance was due to their having been forced
out of the head by boiling.
The general shape of a Prawn (_Palaemon serratus_) is fairly familiar
to everybody. The body is divided into two principal regions--the
carapace, or cephalothorax, as it used to be called (formed by the
union of the head and thorax), and the pleon, or swimming part. The
carapace has a projecting beak or rostrum, and is unsegmented; the
pleon is divided into segments, and the whole may be represented thus:--
---- -- -- -- -- -- -- )
_c_ 1 2 3 4 5 6 _t_
where the long stroke (_c_) stands for the carapace, the shorter ones
(1–6) for the segments of the pleon, and the ) for the telson or tail.
The carapace consists of fourteen united segments, and this will give
twenty or twenty-one segments in all, according as we reckon the telson
an appendage of the sixth segment of the abdomen, or as a distinct
segment. The carapace bears the eyes, two pairs of antennae, six pairs
of mouth appendages, and five pairs of walking legs or perei´opods,
normally with seven joints--in all, fourteen pairs of appendages, that
is, one pair for each of the fourteen segments of which the carapace
is composed. The segments 1–5 of the ple´on bear swimming feet, or
ple´opods, and the female uses these for retaining the eggs, which she
bears about with her. In this fashion the ‘hen’ lobster carries her
‘berries.’
The Prawn is a capital inmate of the aquarium, and as it does well
in confinement, specimens should be kept in order to get a general
acquaintance with their form and external anatomy, and to watch their
habits. The Common Prawn will answer the purpose, but still better is
_Palaemone´tes varians_, an exceedingly common species. It has
this advantage, that it ‘seems to be equally at home in salt water
and fresh.’ The only condition necessary is a good supply of food,
and this may be furnished by putting into the aquarium from time to
time a quantity of water-fleas. If these Prawns are well fed they will
shed their skins at frequent intervals, and this operation will give
us material for examination, for the cast skin will serve our purpose
almost as well as a spirit specimen.
Some of these Prawns are now living in one of my aquaria. They were
taken in a brackish dyke or cut near Newhaven, in Sussex, and in
the mud which was brought back with them were a number of small
bivalves of the genus _Sphaerium_. Most people know Mr. Kew’s
exceedingly interesting book, _The Dispersal of Shells_[44].
In it he relates some extraordinary instances of the way in which
species of shells are carried short distances, and may be carried
from one district or country to another. These Prawns offered a good
illustration of this, and practically confirmed some of the statements
in his book, for on several occasions they were seen with the bivalve
shells attached to their walking legs. The molluscs lay half buried
in the mud and vegetable _débris_ at the bottom of the tank,
and as the Prawns walked about they sometimes trod between the open
valves, which, as they closed, fastened on to the intruding limb. On
one occasion the molluscs did not relax their grasp for days; and had
this incident occurred when the creatures were at liberty the molluscs
might have been carried for a considerable distance. If specimens of
_Sphaerium_ are put into an aquarium containing Prawns of this
kind, it is probable that before very long the crustaceans will have
one or two attached to some of their limbs.
Prawns are exceedingly beautiful, and if we get hold of live specimens,
from salt water or fresh, they should be put into an aquarium--the
smaller, in reason, the better--so that their motions may be watched
with the hand lens. If much weed be put in, the Prawns will use their
walking legs in preference, while if there is little vegetation the
powerful tail-fan will be employed for motion backwards, while the five
pairs of limbs on the abdomen enable their owners to move forwards
through the water.
From Fig. 66 one may get a good notion of a Prawn, and of the points
in which Prawns, in the zoological sense of the word, differ from
Shrimps. The head of the Prawn is armed in front with a long blade-like
beak, studded along its upper and lower edges with a series of teeth
like those of a saw, and the second leg is chelate, that is, armed with
pincers, resembling, in miniature, that of a lobster or crab. In the
Shrimp, on the contrary, there is scarcely a trace of the beak, and
the first leg is incompletely chelate, or sub-chelate[45] (Fig. 67),
its last joint folding back upon the one that supports it, just as the
blade of a pocket-knife closes on its handle. These two distinctions
hold good between all Prawns and all true Shrimps.
[Illustration: FIG. 66.--Prawn.]
Now let us go over our Prawn--a spirit specimen--in detail. The
antennae may be separated, and examined, and the appendages of the
inner pair distinguished, for at first it may be thought that there
are more than two pairs. This, however, is not the case, as should be
ascertained by actual investigation. A needle inserted at the base of
the outer antennae will separate the first three segments, bearing
respectively the eyes, and the first and second pairs of antennae.
The eye should be carefully looked at to make out that it is really
compound. Then the joints of the antennae, each with its circle of
sense-hairs, are to be noticed. Last of all, the inner pair of antennae
deserve attention, for these carry in the basal joint an organ of
hearing. This joint is large and sac-like, and contains an opening
through which grains of sand are introduced by the animal itself. The
grains serve to transmit the vibrations of the water in the sac to the
auditory hairs, to each of which a branch is sent off from the auditory
nerve. If the joint is opened the sand will be found. The first
antennae of a lobster or crayfish may also be examined and compared.
The mouth organs, of which there are six pairs, will offer some
difficulty, and for this reason it may be well to pass them over in
this case and to deal with these organs generally when treating of the
Crab.
Beneath the outer foot-jaws are the first pair of walking feet, which
are used as cleansing organs. Gosse describes them as ‘beset with hairs
which stand out at right angles to the length of the limb, radiating in
all directions like the bristles of a bottle-brush.’ If we watch our
Prawn in life, we shall frequently see these limbs in active operation.
They are brought to bear on every part of the body within reach.
Sowerby says[46]: ‘The prawn loves to be clean, and he takes surprising
pains to keep himself so. Drawing up his tail and abdomen, he subjects
their under surface to the most careful revision, scrubbing and poking
between the lappets of the shell and body, diving into every crevice,
and with the pincer-hand picking out every speck too large to brush
away.’ The next pair of legs are also chelate; but the three following
pairs are armed with claws, and it is upon the points of these that the
animal walks on the bottom. The pincers of the second pair of legs are
used to pick up food and bring it up to the mouth organs, where it is
taken by the outer foot-jaws, and passed into the mouth. The swimming
feet carry two branches, finely fringed with hairs.
[Illustration: FIG. 67. First walking leg of Shrimp
(enlarged).]
If the carapace be removed the gills at the base of the walking feet
will be exposed. These consist of thin leaf-like plates attached to a
central stalk, and they are aërated by water passing in behind and out
in front.
After what has been said of the Prawn, little space need be devoted
to the Shrimp, for it may be gone over in precisely the same way. It
will be sufficient to call attention to the difference in the antennae,
to the rudimentary rostrum or beak, and to emphasize the distinction
between the terminal joints of the first leg in the two creatures. The
leg shown in Fig. 67 corresponds to the limb used for cleansing by the
Prawn.
There is a great difference in their habits, for Shrimps burrow in the
sand for concealment. In doing this the swimming feet, as well as the
walking legs, are brought into action, and when the Shrimp is settling
down, sand is swept over its back by the antennae, to render the
concealment complete.
In many of the rock-pools round the coast, and also in brackish water,
Mysis, or the Opossum Shrimp, may be met with. It is not, so far as my
experience goes, a good inmate of the aquarium, but it is extremely
interesting from the fact that, unlike its higher relations, the
auditory apparatus is not situated in the antennae, but in the plates
of the telson (Fig. 68E).
[Illustration: FIG. 68.--Mysis, or the Opossum Shrimp.]
Mysis is shrimp-like in general appearance but differs from Shrimps
in the structure of the legs, in the absence of gills, and in other
particulars.
The telson consists of five pieces. In each of the two inner and
smaller pieces is an oval sac, like that described in the basal joints
of the first antennae of the Prawn, containing a single lens-shaped
otolith, consisting of chalky matter embedded in some organic substance.
‘The vibration of the hairs [in this sac] is mechanical, not depending
on the life of the animal. Hensen took a Mysis, and fixed it in such
a position that he could watch particular hairs with a microscope. He
then sounded a scale; to most of the notes the hairs remained entirely
passive, but to some one it responded so violently and vibrated so
rapidly as to become invisible. When the note ceased the hair became
quiet; as soon as it was re-sounded, the hair at once began to vibrate
again. Other hairs, in the same way, responded to other notes. The
relation of the hairs to particular notes is probably determined by
various conditions; for instance, by the length, thickness, &c.[47]’
We shall not be able at present to repeat Hensen’s experiment, but we
may break up the sac and extract the otolith, which may be seen with
the lenses at our command.
Small specimens of the Shore Crab (_Car´cinus mae´nas_) are fair
game for us. They will interest us while living in the aquarium, and
when dead we can put them into pickle, and break them up at our leisure.
The broad shell of the Crab--the crab-cart of children--corresponds
to the carapace of the Lobster, the Prawn, and the Shrimp, and bears
the same number of appendages--fourteen pairs. To make out the pleon
or swimming part, it is only necessary to lay the crab on its back,
and, with a needle, or small knife, turn back the flap--or ‘apron,’
as fishermen call it--which lies in a groove on the under surface.
Here we shall find the pleopods, or swimming feet, though they are not
really used for that purpose. The eyes, the two pairs of antennae,
and the five pairs of walking legs will offer no difficulty. It is
only necessary to remark that the terminal joints of the last pair of
walking legs are flattened and fringed with hair, showing some approach
to the swimming crabs, which use those organs to swim with.
Now we may examine the mouth organs, of which there are six pairs.
To do this, the crab may be fixed, with the back downwards, or held
lightly but firmly in the left hand. The latter plan is perhaps the
more convenient. The index and middle fingers should support the
carapace, and the thumb should be placed on the pleon. The outer pair
of mouth organs are the third maxillipedes, or jaw-feet. These close
the area of the mouth, somewhat after the fashion of the double-doors
of a cupboard, though the hinging, of course, is different. To open
these jaw-feet, a needle should be inserted at the top, with a gentle
pressure downwards and outwards. The back of the crab is turned away
from us, so that the left jaw-foot should be pressed outwards to the
right, and the right jaw-foot to the left.
Theoretically these limbs consist of the same number of joints as
the perei´opods or walking legs; and this is to be borne in mind,
even if we do not succeed--and we probably shall not--in tracing the
full number of seven joints. But we may notice and count the terminal
joints, and observe the fringing of the limb with hair.
A similar method of using the needle will enable us to raise the second
and first pairs of maxillipedes, which are of smaller size and softer
structure.
Having raised these organs, it is well to replace them--to close the
doors, as it were--and then to raise them again, to observe how they
work. They may then be detached and fastened to a small piece of card,
for comparison with similar organs in the lobster and the crayfish, and
with the mouth organs of insects.
Beneath the maxillipedes are the second and first maxillae--thin,
leaf-like organs. The first-named are furnished with spoon-like scoops,
which serve to carry out from the gill-chamber the water that has
parted with its oxygen in aërating the gills.
Immediately below the maxillae lie the mandibles, with hard, cutting
edges, by means of which the food is broken up. Each carries a palp.
[Illustration: FIG. 69.--Maxillipedes and Maxilla
(_b_) of Shore Crab. (_d_) First maxillipede. (After
Savigny.)]
These inner three pairs should also be detached, and the whole of the
mouth organs arranged on a card thus:--
MANDIBLES.
FIRST MAXILLAE.
SECOND MAXILLAE.
FIRST MAXILLIPEDES.
SECOND MAXILLIPEDES.
THIRD MAXILLIPEDES.
The first attempt will certainly be unsuccessful; and the first few
attempts will probably be unsatisfactory; but we shall gain knowledge
with each successive trial. And knowledge is worth the winning.
The stomach is interesting, and the gastric mill may be easily
examined. When the mouth organs are removed, there will be no
difficulty in taking out the stomach. This should be cut open with a
needle, and then we shall see the gastric teeth (_g_ _g_)
which grind up the food against the fixed calcareous plate (_b_
_b_). The lower end of the stomach is set with fine hairs, which
prevent the passage of food into the intestines until it has been
ground fine between these living millstones. A similar arrangement is
found in all the higher Crustacea. The time spent in comparing the
gastric mill of the Crab with the ‘gizzard’ of the Cockroach will not
be thrown away.
The Broad-clawed Porcelain Crab (_Porcella´na platyche´les_) is
also worth keeping, for it is a droll little creature. These crabs
are generally to be found clinging to the under surface of stones or
of ledges of rocks overhanging small pools. The chief interest of
these crabs, for us, lies in the exceedingly beautiful arrangement for
procuring food with which the outer pair of foot-jaws is furnished.
[Illustration:
FIG. 70.--Stomach of Crab laid open. _b_ _b_
_b_, fixed plate, against which the gastric teeth _g_
_g_ are opposed; _b´_ _b´_, gastric teeth
enlarged.]
‘Watching a Broad-claw beneath a stone close to the side of my tank, I
noticed that his long antennae were continually flirted about; these
are doubtless sensitive organs of touch, or some analogous sense,
which inform the animal of the presence, and perhaps of the nature,
of objects within reach. At the same time I remarked that the outer
foot-jaws (pedipalps) were employed alternately in making casts, being
thrown out deliberately, but without intermission, and drawn in,
exactly in the manner of the fringed hand of a Barnacle, of which both
the organ and the action strongly reminded me. I looked at this more
closely with the aid of a lens: each foot-jaw formed a perfect spoon
of hairs, which at every cast expanded and partly closed. That you may
understand this better, I must say that the foot-jaw resembles a sickle
in form, being composed of five joints, of which the last four are
curved like the blade of that implement. Each of these joints is set
along its inner edge with a row of parallel bristles, of which those
of the last joint arch out in a semicircle, continuing the curve of
the limb; the rest of the bristles are curved parallel or concentrical
with these, but diminish in length as they recede downwards. It will be
seen, therefore, that when the joints of the foot-jaw are thrown out,
approaching to a straight line, the curved hairs are made to diverge;
but as the cast is made they resume their parallelism, and sweep in, as
with a net, the atoms of the embraced water[48].’
All this description may be verified from a spirit specimen, if the
foot-jaws be carefully removed. And the examination with the lens will
also show that these hairs are plumose, that is, set with smaller hair,
like the barbs of a feather.
At this point we may conveniently take leave of the Stalk-eyed, and
pass on to the Sessile-eyed, Crustacea. Leaving the Cuma´cea out
of the question, we have two Sub-orders from which to choose our
subjects--the Amphip´oda and the Isop´oda--conveniently Englished,
Am´phipods and I´sopods. We learn from Mr. Stebbing[49] that ‘the
Amphip´oda, which are common in fresh as well as in salt water, were
so named by the French naturalist Latreille, as having feet extending
in all directions, their limbs at the same time having much diversity
of form, in correspondence with diversity of function. The Isop´oda,
or equalfooted animals, besides being found both in fresh and salt
water, have more decidedly than the Amphip´oda extended their range
to the dry land. The name was invented by Latreille in ignorance of
the great number of species, since investigated, in which the feet are
strikingly unlike and unequal. Nevertheless, the name may stand, just
as a rose remains a rose even when it is not rose-coloured.’ These last
two sentences must be borne in mind, for they throw great light on a
subject that may give us some trouble--the question of priority in
nomenclature.
The majority of the Amphipods live in salt water, but a few are found
in ponds and streams, and some dwell on the shore, near, but not in,
the sea. The animals of this Sub-order are distinctly segmented, and
three regions may be distinguished thus [image], where C stands for
the Cephalon, or head, Per. for the Perei´on, or body, and Pl. for
the Ple´on (literally, the swimming part), or tail. On the head we
shall find two pairs of antennae, the eyes, and the mouth appendages.
Each segment of the perei´on bears a pair of appendages; the first
two pairs are called respectively the first and second gnath´opods
(or jaw-feet), and the other five pairs perei´opods, or walking feet.
The pleon carries three pairs of ple´opods, or swimming feet, on the
first three segments, and each of the following three has a pair of
uropods or tail appendages. It is well to make out these parts in every
specimen that comes in our way. More is learnt by breaking up one
specimen than by reading the clearest description so often that one
knows it by heart.
[Illustration: FIG. 71.--Gammarus. (After Sars.)]
We may begin with the Fresh-water Shrimp (_Gam´marus pulex_),
which may be taken abundantly in running water where there is plenty
of vegetation. Willow-moss affords these Crustaceans a favourite
hiding-place. It is an excellent plan to gather a quantity of
weed and shake it over a newspaper or a piece of mackintosh. The
‘Shrimps’--which, by the way, are not really Shrimps--will be dislodged
from the weed, and we shall see them wriggling along on their sides,
from which habit they and their near relatives are often called
‘Scuds,’ and ‘Screws.’ They are useful inmates of an aquarium, because
they feed on decaying animal matter, and so keep the water pure and
sweet. Opinions are divided as to whether these animals feed on
water-plants; it is probable that when their natural food fails them,
they take what comes in their way. I have kept marine and fresh-water
species of Gammarus (the genus to which the Fresh-water Shrimp belongs)
in tanks which contained no other animals, but plenty of vegetation,
and both lived and did well for a considerable time. They are by no
means unwilling to make a meal off the dead body of one of their own
species; but it is exceedingly doubtful if they prey on each other, as
some old writers have asserted. This notion probably arose from the
fact that the male carries the female, which is much smaller, about
with him, during the period of courtship, holding her tightly beneath
his body by means of the fingers of its first two pairs of hands. The
habit is not confined to this genus, nor even to this Sub-order; for
some, if not all, the species of Idotea carry on their courtship in
similar fashion, as does also the Water Woodlouse. For the verification
of statements such as these, a small aquarium is necessary, but the
animals will not be under observation long before the observer will be
convinced of their truth.
All species of Gammarus, whether living in the sea or fresh water,
may be readily distinguished by the rows of small spines on the three
hinder segments of the pleon, for this is one of the characteristic
marks of the genus. After we have kept specimens in the aquarium for
a time, so as to become familiar with their general appearance and
habits, we will put them to practical use by breaking them up.
Our first task is to work over the animal as a whole, and to make
out the three regions--ceph´alon, or head; pereī´on, or body; and
plē´on, or swimming part, or tail--into which it is divisible. It
will not be sufficient to do this once, and then to imagine we have
the whole matter fixed in our memory. It should be repeated over and
over again, with every specimen that comes into our hands, till we know
these regions practically, and the number and kind of appendages they
carry. And then the three rows of spines are to be looked for. For all
this the inch lens will be quite sufficient.
Now let us separate the head. When this is done, and if we use the
lens, we shall at once be able to account for the name ‘Sessile-eyed
Crustacea,’ for the difference between the eyes of our specimen and
those of a shrimp or a crab will be evident. Nor can there be any doubt
that they are compound eyes, though the outer integument is not divided
into facets. The antennae are next to be considered. Of these there
are two pairs, the superior, or inner, pair being the longer. These
antennae consist of three basal joints and a many-jointed flagellum,
or lash, and on each of the inner pair is a secondary appendage,
arising from the distal (or outer) end of the third basal joint. We
may represent one of the superior antennae thus: [image]. The dashes
represent the three basal joints, the long row of dots the many-jointed
flagellum, and the slanting row of dots stands for the secondary
appendage. The sensory-hairs on the joints of the flagellum should be
looked for, and may be made out with the inch lens. The same power will
show the denticle, or tooth-like projection at the base of the lower
antennae.
Next come the mouth parts. As compared with Crabs, Amphipods seem badly
off in this respect; for the second and third maxillipedes of the
former become the first and second gnathopods of the latter, so that
instead of six pairs of mouth organs the Amphipods have only four.
It is not an easy matter for a beginner to separate the mouth parts
of an Amphipod, but the difficulty is not insuperable, and will grow
‘small by degrees and beautifully less’ with practice. We have to make
out four pairs of organs arranged in the order given at the side of the
page, the mandibles being the innermost.
MANDIBLES.
FIRST MAXILLAE.
SECOND MAXILLAE.
MAXILLIPEDES.
Of course we must begin with the maxillipedes (Fig. 72). The specimen
may be held between the finger and thumb, and the parts picked out with
a needle. It is, however, better and easier to make the dissection
under water. Then we can remove the second and first maxillae, the
latter easily recognizable by its palp or feeler. Last of all come the
mandibles, also bearing a palp. We shall _feel_ these under the
needle, because of their hardened cutting edges. These are distinctly
toothed. When practice has made the separation of these parts fairly
easy, they should be compared with the mouth parts of other members of
the group, so as to utilize the knowledge we have gained.
[Illustration: FIG. 72.--Maxillipedes of _Gammarus
marinus_ (in centre). _A._ First maxilla. _B._
Second maxilla (magnified).]
Next come the two pairs of gnathopods, and in these we have to find
seven joints--which may be denoted by the numbers 1, 2, 3, 4, 5, 6, 7;
1 being the basal joint, or that nearest the body. The sixth joint is
often called the ‘hand,’ and the seventh, the ‘finger.’ The joints vary
greatly in different genera. The walking legs are next to be examined,
and we may notice that the first and second pairs are turned forwards,
and the third, fourth, and fifth pairs backwards. At the bases of these
legs are the breathing apparatus, and the females have leaf-like plates
on the anterior four pairs, forming a pouch in which the eggs are
hatched, and here she shelters her young, and carries them about with
her.
The following account of this habit is taken from Bate and Westwood’s
_Sessile-eyed Crustaceae_ (i. pp. 380, 381), and was furnished to
the authors of that book by Dr. James Salter: ‘On catching a female
with live larvae, nothing is seen of the progeny till the parent has
become at home in the aquarium, when the little creatures leave her
and swim about in her immediate neighbourhood. The plan I have adopted
to watch this curious habit of maternal protection, has been to place
a single individual in a bottle of sea water. After a time, and that
soon, the little crustacean seems at ease and swims slowly about,
when the young fry leave her and swarm around her in a perfect cloud;
they never leave her for more than half or three-quarters of an inch,
and as she slowly moves about they accompany her. If now one taps the
side of the bottle with one’s finger-nail, the swarm of larvae rush
under their parent, and in a second are out of sight. The parent now
becomes excited, and swims about quickly, as if trying to escape;
but by letting the bottle containing her rest quite still she soon
gets composed, when out come the young larvae again and swim about
as before. This may be repeated as often as the observer wishes, and
always with the same result. I have only seen this in one species, but
it is quite a common species in Poole Harbour, and I have watched the
interesting habit many times.’
The swimming legs are, roughly speaking, [image]-shaped--that is, they
consist of a stem, carrying two many-jointed filaments, fringed with
fine plumose hairs. A hair is said to be plumose when it bears smaller
and finer hairs on each side. ‘By folding the tail beneath the body,
and suddenly striking it out again, those animals which exist in the
water, as well as those which live on the shore, are enabled to dart or
leap to a considerable distance[50].’
[Illustration: FIG. 73.--Nest-building Amphipod. (From
life.)]
Our hand lens may be well employed in watching some of the
nest-building Amphipods at work in the aquarium. There can be no
difficulty in keeping these creatures in captivity, and under
observation, as they build their tubes and rear their families. They
are plentiful in every rock pool round the coast, and it would be a
hard matter to dip the net into any such pool without getting a few
specimens.
They need absolutely no care. The aquarium of the specimen figured
was a four-ounce bottle, tightly corked; and in it was a spray of
Cladophora, on which the animal fed, and the growth of which broke up
the carbon dioxide and set free good store of oxygen. Here it lived for
some months, and built more than one tube for itself against the side
of the bottle.
It is easy enough through the pocket lens to watch the Amphipod at
work. Like other builders, the first thing it does is to get its
materials ready. Lying on its side, with its back against the glass,
it will rake together with its antennae and jaw-feet a good store of
vegetable _débris_, or if there be no supply of this, will break
off branches from the growing weed.
But gathering vegetable _débris_, or even filaments of living
weed, is very far from being tube-building. Something more is needed
to bind the mass into a coherent structure. This the animal itself
supplies. The bases of the first two pairs of walking feet are large,
and contain glands which secrete a glutinous cement, that can be spread
like mortar, or spun out into threads.
An American observer devoted much time to the observation of these
animals. He says[51]: ‘When captured and placed in a small zoophyte
trough, with small branching algae, the individuals almost always
proceeded at once to construct a tube, and could very readily be
observed under the microscope.... The branches were not usually at once
brought near enough together to serve as the framework of the tube, but
were gradually brought together by pulling them in and fastening them
a little at a time until they were brought into the proper position,
where they were firmly held by means of a thick network of fine threads
of cement spun from branch to branch. After the tube had assumed
very nearly its completed form, it was still usually nothing but a
transparent network of cement-threads woven among the branches of the
weed.’
Then he describes the method in which the Amphipod works up bits
of weed and its own droppings into the framework of the tube. In
putting its foecal pellets to this use, it reminds one of a species of
Melicerta (_Melicerta janus_)[52], which employs the same material
to coat its gelatinous sheath.
In breaking up weed and pellets with its foot-jaws and (probably) its
mandibles, the Amphipod recalls the practice of some of the Masking
Crabs, which have been seen to apply to the mouth the material they
were using to deck themselves. Dr. Aurivillius suggests that in the
case of the Crabs there may be an adhesive secretion from the mouth,
as there is possibly in the Amphipods. ‘The spinning was done wholly
with the first and second pereiopods, the tips of which were touched,
from point to point over the inside of the skeleton tube, in a way
that recalled strongly the movements of the hands in playing upon a
piano. The cement adhered at once to the points touched, and spun out
between them in uniform delicate threads. The threads seemed to harden
very quickly after they were spun, and did not seem, even from the
first, to adhere to the animal itself. In one case, in which the entire
construction of the tube was watched, the work was apparently very
nearly or quite completed in little more than half an hour.’
The species we are likely to meet with in rock-pools fashion their
tubes in a similar way. The only difference to be noted is that they
employ less cement, and a larger proportion of broken-down weed and
other matters.
The Sand-hopper (_Tali´trus locus´ta_) and the Shore-hopper
(_Orches´tia littorea_) are so exceedingly plentiful that it may
be well to collect and preserve some during any visit to the seaside.
Both are of fairly large size, and present no great difficulty to us in
making out their several parts. Let us take the Sand-hopper first.
Sand-hoppers swarm on most sandy shores, where they perform the useful
part of scavengers. They are always found above high-water mark, and
do not enter the sea of their own accord. In hunting for them it is a
good plan to turn over decaying masses of sea-weed, for under them the
Sand-hoppers are sure to swarm.
Strange tales have been told of their voracity. Bate and Westwood[53]
record the story of a correspondent who says that at Whitsand he ‘saw
“not millions, but cartloads,” of this species lying piled together
along the margin of the sea. They hopped and leaped about, devouring
each other, as if for very wantonness. A handkerchief, which a lady let
fall amongst them, was soon reduced to a piece of open work by the
minute jaws of these small creatures.’
This statement has been copied into a good many books, without
criticism. At last Mr. David Robertson tried various experiments with
a view to discover if these creatures would feed on each other, or,
failing other food, put up with cambric or muslin. The specimens upon
which he made his observations did neither the one nor the other. Mr.
Robertson embodied the results of his experiments in a paper which he
read before the Natural History Society of Glasgow[54]. And the story
may be read in an abbreviated form in the Rev. T. R. R. Stebbing’s
_Naturalist of Cumbrae_, p. 329.
In Gammarus we have a standard with which to compare our Sand-hopper.
The first thing to notice is the difference in the antennae. Here the
superior pair are very short, and carry no secondary appendage, and
the lower pair have no denticle or tooth-like process. There is also
considerable difference in the gnathopods, or jaw-feet, the sixth joint
of which, in the Sand-hopper, does not form a ‘hand.’ The pleopods, or
swimming feet, are small, and are used for leaping. We shall also find
some difference in the details of the mouth parts, especially in the
toothing of the mandibles.
We now come naturally to the Isop´oda, which are distinguished by the
nearly uniform size of the seven segments which constitute the trunk,
and the seven pairs of limbs borne by these segments. The head is
distinct, and the breathing apparatus is carried on the under side of
the pleon--in these animals not the ‘swimming’ part--five pairs of
plates lying one over another, sometimes covered by a larger outer pair.
A normal I´sopod may be represented [image:--·······--], where the
small dash will stand for the cephalon, or head; the seven dots for the
segments of the perei´on, and the long dash for the pleon.
The Common Asellus (_Asellus aquaticus_) of ponds and ditches
is an excellent subject. It lives well in confinement, and if the
conditions are fairly favourable, will increase and multiply. These
animals will forage for themselves, and pick up a comfortable living
from the vegetable _débris_ that always accumulates at the bottom
of an aquarium, and they are not averse from an occasional meal of
animal food.
[Illustration: FIG. 74. Water Woodlouse.]
While our specimens of Asellus are moving about in any convenient
vessel, we may verify with the hand lens what has been said about the
general form. Then we may notice the antennae, the inner pair being
much the smaller. There can be no difficulty in discriminating the head
and the eyes; the seven segments of the perei´on, each bearing a pair
of limbs; and the pleon with its two terminal appendages. These last
consist of a stalk bearing two longer filaments, armed with spines, and
ending in a small pencil of hairs.
It is easy to see that the segments of the pleon have coalesced, so as
to form a continuous plate or shield on the upper surface.
If we now take our dissecting microscope and place an Asellus in some
water in an excavated 3 in. by 1 in. slip on the stage, examination
with an inch lens will show us a considerable amount of detail. With
the half-inch Leitz lens (see p. 18) one may see quite clearly the
beautifully annulated form of the flagella of the antennae, the sensory
hairs with which these organs are set, and the circulation of the blood
in the limbs and the antennae--the corpuscles moving in a continuous
stream. More than this: we shall be able with the same power to
distinguish tufts of Vorticellids that settle on the Asellus, and the
commensal rotifers that roam about on the body of their host, generally
on the limbs and under surface.
Now we may turn the Asellus on its back, to examine the breathing
apparatus more closely than we were able to do when the creature was
moving about in the bottle. It will be easy to make out the opercular
plates--modified tail appendages--that open and shut to admit water to,
or allow it to flow out from, the true breathing-plates which function
as gills, and correspond to the swimming feet of the Amphipods.
In the female there is a pouch beneath the perei´on, in which the eggs
are carried till they are hatched, and which serves as a nursery and
refuge for the young.
If a good number of these animals be collected they will probably
breed, and then there will be the opportunity of seeing for ourselves
the young carried about in the incubatory pouch.
There are two other aquatic I´sopods which will make good subjects for
us on account of their great abundance, and the ease with which they
may be kept in any improvised aquarium, with a little weed. They may
both be taken in brackish water, and will live and thrive in fresh
water, without any admixture of salt. Indeed, both have lived for some
months in a small bottle of New River water, in which the only weed is
some willow moss. They feed on this and on the vegetable _débris_
that accumulates at the bottom of the bottle, and both species have
bred.
The first is Idot´ea (_I. pelag´ica_), a long, narrow creature,
with very short inner antennae. The last four segments of the pleon
form a plate on the upper surface; and on the under surface the
opercular plates may be opened like tiny folding-doors, to show the
breathing plates.
These vary greatly in colour. Of another species, Spence, Bate, and
Westwood say: ‘According to our experience the colour of the animal is
dependent upon that of the weed on which it lives. Those that live on
the black fucus are generally very dark purple, while those that we
find on the green algae are brightly verdant; and it has always been
our opinion that this change was due to the food[55].’
The other little creature is called Sphaero´ma--it has no English
name--from the fact that it can roll itself into a ball. It is not
difficult to identify, from the fact that all the segments of the pleon
are joined into one plate, the hinder margin of which is entire, thus
[image].
The garden will afford us a hunting-ground for the last specimen of
this group for which we have space--the Woodlice. Enough has been said
of the method of looking over and breaking-up I´sopods generally to
render detailed description unnecessary. The inner pair of antennae,
however, are so small as to be readily overlooked: indeed, on first
sight these creatures seem to have but a single pair. Some have, and
others have not, the power of rolling themselves into a ball; and,
concerning the former, Swammerdam tells the following story:--
‘One of our maidservants had at one time found a great number of
Woodlice in the garden, contracted into round balls ..., and thinking
she had found a kind of coral beads, she began to put them one after
another on a thread, but it soon happened that these little creatures,
which roll themselves up in such a manner only for fear of harm, and
appear as if they were dead, being obliged to throw off their mask,
resumed their motions. On seeing which, the maidservant was so greatly
astonished, that she threw away the Woodlice and the thread, and cried
out, and ran away[56].’