CRUSTACEA AS PARASITES AND MESSMATES
The life of every animal is in more or less intimate relation with that
of all the living creatures which surround it. Some serve for its food,
or supply it with shelter or foothold; others prey upon it, or compete
with it for the necessaries of life; and others, again, influence it for
good or evil in countless ways more subtle than these, but equally
important. There are some associations of a closer and more enduring
nature, to which the names of Symbiosis, Commensalism, and Parasitism,
are applied, and it is with examples of these that the present chapter
is concerned.
The term Symbiosis is strictly applied to an intimate physiological
partnership, such as we find in some of the lower animals and plants,
and in this sense there are no truly symbiotic Crustacea. The word,
however, is sometimes used, in its literal sense of a "living together,"
to embrace all cases of animals living together for mutual advantage.
Commensalism means, literally, "sitting at the same table," and ought
to be applied only to cases where two or more animals, living together
as "messmates," partake of the same food; but it is sometimes used more
loosely to include instances where one of the animals does not actually
share in the food-supply of the other. Parasitism, again, implies that
the parasite lives permanently at the expense of its host, by sucking
its juices or otherwise, and in this case also there are innumerable
degrees and varieties of dependence, which defy inclusion in a strictly
logical scheme of classification. Even such typical parasites as
Tape-worms, for example, might strictly be regarded as commensals,
sharing in the host's food only after it has entered the alimentary
canal. Finally, in all these kinds of interrelation, we find cases where
the association is temporary, intermittent, or almost accidental, and
where there are no perceptible adaptations of structure directed to its
maintenance in either of the partners. From these we may trace a series
of gradations leading to cases where the associated organisms are never
found apart, and where the structure of both is profoundly modified in
adaptation to the particular form of association.
Perhaps the simplest form of association between two animals is found
where one utilizes the other as a means of transport. The little
Gulf-weed Crab, previously mentioned, is very often found clinging to
the carapace or skin of large marine Turtles. It is not a parasite,
since it can hardly derive any food from the Turtle itself; neither
is it a commensal, for there is no evidence that it shares in the
Turtle's meals. It probably takes to a Turtle, when it can find one, as
giving it a wider range of operations than is afforded by its usual
drift-log or tuft of sargasso-weed. A somewhat similar case is afforded
by some of the Barnacles that are found on the skin of Whales. The
species of _Conchoderma_, for instance, are often found on certain
Whales, but they may also occur on inanimate floating objects. Other
Whale-infesting Cirripedes, however, are specially adapted to their
habitat, and never occur elsewhere. For example, _Coronula_ (Plate
XXVIII.) is a genus of sessile Barnacles in which the shell is
elaborately folded, forming a series of chambers into which
prolongations of the Whale's epidermis grow, securely fixing the shell.
_Tubicinella_ is even more effectively protected against dislodgment,
for its shell is sunk in the thickness of the Whale's skin, with only
the opening exposed. Other genera of sessile Barnacles (_Chelonobia_,
etc.) are found adhering to the shell of Turtles. The increased
food-supply made available by the host's movements through the water is
probably the chief advantage that the Barnacles gain in such cases. This
is indicated by the fact that certain small stalked Barnacles
(_Dichelaspis_, etc.), found on large Crabs and Lobsters in tropical
seas, generally cluster on the mouth parts of their hosts, near the
entrances to, or even within, the gill chambers, profiting no doubt by
the respiratory currents and the food particles they carry.
[Illustration: _PLATE XXVIII_
GROUP OF BARNACLES, _Coronula diadema_, ON THE SKIN OF A WHALE.
JAPAN. (REDUCED)]
A great variety of Crustacea find shelter and defence in association
with Sponges, Corals, and other more or less sedentary animals. Sponges
are not eaten by many marine animals, the needle-like spicules which
often form their skeleton no doubt helping to render them distasteful,
and many small Crustacea, Amphipods, Isopods, Prawns, etc., profit by
their immunity from attack, and take up their abode in the internal
channels and cavities of the Sponge. The beautiful siliceous Sponge
known as "Venus's Flower-basket" (_Euplectella_) very often contains
imprisoned within it specimens of a delicate little Prawn (_Spongicola
venusta_) or of an Isopod (_AEga spongiophila_). As these Crustacea share
with the Sponge the food particles drawn in by the currents of water
passing through the pores in its walls, they are in the strict sense
commensals.
The Corals and various other animal organisms commonly known as
"Zoophytes," forming together with the Jellyfishes the group Coelentera,
are very effectively protected against the attacks of most predatory
animals by the possession of "stinging cells," and this protection is
shared by many other animals which shelter among them. Thus, the
branching Coral stocks which grow in great luxuriance on tropical coasts
support a rich and varied assemblage of animals, some of which may
actually prey upon the Coral polypes, but all of which profit by the
fact that few enemies venture to pursue them in their retreats.
Innumerable prawn-like animals of the Alpheidae and other families, and
many kinds of Crabs, are found among living Corals. The Crabs of the
family Trapeziidae are especially characteristic of such habitats, and
their thin, flat bodies seem to be adapted to slip into slits and
crannies of the Coral blocks. The most highly specialized of all Coral
Crabs, however, are the species of the family Hapalocarcinidae, which
modify in various ways the growth of the corals on which they live. In
some of the more delicately branched kinds of Coral there may sometimes
be found hollow bulbous growths, each of which contains imprisoned
within it a little Crab--_Hapalocarcinus marsupialis_ (Fig. 66). It
seems that the female Crab (the habits of the male are not definitely
known) settles down among the branches of the Coral, and that the
irritation of its presence causes the branches to grow up and surround
it, coalescing with each other to form a kind of cage, and ultimately
leaving only one or two small openings. Through these openings water can
enter to enable the Crab to breathe, and no doubt food particles find
their way in, but it is not possible for the Crab to leave its prison.
The production of these abnormal growths of the Coral is closely
analogous to the formation of "galls" on plants as a result of the
irritation set up by the presence of insect larvae or other parasites,
and it is not inappropriate, therefore, to speak of them as "Coral
galls."
[Illustration: FIG. 66--TWO BRANCHES OF A CORAL (_Seriatopora_) SHOWING
"GALLS" INHABITED BY THE CRAB _Hapalocarcinus marsupialis_. ON THE RIGHT
THE FEMALE CRAB, EXTRACTED FROM THE GALL AND FURTHER ENLARGED]
The Medusae, or Jellyfishes, like other Coelentera, are provided with
poisonous stinging cells, which, in the larger species of our own seas,
are powerful enough to cause discomfort to bathers who come in contact
with them. The protection thus afforded is no doubt of advantage to the
little globular Amphipods of the genus _Hyperia_ (Fig. 67), which are
almost always to be found sheltering under the bells of the larger
Medusae. In what way the Amphipods escape injury from the stinging cells
of their host is not known.
[Illustration: FIG. 67--_Hyperia galba_, FEMALE. ENLARGED. (After
Sars.)]
In all the cases mentioned, the advantages of the partnership seem to be
all on one side, but there are numerous instances in which both partners
seem to reap some benefit. A species of Hermit Crab very common in
moderately deep water on many parts of the British coasts, _Eupagurus
prideauxi_, is always found to have a Sea-anemone (_Adamsia palliata_)
attached to the shell which it carries. The Anemone has a broad base
which is wrapped round the shell, the mouth, surrounded by the
tentacles, being on the under-side next the opening of the shell. There
seems no reason to doubt that the presence of the Anemone does afford
some degree of protection to the Hermit, and that, on the other hand,
the Anemone benefits by being carried about, and shares in the crumbs
from the Hermit's meals. It is stated that, when the Hermit removes to a
new shell, it detaches the Anemone from the old shell with its pincers
and places it in position on the new one. It appears, however, that it
is not always necessary for the Hermit to remove to a larger shell as it
grows, for the enveloping Anemone, as it increases in size, extends
beyond the mouth of the shell, and so enlarges the shelter. Further, the
Anemone in course of time dissolves the shell almost entirely away, and
the Hermit is enveloped only by the soft fleshy mantle which it forms.
In a similar way the deep-sea Hermit Crab _Parapagurus pilosimanus_ (see
Plate XVI.) is always found lodged in a fleshy mass formed by a colony
of Sea-anemones (_Epizoanthus_), within which, when it is cut open, may
be found the remains of the shell which the Hermit first inhabited. A
further development of the same habit is given by _Paguropsis typica_,
found in deep water in Indian seas, which does not inhabit a shell at
any time, but carries a fleshy blanket formed by a colony of Anemones.
In dredging off the British coasts, we often find smooth rounded lumps
of a Sponge (_Suberites ficus_), generally yellowish-grey in colour,
having a round opening in which the claws of a small Hermit Crab
(_Eupagurus cuanensis_) may be seen. On cutting open the Sponge, the
body of the Hermit is seen to be lodged in a spiral cavity, and at the
apex may be found the remains of a shell that has been corroded away by
the Sponge which settled on and replaced it. Other species of Hermit
Crabs constantly have their shells covered with a horny crust formed by
Hydroid zoophytes (_Hydractinia_, etc.), and in this case also the
extension of the Hydroid colony beyond the lip of the shell relieves the
Hermit from the necessity of so frequently changing to a larger shell as
it grows.
A number of other animals are found associated with Hermit Crabs,
without, as far as we can see, rendering any service in return for the
house-room. The Whelk-shells inhabited by _Eupagurus bernhardus_ (see
Plate VII.) often contain one of the bristle-footed worms (_Nereilepas
fucata_), which may sometimes be observed to protrude its head from the
shell when the Crab is feeding, and to snatch away fragments of the prey
from the very jaws of its host. It is thus, in the strict sense of the
word, a commensal. Species of Copepods, Amphipods, Porcelain Crabs, and
even a Mysid, have been found sharing the lodging of Hermit Crabs in a
similar way, and in addition there are various parasites, presently to
be mentioned, found on the Crabs themselves, so that each Crab forms the
centre of a whole community of widely diverse organisms all more or less
directly dependent on it.
A habit similar to those of some Hermit Crabs is that of the Crab
_Dromia_ (see Plate IX.), mentioned in a previous chapter, which
carries, as a cloak, a mass of living sponge, holding it in position by
means of the last two pairs of legs. Even the "masking" habit of the
Spider Crabs, already described (p. 96), may be regarded as a kind of
symbiosis, since the sponges, zoophytes, etc., which grow on the Crabs
no doubt benefit by being carried about in return for the protection
they give.
[Illustration: FIG. 68--A, THE CRAB _Melia tessellata_ CLINGING TO A
BRANCH OF CORAL, AND CARRYING IN EACH CLAW A LIVING SEA-ANEMONE; B, ONE
OF THE CLAWS FURTHER ENLARGED TO SHOW THE WAY IN WHICH THE ANEMONE IS
HELD. (After Borradaile.)]
One of the strangest habits is that of certain little tropical Crabs, of
which _Melia tessellata_ (Fig. 68) is the best known, which carry in
each claw a living Sea-anemone and use it as a weapon. The claws or
chelipeds are in this case of small size, so that they would be of
little use by themselves for attack or defence; but the fingers are
provided with recurved teeth, enabling them to take a firm hold of the
slippery body of the Anemone. Particles of food caught by the tentacles
of the Anemone are removed and eaten by the Crab, which uses for the
purpose the long walking legs of the first pair. The same limbs are also
used in the process of detaching the Anemones from the stone on which
they may be growing. The Anemones do not appear to suffer from the rough
treatment to which they are subjected, but whether they can reap any
benefit from the partnership is very doubtful.
[Illustration: FIG. 69--THE COMMON PEA CRAB (_Pinnotheres pisum_),
FEMALE. NATURAL SIZE.]
From remote antiquity it has been known that a little Crab (Fig. 69) is
frequently found living within the shells of bivalve Molluscs, such as
Oysters, Mussels, and especially the large mussel-like _Pinna_, which is
common in the Mediterranean. Ancient writers regarded this as a case of
association for mutual advantage, believing that the _Pinnotheres_
warned the _Pinna_ of the approach of enemies or of the entrance of prey
between its gaping valves. It is even stated that the Pinna and Crab
were depicted in Egyptian hieroglyphics to symbolize the dependence of a
man on his friends.
As a matter of fact, however, there is no reason to believe that the
Molluscs which harbour species of _Pinnotheres_ and allied genera
benefit in any way by the presence of the Crabs. The latter probably
feed, as their hosts do, on particles brought in by the current of water
entering the mantle cavity. They are therefore strictly "commensals,"
though it is usual, and perhaps equally correct, to speak of them as
"parasites." The case is, indeed, an example of the difficulty of
defining these two terms. At all events, the Pinnotherid Crabs show one
of the characteristics of parasites in being to some extent degenerate
in their structure. The carapace and the rest of the exoskeleton, no
longer needed for protection, have become soft and membranous, and the
eyes and antennules, the chief organs of sense, are very minute. As in
many parasites, also, the eggs produced by the female are very numerous,
and the abdomen is very broad and deeply hollowed for their reception.
While most of the Pinnotheridae live in bivalve Molluscs, some species
are associated with other invertebrate animals. _Pinnaxodes chilensis_
is found in a species of Sea-urchin (_Strongylocentrotus gibbosus_) on
the coast of Chili. On opening the shell of the Urchin, the Crab is
found enclosed in a thin-walled bag formed by enlargement of the
terminal part of the host's intestine.
It did not escape the notice of Aristotle that a little Shrimp sometimes
occurred in the _Pinna_ in place of the Crab. This is _Pontonia custos_,
and other species of the same and allied genera have similar habits.
The order Isopoda includes a very large number of parasitic species. The
extensive family Cymothoidae presents a whole series of gradations in
habits and structure between actively swimming predatory species and
others which in the adult state are permanently fixed to their host,
usually a fish, and are incapable of movement. At one end of the series
are the species of _Cirolana_, which have powerful biting jaws. Of _C.
borealis_ (Fig. 70), Mr. Stebbing remarks that "it is a good swimmer,
tenacious of life, a savage devourer of fish, and not to be held in the
human hand with impunity." The species is not uncommon in British seas,
and numerous individuals will sometimes attack a Cod or other large
fish, perhaps after it has been caught on a hook, and gnaw their way
into its body, so that when brought to the surface the fish consists of
little more than skin and bone.
[Illustration: FIG. 70--_Cirolana borealis._ ABOUT TWICE NATURAL SIZE.
(After Sars.)]
The little _Eurydice achatus_, belonging to the same subfamily,
Cirolaninae, is commonly taken in the tow-net in sandy bays on our own
coasts. It is said sometimes to attack bathers, and to "nip most
unpleasantly."
More definitely parasitic are the species of _AEga_ and allied genera,
which have piercing and suctorial mouth parts, and suck the blood of
fish. They are usually found adhering closely to the skin of their
victim by means of the strong hooked claws of the anterior pairs of
legs; but they have not lost the power of locomotion, and, as females
bearing eggs are never taken on fish, it would appear that they drop off
after gorging themselves with blood, and probably seek a retreat at the
bottom of the sea, where they may hatch their young in safety. The
digestive canal of _AEga_ dilates into a large bag, which becomes
distended with a semi-solid mass of blood. This mass, when extracted and
dried, is the "Peter's stone" of old Icelandic folklore, to which
magical and medicinal virtues were attributed. The species _AEga
spongiophila_, already mentioned, differs in its habits from all the
other species of the genus, since it lives, not on fish, but in the
interior of a sponge.
[Illustration: _PLATE XXIX_
_Cymothoa oestrum_, AN ISOPOD PARASITE OF FISH (SLIGHTLY ENLARGED)
_Sacculina carcini_ ATTACHED UNDER THE ABDOMEN OF A COMMON SHORE-CRAB
(REDUCED)]
The most completely parasitic members of the Cymothoidae are found in the
subfamily Cymothoinae, including the typical genus _Cymothoa_ (Plate
XXIX.) and many others. The adult animals are found clinging to the skin
of fishes, the legs being provided with strong hook-like claws that give
them a very firm hold. Some species, especially common on Flying-fishes,
cling to the tongue of the fish, and almost prevent it from closing its
mouth. When young, the Cymothoinae swim freely, and the shape of the body
is not unlike that of the Cirolaninae; but after they have settled on a
host the body often becomes distorted and unsymmetrical. A still more
remarkable change occurs in the reproductive organs in some, if not in
all members of this subfamily. Each individual, when it first
attaches itself to a host, presents the characters of the male sex.
Later it becomes a female, develops a brood-pouch, and produces eggs.
The animals are, in fact, hermaphrodite; but it is to be noted that the
hermaphroditism is of a different kind from that presented by the
Cirripedia, since the organs of the two sexes are successively, not
simultaneously, developed. Where, as in this case, the male phase comes
first in the life-history of the individual, the condition is known as
"protandrous" hermaphroditism.
Another large group of parasitic Isopods is the suborder Epicaridea, all
the species of which are parasitic on other Crustacea. It is not
uncommon to find specimens of the common Prawn (_Leander serratus_)
which have a large swelling on one side of the carapace. If the lower
edge of the carapace be raised, it will be seen that this swelling is
due to the presence in the gill cavity of an Isopod parasite (_Bopyrus
squillarum_). A closely similar form, found on Prawns of the genus
_Spirontocaris_, is _Bopyroides hippolytes_, represented in Fig. 71.
Other allied species are found on Hermit Crabs and other Decapods. When
extracted, the parasite is seen to have a flat and curiously distorted
body, with extremely short legs ending in hooked claws. The under-side
is generally occupied by a relatively enormous mass of eggs, which is
only partly covered in by the small brood-plates. The mouth parts form
a short piercing beak with which the parasite sucks the blood of its
host. On the under-side of the abdomen may usually be found the minute
male, attached, like a secondary parasite, to the body of the female.
[Illustration: FIG. 71--A, FRONT PART OF BODY OF A PRAWN (_Spirontocaris
polaris_), FROM ABOVE, SHOWING ON THE RIGHT SIDE A SWELLING OF THE
CARAPACE CAUSED BY THE PRESENCE OF THE PARASITE _Bopyroides hippolytes_
IN THE GILL CHAMBER; B, THE FEMALE PARASITE EXTRACTED AND FURTHER
ENLARGED; C, THE MALE PARASITE ON SAME SCALE AS THE FEMALE. (After
Sars.)]
The species of Epicaridea are very numerous, and they infest Crustacea
belonging to nearly all the chief groups of the class, a few even being
parasitic on other Epicaridea. Many of them differ greatly from the
_Bopyrus_ just described, and in some cases it would be impossible to
guess from the structure of the adult animals that they were Isopoda, or
even Crustacea at all. The life-history is not yet completely known.
When hatched from the egg, the free-swimming larvae have a short and
broad body, and, as in other Isopod larvae, have only six instead of
seven pairs of legs. A later larval stage, just before attachment to the
final host, has a long narrow body and the full number of legs. It has
lately been shown, however, that, in all probability, between these two
free-swimming stages there intervenes a stage in which the larvae is
temporarily parasitic on certain Copepoda. Further, some of the
Epicaridea, like the Cymothoinae described above, are protandrous
hermaphrodites, developing the male organs when in the last larval
stage, and passing into the female phase after they have become attached
to the host. In _Bopyrus_ and many other genera, however, there is no
evidence that the males ever develop into females.
Some of the most remarkable Epicaridea are those belonging to the family
Entoniscidae, which are parasitic on Crabs. In these the parasite
penetrates from the gill chamber into the interior of the body of the
host, remaining enveloped, however, by a delicate membrane which grows
in with it from the wall of the gill chamber. The body is distorted in
an extraordinary fashion, so that at first sight it seems impossible to
trace any resemblance to the form of a typical Isopod.
Among the Amphipoda there are a few species belonging to various
families of the Gammaridea which have suctorial mouth parts, and lead a
semi-parasitic existence; but the only completely parasitic forms are
the Whale-lice, forming the family Cyamidae (see Fig. 23, p. 55) in the
suborder Caprellidea. Although differing greatly in the broad, flattened
shape of the body from the slender, thread-like Caprellidae, they closely
resemble them in structure, particularly in having the abdomen reduced
to a mere knob. The fourth and fifth pairs of thoracic limbs have
disappeared, although the gills corresponding to them are very large;
and the last three pairs of legs have long curved claws which enable the
Whale-louse to cling firmly to the skin of its host. The mouth parts are
adapted for biting, not for sucking blood, and the animals seem to live
by gnawing the skin of the Whales. In one respect the Whale-lice are
unique among Crustacean parasites: they have not the power of swimming
at any period of their life-history. The young settle down near their
parents, and masses of many hundred individuals of all sizes are found
clinging close together on the skin of the host.
No group of Crustacea exhibits more numerous or more varied examples of
parasitism than the Copepoda. Every grade of transition between a free
predatory habit of life and the most complete dependence upon a host may
be traced in various families of the subclass. Only a few examples can
be mentioned here.
[Illustration: FIG. 72--A FISH-LOUSE (_Caligus rapax_), FEMALE. x 5.
(After Wilson.)]
The commonest "fish-lice" are the numerous species of the family
Caligidae, many of which, belonging to the genera _Caligus_ (Fig. 72),
_Lepeophthirus_, etc., are found on marine fishes on our own coasts. In
these the body is broad and flat, but in many of them the resemblance,
even in general form, to the free-living Copepoda is easily traceable.
The maxillipeds form powerful hooked claws, by means of which the
animals cling to the skin of the fish they infest, and in _Caligus_ the
basal segments of the antennules have a pair of suckers which aid in
adhesion. The mouth parts are adapted for piercing, and are enclosed in
a suctorial proboscis.
When the young Caligid, after passing through the free-swimming larval
stages, first becomes attached to a fish, it adheres by means of a
thread-like process issuing from the front of the head, and formed by
the secretion of a gland. At this stage, formerly described as an
independent species under the generic name of _Chalimus_, the parasite
is unable to detach itself from its host; but later, in many species,
it re-acquires the power of swimming, and specimens of _Caligus_, for
instance, are commonly found free in tow-net gatherings.
[Illustration: FIG. 73--STAGES OF DEVELOPMENT OF _Lernaea branchialis_. F
IS SLIGHTLY, THE OTHER FIGURES GREATLY, ENLARGED. (After A. Scott.)
A, Nauplius, just hatched; B, young female taken from gills of Flounder;
C, free-swimming stage of female, after leaving Flounder; D,
free-swimming male; E, female just after settling on gills of Whiting;
F, fully-developed female.]
On the gills of Cod, Haddock, and other common fish, we often find a red
worm-like parasite, _Lernaea branchialis_ (Fig. 73, F), which at first
sight seems to bear no sort of resemblance to a Crustacean. The soft
body is curiously doubled up, and is attached to the host by a narrow
neck; while dissection will reveal a small head buried in the flesh of
the fish's gills, and having three branched outgrowths, which penetrate
into the surrounding tissues and make the attachment of the parasite
more secure. Near the hinder end of the body are two coiled threads,
which are the egg-masses. The reduced mouth parts and the microscopic
vestiges of the swimming feet may be detected on and near the head, but
apart from these it would be hard to find any characters to show that
the animal is a Crustacean.
The life-history of _Lernaea_ is very remarkable. The young are hatched
in the nauplius stage (Fig. 73, A), and after passing through some
further free-swimming stages they become parasitic on a fish. Curiously
enough, however, they choose a very different host from that on which
the adults are found, for at this stage (Fig. 73, B) they attach
themselves to the gills of one of the Flat-fishes (Pleuronectidae), such
as the Flounder, Plaice, etc., attachment being effected by a frontal
cement gland similar to that of the larval Caligidae, already mentioned.
The animal is now without the power of swimming, its appendages becoming
reduced to stumps and losing their setae. After passing some time in
this condition, the larva again acquires the power of swimming, and
leaves its host. Both sexes become mature in this free-swimming stage
(Fig. 73, C, D), and impregnation is effected. The males die without
developing further, but the females seek a second host, a fish of the
family Gadidae, such as the Cod, Haddock, etc., and, settling on the
gills, become metamorphosed (Fig. 73, E) into the adult form described
above.
Within the gill cavities of the strange-looking fish known as the Angler
or Fishing-frog (_Lophius piscatorius_) there may often be found
specimens of another parasitic Copepod, _Chondracanthus gibbosus_. It
has a soft, unsegmented body about half an inch long, provided with
numerous blunt lobes which give it a very irregular shape. On the
under-side, near the front, are forked lobes representing two pairs of
the swimming feet. At the hinder end are usually attached a pair of long
thread-like egg-masses. Just at the point where the egg-masses are
attached, close inspection of the under-side of the body will reveal a
very minute maggot-like object. This is a male individual, which is
attached, like a secondary parasite, to the body of the enormously
larger female.
[Illustration: FIG. 74--STAGES IN THE LIFE-HISTORY OF _Haemocera danae_,
ONE OF THE MONSTRILLIDAE. (From Lankester's "Treatise on Zoology," after
Malaquin.)
A, Free-swimming nauplius larva; B, embryo after penetrating into the
body of the worm _Salmacina_; C, D, E, successive stages in the body of
the host; F, free-swimming adult female. (All greatly enlarged, not to
same scale.) _a'_, Antennule; _br_, brain; _e_, nauplius eye; _f_,
swimming feet; _g.s._, hairs on which the eggs are carried; _m_,
position of mouth; _md_, hooked mandible of nauplius; _n_, nerve cord;
_ov_, mass of eggs carried by female; _ovy_, ovary; _pr_, absorptive
processes.]
In all the cases mentioned, the animal is parasitic in the final state
of its existence--at least in the female sex--but there are a few
Copepoda which are free-swimming, both when young and when adult, but
parasitic in the intermediate stages. Among the Copepoda taken by the
tow-net in British seas, there may sometimes be found species of the
family Monstrillidae (Fig. 74, F), which are remarkable for having no
appendages between the antennules and the first pair of swimming feet.
They have no trace of jaws, and only a minute vestige of a
mouth-opening; while internally there is no food-canal, so that the
animals are incapable of taking nourishment. Their development was for
long a mystery, but it is now known that the greater part of their life
is passed as internal parasites in certain bristle-footed worms
(Polychaeta). The young are hatched as nauplius larvae (Fig. 74, A)
without mouth or food-canal, but capable of swimming, and having the
third pair of appendages (mandibles) furnished with strong hooks, by
means of which they fasten on to the worm which is to serve as their
host. The nauplius bores through the skin of the worm, casting its
cuticle and losing all its appendages in the process, and making its way
into one of the bloodvessels in the form of a little oval mass of cells
(Fig. 74, B), within which no organs except the degenerating nauplius
eye can be detected. It later becomes enclosed in a delicate cuticle,
and from one end two long finger-like processes grow out, which are
believed to have the function of absorbing nourishment from the blood of
the host (Fig. 74, C, D). Within the cuticle the organs of the adult
animal are gradually differentiated (Fig. 74, E), and when fully formed
it bores its way through the tissues of its host by means of rows of
hook-like spines surrounding the pointed posterior end of the sac. On
reaching the surface the enclosing membrane bursts, and the adult animal
is set free.
Of all Crustacean parasites, however, perhaps the most remarkable in
their structure and life-history are the Cirripedes of the order
Rhizocephala. It is not uncommon on the British coasts to find specimens
of the common Shore Crab or other Crabs which carry under the abdomen an
oval fleshy body. This is the Rhizocephalan _Sacculina carcini_ (Plate
XXIX.), and it would hardly be possible to guess, from its appearance or
structure, that it was a Cirripede or a Crustacean at all. It is
attached to the under-side of the Crab's abdomen by a short stalk, and
in the middle of its opposite surface is a small opening which leads
into a cavity separating the outer "mantle" from the body of the animal.
Very often this mantle cavity will be found to be full of eggs enclosed
in sausage-shaped packets. At the point where the short stalk enters the
abdomen of the Crab, it gives off an immense system of fine branching
roots, which penetrate throughout the body of the Crab, and even into
its legs and other appendages. By means of these roots the _Sacculina_
absorbs nourishment from the body-fluids of its host. Like most
Cirripedes, _Sacculina_ is hermaphrodite, and the body within the mantle
cavity contains only the reproductive organs of the two sexes and a
small nerve ganglion representing the whole of the nervous system. There
is no mouth, no food-canal, and no trace of appendages. Another
Rhizocephalan, _Peltogaster_, is not uncommonly found attached to the
abdomen of Hermit Crabs. Although the nauplius larva of _Sacculina_ was
described, and its resemblance to that of the Cirripedia pointed out, as
long ago as 1836, by that acute observer, J. Vaughan Thompson, it is
only recently that the full life-history has been made known by the
researches of Professor Delage and Mr. Geoffrey Smith. The nauplius
larva (Fig. 75, A) resembles that of the normal Cirripedes, especially
in the shape of the dorsal shield, which is drawn out on either side in
front into a pair of fronto-lateral horns. It has, however, no mouth,
and the food-canal is quite absent. As in the normal Cirripedes, the
nauplius is followed by a _cypris_ stage (Fig. 75, B), also mouthless,
and it is in this form that the _Sacculina_ seeks the Crab on which it
is to become parasitic. It would be almost impossible for the _cypris_
larva to settle on that part of the Crab where the adult _Sacculina_ is
afterwards to appear, since the Crab usually has its abdomen closely
pressed against the under-side of its thorax. The larva therefore
attaches itself on some exposed part of the Crab, often on one of the
legs, clinging to a hair by means of its antennules. It bores through
the cuticle at the base of the hair, and the contents of its body pass
into the interior of the Crab as a little mass of cells, the empty
_cypris_ shell being cast off. This mass of cells, which constitutes the
embryo _Sacculina_, is carried about by the blood-currents of the Crab
till it reaches the under-side of the intestine, where it becomes
attached. It now begins to send out roots (Fig. 76), and as it grows the
central mass travels backwards along the intestine of the Crab till it
reaches the place where the adult parasite is to emerge. As the mass
increases in size, and the organs of the _Sacculina_ become
differentiated within it, its presence causes the living tissues
between it and the external cuticle to degenerate, so that when the Crab
moults an opening is left through which the body of the parasite
protrudes. Owing, no doubt, to the drain on its system due to the
presence of the _Sacculina_, the Crab ceases to grow, and it does not
moult again as long as the parasite remains alive.
[Illustration: FIG. 75--FREE-SWIMMING STAGES OF _Sacculina carcini_.
MUCH ENLARGED. (After Delage.)
A, Nauplius; B, cypris stage.]
[Illustration: FIG. 76--EARLY STAGE OF _Sacculina_ WITHIN THE BODY OF A
CRAB. (After G. Smith.)
_i_, Intestine of the Crab; _s_, body of the _Sacculina_, which
afterwards emerges on the under-surface of the Crab's abdomen; _r_,
roots of the _Sacculina_.]
In addition to this arrest of growth, _Sacculina_ produces in its hosts
other changes, which affect chiefly the reproductive organs and the
structures associated therewith. Crabs of either sex infected with
_Sacculina_ are incapable of breeding; the genital gland (ovary or
testis) is found on dissection to be shrivelled up, and the external
characters indicative of sex become strangely modified. The changes
have been most fully studied in the case of a kind of Spider Crab
common at Naples--_Inachus mauritanicus_. In this species it is found
that females infected with _Sacculina_ show no conspicuous external
modification, except that the abdominal appendages, which in the normal
females serve for the attachment of the eggs, are greatly reduced in
size. Infected males, however, may assume to a greater or less degree
the characters proper to the female sex. Some males show little change,
except that the chelipeds remain small and flattened, as in the females
and non-breeding males. Other specimens have, in addition, the abdomen
much broader than in normal males, and sometimes as broad as in the
females. Finally, some males develop on the abdomen, in addition to the
rod-like appendages on the first and second somites, characteristic of
the male sex, two-branched appendages on the next three somites, as in
the females; these individuals are, in fact, so completely intermediate
in character between the two sexes that it is only by dissection that it
is possible to recognize them as modified males.
An indication of the way in which the degenerate Rhizocephala have been
derived from normal Cirripedes is given by a peculiar species of
pedunculate Barnacle, _Anelasma squalicola_, which lives attached to
Sharks and Dogfish in the North Sea. In _Anelasma_ the peduncle becomes
deeply buried in the flesh of the Shark, and its surface is covered with
short branching, root-like filaments. As in the case of the
Rhizocephala, these roots appear to absorb nutriment from the host, and,
although _Anelasma_ possesses a food-canal and mouth, the cirri are
reduced in size and devoid of hairs, so that they cannot be used for
obtaining food as in ordinary Barnacles.