CRUSTACEA (_CONTINUED_)—EUMALACOSTRACA (_CONTINUED_): EUCARIDA—
EUPHAUSIACEA—COMPOUND EYES—DECAPODA
DIVISION 4. EUCARIDA.
The carapace fuses with all the thoracic segments. The eyes are
pedunculate. The mandible is without a lacinia mobilis. There are no
oostegites, the eggs being attached to the endopodites of the pleopods.
The hepatic caeca are much ramified, the heart is abbreviated and
saccular, the spermatozoa are spherical with radiating pseudopodia, and
development is typically attended by a complicated larval metamorphosis.
=Order I. Euphausiacea.=
[Illustration:
FIG. 101.—Calyptopis larva of _Euphausia pellucida_, × about 20.
_A.1_, 1st antenna; _Ab.6_, 6th abdominal segment; _E_, eye; _M_,
maxillipede. (After Sars.)
]
The =Euphausiidae=[116] agree with the Decapoda in passing through a
complicated larval metamorphosis. The young hatch out as Nauplii, with
uniramous first antennae and biramous second antennae and mandibles. In
the next stage, or “Calyptopis” (Fig. 101), which corresponds exactly to
the Zoaea of the Decapoda, two pairs of maxillae and a pair of biramous
maxillipedes are added; the hinder thoracic segments are
undifferentiated, but the abdomen is fully segmented, and the rudiments
of the sixth pair of pleopods are already visible.
In the next stage (“Furcilia”) the other abdominal pleopods are added,
the whole series being completed before the thoracic appendages number
more than two or three. This stage corresponds to the Metazoaea of the
Decapoda, and the interference in the orderly differentiation of the
segments with their appendages from before backwards is a phenomenon
which we shall meet again when we treat of Decapod metamorphosis. It is
evidently a secondary modification, furnishing the larva precociously
with its most important swimming organs so as to enable it to lead a
pelagic existence. The frequent violation of the law of metameric
segmentation, that the most anterior segments being the first formed
should be the first to be fully differentiated, leads us to suppose that
the larval stages of the Eucarida at any rate do not represent
phylogenetic adult stages through which the Malacostraca have passed.
Nor do they, perhaps, even represent primitive larval stages, but have
been secondarily acquired from an embryonic condition which used to be
passed through within the egg-membranes, as in _Nebalia_ and the
Mysidacea, when the order of differentiation of the segments was normal.
The case is a little different with the Nauplius larva. This larval
form, in an identical condition, is found both in the Entomostraca as a
general rule, and again in certain Malacostraca, viz. the Euphausiidae
and the Peneidea. Whatever its phylogenetic meaning may be, we may be
quite certain that the ancestor of the two great divisions of the
Crustacea had a free-swimming Nauplius larva, and this conclusion is
confirmed by the probable presence of a Nauplius larva in Trilobites.
The Euphausiidae, in contradistinction to the Mysidae, are frequently
met with in the surface-plankton. _Euphausia pellucida_ (Fig. 102) is of
universal distribution, and is frequently taken at the surface as well
as at considerable depths.
Many noteworthy features in Euphausiid organisation are brought out in
Fig. 102. The shrimp-like appearance of the carapace and antennae
indicate the special Decapodan affinities of the family; noteworthy,
also, are the single series of gills and the biramous thoracic and
abdominal limbs, similar to those of the Mysidacea. The Euphausiidae
also possess phosphorescent organs of a highly developed kind, and these
are usually situated, as in the type figured, upon the outer margins of
the stalked eyes, on the bases of the second and seventh thoracic limbs,
and on the ventral median line on the first four abdominal segments.
These organs are lantern-like structures provided with a lens, a
reflector, and a light-producing tissue, and they are under the control
of the nervous system. Their exact use is not known, any more than is
the use of phosphorescence in the majority of organisms which produce
it; but in certain cases it appears that the Euphausiids make use of
their phosphorescent organs as bull’s eye lanterns for illuminating the
dark regions into which they penetrate or in which some of them
permanently dwell. At any rate, associated with the presence of these
organs in some deep-sea Euphausiids are remarkable modifications of the
eyes; and we may perhaps here fittingly introduce a short discussion of
these visual modifications in deep-sea Crustacea, and the conditions
which call them forth.
[Illustration:
FIG. 102.—_Euphausia pellucida_, female, × 5. _G_, Last gill; _L_,
luminous organ of first leg; _L′_, luminous organ of 2nd abdominal
segment; _T_, biramous thoracic appendages. (After Sars.)
]
[Illustration:
FIG. 103.—=A=, Sections (diagrammatic) of Crustacean compound eye,
=A=, with pigment in light-position for mosaic vision; =B=, with
pigment in dark-position for refractive vision. _c_, Corneal lens;
_c.g_, corneagen cells; _cr_, crystalline cone; _f_, basal membrane,
or membrana fenestrata; _ip_, irido-pigment; _n_, nerve; _r_,
retinula; _rh_, rhabdom; _rp_, retino-pigment; _v_, vitrella.
]
The =compound eyes of Crustacea= resemble those of Insects in that they
are composed of a very large number of similar elements or “ommatidia,”
more or less isolated from one another by pigment. Each ommatidium
consists typically of a corneal lens (Fig. 103, _c_), secreted by flat
corneagen cells (_c.g_) below; beneath the corneal lens is a transparent
refractive body called the “crystalline cone” (_cr_), which is produced
by a number of cells surrounding it called the “vitrellae” (_v_). Below
the crystalline cone comes the “rhabdom” (_rh_), produced and nourished
by “retinulacells” (_r_). The rhabdom is a transversely striated rod,
constituting the true sensory part of each ommatidium, and is in
connexion at its lower end with a nerve-fibre (_n_), passing to the
optic ganglion. The rhabdoms rest upon a membrane (_f_) called the
“membrana fenestrata.” Each ommatidium is isolated from its fellows
which surround it by a complete cylinder of pigment, part of which is
especially crowded round the crystalline cone, and is known as
“irido-pigment” (_ip_), while the part which surrounds the rhabdom is
called “retino-pigment” (_rp_).
When the pigment is arranged in this way, as in Fig. A, only those rays
of light which strike an ommatidium approximately at right angles to the
corneal surface can be perceived, since only these can reach the top of
the rhabdom; the others pass through the crystalline cones obliquely,
and are absorbed by the cylinder of pigment surrounding each ommatidium,
so that they neither reach the rhabdom of the ommatidium which they
originally entered, nor can they penetrate to the rhabdom of
neighbouring ommatidia. This gives rise to what is known as “mosaic
vision,” that is to say, each ommatidium only perceives the rays of
light which are parallel to its long axis, and in this way an image is
built up of which the various points are perceived side by side by means
of separate eye-elements. The distinctness and efficiency of this mode
of vision depends chiefly upon the number of ommatidia present, and the
completeness with which they are isolated from one another by the
pigment. Now this form of vision, depending as it does upon the
absorption of a great number of the light-rays by pigment, and the
transmission of only a limited number to the sensory surface, is only
possible when there is a strong light, and there is no need for
economising the light-rays. The most important discovery was made by
Exner,[117] that the majority of animals with compound eyes had the
power of so arranging the pigment in their eyes as to enable them to see
in two ways. In bright light the pigment is situated as in Fig. 103, A,
so as completely to isolate the rhabdoms from one another
(day-position); but in the dusk the pigment actively migrates, the
irido-pigment passing to the surface (B) near the tops of the
crystalline cones, and the retino-pigment passing interiorly to rest on
the membrana fenestrata at the bases of the rhabdoms (night-position).
When this happens the rays of light which strike the ommatidia at all
sorts of angles, instead of being largely absorbed by the pigment, are
refracted by the crystalline cones and distributed over the tops of the
rhabdoms, passing freely from one ommatidium to another. In this way the
eye acts on this occasion, not by mosaic vision, but on the principle of
refraction, as in the Vertebrate eye. Of course the distinctness of
vision is lost, but an immense economy in the use of light-rays is
effected, and the creature can perceive objects and movements dimly in
the dusk which by mosaic vision it could not see at all. The pigment is
contained in living cells or chromatophores, and it is carried about by
the active amoeboid movements of these cells with great rapidity.
Now, besides the active adaptability to different degrees of light
brought about in the individual by these means, we find Crustacea living
under special conditions in which the eyes are permanently modified for
seeing in the dusk, and this naturally occurs in many deep-sea forms.
Doflein[118] has examined the eyes of a great number of deep-sea
Brachyura dredged by the _Valdivia_ Expedition, and as the result of
this investigation he states that the eyes of deep-sea Brachyura are
never composed of so many ommatidia, nor are they so deeply pigmented as
those of littoral or shallow water forms. At the same time an immense
range of variation occurs among deep-sea forms which are apparently
subjected to similar conditions of darkness, a variation stretching from
almost normal eyes to their complete degeneration and the fusion of the
eye-stalks with the carapace; and this variation is very difficult to
account for. A very frequent condition for crabs living at about 100
fathoms, and even more, is for either the irido-pigment or the
retino-pigment to be absent, for the number of ommatidia to be reduced,
and for the corneal lenses to be greatly arched. There can be little
doubt that these crabs use their eyes, not for mosaic vision, but to
obtain the superposition-image characteristic of the Vertebrate eye. In
deeper waters, where no daylight penetrates at all, this type of eye is
also met with, and also further stages in degeneration where all pigment
is absent, and the ommatidia show further signs of reduction and
degeneration, e.g. _Cyclodorippe dromioides_. In a few forms, e.g.
_Cymonomus granulatus_ among Brachyura, and numerous Macrura, the
ommatidia may entirely disappear, and the eye-stalks may become fused
with the carapace or converted into tactile organs.
Progressive stages in degeneration, correlated with the depth in which
the animals are found, are afforded by closely related species, or even
by individuals of apparently the same species. Thus in the large
Serolidae of Antarctic seas, _Serolis schytei_ occurs in 7–128 metres,
and has well-developed eyes; _S. bronleyana_, from 730 to 3600 metres,
has small and semi-degenerate eyes; while _S. antarctica_ in 730–2920
metres is completely blind. _Lispognathus thompsoni_ is a deep-water
spider-crab, and the individuals taken at various depths are said to
exhibit progressive stages in degeneration according to the depth from
which they come.
At the same time many anomalies occur which are difficult to explain. In
the middle depths, _i.e._ at about 100 fathoms, side by side with
species which have semi-degenerate or, at any rate, poorly pigmented
eyes, occur species with intensely pigmented eyes composed of very
numerous ommatidia, _e.g._ the Galatheid _Munidopsis_ and several
shrimps, while in the true abysses many of the species have quite normal
pigmented eyes. This is especially the case with the deep-sea Pagurids,
of which Alcock describes only one species, _Parapylocheles scorpio_, as
having poorly pigmented eyes. An attempt to account for this was made by
Milne Edwards and Bouvier,[119] who pointed out that the truly deep-sea
forms with well-developed eyes were always Crustacea of a roving habit,
which were perhaps capable of penetrating into better lit regions, and
to whom well-developed eyes might be useful, while the degenerate forms
were sluggish. This explanation cannot be held to account for the
phenomenon, as too many deep-sea forms with fairly normal eyes are known
which are never taken outside deep waters. Doflein (_loc. cit._) points
out that in the Brachyura of the deep sea there is a remarkable
correlation between the degree of degeneration of the eye and the size
of the eggs—the large-egged forms having unpigmented and degenerate
eyes, while the species with small eggs have pigmented eyes. He supposes
that the species with large eggs undergo a direct development without
pelagic free-swimming larvae, and that since they never reach the
surface their eyes never meet with the necessary stimulus of light for
the development of pigment; whereas the small-egged species undergo a
pelagic larval existence when this stimulus is present and gives the
necessary initiative for the development of the pigment.
Another factor enters into the question of eye-degeneration in the
Crustacea. The great majority of deep-sea animals, including many
deep-sea Crustacea, are phosphorescent, and it is certain that although
daylight never penetrates into the abysses of the ocean, yet there is
considerable illumination derived from the phosphorescence of the
inhabitants of these regions.
Alcock[120] points out in this connexion that the Pagurids, which are
conspicuous in the great depths as animals with normally developed eyes,
carry about anemones with them, and these organisms are very frequently
phosphorescent to a high degree. It may well be, therefore, that the
Pagurids are enabled to use their eyes in the normal manner owing to the
phosphorescent light which they carry about with them, and this use of
phosphorescent light may apply to a number of deep-sea Crustacea whose
eyes are not at all or only partially degenerate.
An extremely interesting case of the use of phosphorescent light is
given by Chun.[121] In a number of Euphausiids occurring in deep waters
each compound eye is divided into two parts—a frontal and
ventro-lateral—which differ from one another very greatly in the nature
and disposition of their ommatidia.
[Illustration:
FIG. 104.—Section of eye of _Stylocheiron mastigophorum_. =A=, Frontal
portion; =B=, ventro-lateral portion; =C=, phosphorescent organ;
=D=, entrance of optic nerve; _c_, corneal lens; _cr_, crystalline
cone; _pg_, pigment; _ret_, retinula; _rh_, rhabdom. (After Chun.)
]
In the frontal portion (Fig. 104, A) the ommatidia are few in number and
long, the corneal lenses are highly arched, and the pigment is reduced
to a few clumps in the iris. This part of the eye is evidently adapted
for forming a vague superposition-image in the dusk. The ventro-lateral
part (B), on the other hand, is composed of numerous small ommatidia,
the crystalline cones of which can be completely isolated from one
another by the irido-pigment. Immediately below this part of the eye is
a phosphorescent organ (C) provided with a lens and tapetum. Chun
suggests that the ventro-lateral part of the eye is used for obtaining a
clear mosaic image of objects illuminated by the phosphorescent organ,
while the frontal part of the eye is used for obtaining general visual
impressions in dimly lit regions. This curious differentiation of the
eye into two parts apparently only occurs in predaceous animals, which
capture their prey alive upon the bottom, and to whom a clear vision of
moving organisms is a necessity.
Another instance of Crustaceans making use of their own light is given
by Alcock,[122] who found two deep-sea prawns, _Heterocarpus alphonsi_
and _Aristaeus coruscans_, at about 500 fathoms in the Indian Ocean.
These animals produce a highly phosphorescent substance which they eject
from the antennary glands, and they possess very large, deeply-pigmented
eyes.
The whole subject of the modification of the pigment and structure of
Crustacean eyes is an interesting one, because it presents us with one
of those cases in which the direct response to a stimulus acting within
the lifetime of the individual seems to run parallel to the fixed
adaptations of a whole species, which have become hereditary and
apparently independent of the external stimulus of light or of the
absence of light. As far as is known, however, the direct response of
the individual to the absence of light is limited to the reduction or
disappearance of the pigment, and does not extend to those structural
changes in the ommatidia which are characteristic of so many deep-sea
forms.
=Order II. Decapoda.=[123]
The Decapoda, together with the Euphausiidae, make up the Division
Eucarida, the members of which differ from the Orders hitherto described
in a number of characters, _e.g._ the presence of a carapace covering
the whole of the thorax, the absence of a brood-pouch formed of
oostegites, the presence of a short heart, of spermatozoa with radiating
pseudopodia, and of a complicated larval metamorphosis, of which the
Zoaea stages are most prominent.
The Decapoda differ from the Euphausiidae chiefly in the anterior three
thoracic limbs being turned forwards towards the mouth to act as
maxillipedes, and in the five succeeding thoracic limbs being nearly
always uniramous and ambulatory or chelate; there are typically present
three serial rows of gills attached to the thoracic segments, an upper
series (“pleurobranchiae”) attached to the body-wall above the
articulation of the limbs, a middle series (“arthrobranchiae”) attached
at the articulation of the limbs, and a lower series (“podobranchiae”)
attached to the basal joints of the limbs. These gills are enclosed in a
special branchial chamber on each side of the thorax, formed by lateral
wings of the carapace known as “branchiostegites.” The gills of each
series are never all present in the same animal, the anterior and
posterior members showing a special tendency to be reduced and to
disappear. In this manner “branchial formulae” can be constructed for
the various kinds of Decapods, which differ from the ideal formula in a
manner distinctive of each kind. The second maxilla is always provided
with an oar-like appendage on its outer margin (exopodite), known as the
“scaphognathite,” which, by its rhythmical movement, keeps up a constant
current of water through the gill-chamber.
A complicated auditory organ is present on the basal joint of the first
antennae; this is a sac communicating with the exterior and lined
internally with sensory hairs. The animal is said to place small pieces
of sand, etc., in its ears to act as otoliths. _Anaspides_ (see p. 116)
is the only other Crustacean which has an auditory organ in this
position.
The larval histories of the Decapods[124] are of great interest, and
will be given under the headings of the various groups. The first
discoverer of the metamorphosis of the Decapoda was the Irish naturalist
J. V. Thompson, certainly one of the ablest of British zoologists. In
1828, in his _Zoological Researches_, he describes certain Zoaeas of the
Brachyura and proves that these animals are not an adult genus, as
supposed, but larval forms. But Rathke, in 1829, described the direct
development of the Crayfish; and Westwood, after describing the direct
development of _Gecarcinus_, utterly denied Thompson’s assertions
concerning metamorphosis. Thompson replied in the _Royal Society
Transactions_ for 1835, and described the Megalopa stage of _Cancer
pagurus_. Rathke,[125] although previously an opponent of Thompson,
subsequently made confirmatory observations upon the larvae of the
Anomura; and Spence Bate clinched the matter by describing Brachyuran
metamorphosis with great accuracy in the _Philosophical Transactions_
for 1859. Since then a mass of work has been done on the subject, though
much detail still remains to be elucidated.
The Decapoda fall into three sub-orders, which graduate into one
another—(i.) the Macrura, including the Lobsters, Crayfishes, Shrimps,
and Prawns; (ii.) the Anomura, including the Hermit-lobsters and
Hermit-crabs; and (iii.) the Brachyura or true Crabs.
=Sub-Order 1. Macrura.=
This sub-order[126] is characterised by the large abdomen, furnished
with five pairs of biramous pleopods, and ending in a powerful tail-fan
composed of the telson and the greatly expanded sixth pair of pleopods,
the whole apparatus being locomotory. The second antennae are furnished
with very large external scales, representing the exopodites of those
appendages. Some of the Shrimps and Prawns closely resemble the
“Schizopods,” but the pereiopods are nearly always uniramous.[127]
Several subdivisions of the Macrura are recognised.
=Tribe 1. Nephropsidea.=
This tribe includes the Lobsters and Crayfishes, animals well known from
their serviceableness to man. There are three families, which will be
treated separately.
=Fam. 1. Nephropsidae.= The podobranchs are not united with the
epipodites, and the last thoracic segment is fixed and fused to the
carapace. The chelae are generally asymmetrical. The most important
Lobsters are the European and the American species—_Homarus vulgaris_ (=
_Astacus gammarus_) and _H. americanus_ respectively; these animals
engage a large number of people in the fisheries. It is estimated that
in America about £150,000 are spent every year on Lobsters.
The genus _Nephrops_ contains the small Norwegian lobster and other
forms.
Herrick[128] gives some interesting particulars with regard to the
life-history of the American species. The largest recorded specimen
weighed about twenty-five pounds, and measured twenty inches from
rostrum to tail; similar European specimens have been recorded, but, on
the average, they are not so large as the American forms.
The Lobster, like all Crustacea, undergoes a series of moults as the
result of increase in size, shedding the whole of the external
integument in one piece. This is accomplished by a split taking place on
the dorsal surface at the junction of thorax and abdomen; through the
slit so formed the Lobster retracts first his thorax with all the limbs,
and then his abdomen. When first issuing from the old shell the animal’s
integument is soft and pulpy, but the increase in size of the body is
already manifest; this increase per moult, which is approximately the
same in young and adult animals, varies from 13 to 15 per cent of the
animal’s length. According to this computation, a Lobster 2 inches long
has moulted fourteen times, 5 inches twenty times, and 10 inches
twenty-five times, and it may be roughly estimated that a 10–inch
Lobster is four years old. Young Lobsters probably moult twice a year,
and so do adult males, but females only moult once a year soon after the
young are hatched out.
The process of moulting or ecdysis is an exceedingly dangerous one to
the Lobster and to Crustacea in general, and is very frequently fatal.
There is, first of all, the danger of the act not being accomplished
skilfully, when death always ensues. The Lobster remains soft and
unprotected for about six weeks after the ecdysis, and is very apt to
fall a prey to the predaceous fish, such as Sharks, Skates, Cod, etc.,
which feed upon it. There are, however, some peculiar adaptations
connected with the process which are of interest. In order to facilitate
the ecdysis, areas of absorption are formed upon the dorsal and ventral
surfaces of the carapace, on the narrower parts of the chelipedes, and
at other places; in these areas the calcium carbonate is absorbed, and
the old shell becomes elastic and thin, so as to allow a more easy
escape for the moulting Lobster. It has been noticed that while this is
taking place large concretions of calcium carbonate are formed at the
sides of the stomach, known as “gastroliths,” which perhaps represent
the waste lime that has been abstracted from the areas of absorption.
After moulting the Lobster is in great need of lime for stiffening his
shell, and it has been noticed that on these occasions he is very greedy
of this substance, even devouring his own cast-off skin.
The male Lobster is especially prized on account of his larger chelae,
but in both sexes the chelipedes are differentiated into a smaller
cutting pincer and a larger crushing one. Lobsters may be right or left
handed, with the large crushing claw on the right or left hand, and
sometimes specimens occur with the smaller cutting pincers on both
chelipedes, and very rarely, indeed, with crushing claws on both sides.
Crustacea very commonly have the power of casting off a limb if they are
seized by it or if it is injured, and of regenerating a new one. In the
Lobster a so-called breaking joint is situated on each leg at the suture
between the fused second and third segments; a membrane being pushed
inwards from the skin, which not only serves to form a weak joint where
rupture may easily take place, but also to stop excessive bleeding after
rupture. In the newly-hatched larvae there is a normal joint between the
second and third segments; and autotomy, or the voluntary throwing away
of a limb, never occurs until the fourth larval stage, when the breaking
joint is formed. Autotomy is a reflex act under the control of the
segmental ganglion; if a Crab or Lobster be anæsthetised, and then a
limb be injured or broken off below the breaking joint, the animal
forgets to throw the injured leg or stump off at the breaking joint, a
proceeding which always occurs under normal conditions. The regeneration
of a limb starts from a papilla which grows out of the breaking joint,
and after a number of moults acquires the specific form of the limb that
has been lost. A number of interesting observations have been made upon
the regeneration of the limbs in Crustacea. It was in the Hermit-crab
that Morgan[129] proved that regeneration and the liability to injury do
not always run parallel, as Weismann held they should, since the
rudimentary posterior thoracic limbs, which are never injured in nature,
can regenerate when artificially removed as easily as any others.
Przibram[130] has shown that in the shrimp _Alpheus_, whose chelipedes
are highly asymmetrical, if the large one be cut off, the small one
immediately begins to grow and to take on the form of the large one,
while the regenerated limb is formed as the small variety. This
remarkable inversion in the symmetry of the animal clearly ensures that,
if the large chela is injured and thrown away, the least amount of time
is wasted in providing the shrimp with a new large claw.
To return to the Lobster; for the majority of the individuals there is a
definite breeding season, viz. July and August, but a certain proportion
breed earlier or later. A female begins to “berry” at about eight inches
in length, and to produce more and more eggs up to about eighteen
inches, when as many as 160,000 eggs are produced at a time; after this
there is a decline in numbers. A female normally breeds only once in two
years. Strict laws are enforced forbidding the sale of Lobsters and
Crabs “in berry” in both England and America. The period of incubation,
during which the developing eggs are attached to the swimmerets of the
female, lasts about ten or eleven months, so that the larvae are hatched
out approximately in the following June. On hatching, the larva, which
measures about one-third of an inch, and is in the Mysis stage (_i.e._
it possesses all the thoracic limbs in a biramous condition, but is
without the abdominal limbs), swims at first on the surface. After five
or six months of this life, during which the abdominal pleopods are
added from before backwards, it sinks to the bottom, loses the
exopodites of the thoracic limbs, and is converted into the young
Lobster, measuring about half an inch in length. The little Lobster
starts in deepish water, and gradually crawls towards the shore; here it
passes its adolescence, but on coming to maturity it migrates out again
into the deep water.
=Fam. 2. Astacidae.=—In this family, which includes all the European and
North American Crayfishes, _Astacus_ (_Potamobius_) and _Cambarus_, the
podobranchs are united with the epipodites, the last thoracic segment is
free, there is only one pleurobranch or none at all, the gills have a
central lamina, but the filaments are without terminal hooks, and the
endopodites of the first two pairs of abdominal appendages in the male
serve as copulatory organs. For the distribution, etc., of these forms
see p. 213.
=Fam. 3. Parastacidae.=—This family includes the Crayfishes of the
Southern Hemisphere, viz. _Parastacus_ from South America, _Astacopsis_
and _Engaeus_ from Australia, _Paranephrops_ from New Zealand, and
_Astacoides_ from Madagascar. These genera agree with the Potamobiidae
in the union of the podobranchs with the epipodites, and in the free
condition of the last thoracic segment, but there are generally four
pleurobranchs, the gills are without a lamina, the filaments have
terminal hooks, and there are no sexual appendages in the male. For
distribution, etc., see also p. 213.
The larval development in the Crayfishes is still more abbreviated than
in the Lobsters, the Mysis stage being passed through within the
egg-membranes. The young, when they hatch out, are furnished with hooks
upon the chelipedes, by which they anchor themselves to the pleopods of
the mother.
=Tribe 2. Eryonidea.=
[Illustration:
FIG. 105.—_Willemoesia inornata_, × ⅓. (From a figure prepared for
Professor Weldon.)
]
These are remarkably archaic animals of great rarity, though they were
common enough in Triassic seas, and have come down to us as fossils from
those times, being thus among the oldest Decapoda known. They only
survive now as deep sea species, and the genus discovered by the
_Challenger_,[131] _Willemoesia_ (Fig. 105), confirmed the expectations
of the _Challenger_ naturalists that the abysses of the ocean would
contain relics from older periods which had managed to survive where the
competition was not so keen. The genus _Willemoesia_ is very widely
distributed, being dredged up from below a thousand fathoms in the
Indian Ocean, the Mediterranean, North and South Atlantic, and the
Pacific oceans. All the walking legs are chelate, and the animal is
quite blind, as are all the Eryonidea, the eye-stalks being fused with
the carapace.
Only a single family =Eryonidae= is recognised.
=Tribe 3. Peneidea.—Tribe 4. Caridea.=
We will now consider the Shrimps and Prawns, since in them occurs the
most complete metamorphosis found in the Decapoda. The Peneidea are
distinguished from the ordinary Prawns and Shrimps (Caridea) by having
the first three instead of the first two pereiopods chelate. The genus
_Peneus_ affords several species which are of commercial value as
objects of food; the edible Prawns of the Mediterranean belong to this
genus, while in the North Sea two of the Caridea, viz. the Shrimp,
_Crangon vulgaris_, and the Prawn, _Palaemon serratus_, are the forms
very commonly eaten. Both subdivisions are well represented in the deep
sea fauna from all parts of the world. _Glyphocrangon spinulosa_ (Fig.
110, p. 164) is a deep sea Shrimp with eyes that have lost their
pigment, and with the body covered with spines, while the last abdominal
segment is fused with the telson to form a sharp bayonet-like process at
the hind end of the body. Some of the deep-sea Prawns of the Indian
Ocean are described by Alcock[132] as possessing peculiar secondary
sexual characters. Thus _Parapeneus rectacutus_ ♂ has one lash of the
first pair of antennae peculiarly bent to form a clasping organ, while
_Aristaeus crassipes_ has a hook on the end of the third maxillipede. In
the latter the females have much longer rostra than the males, and are
in general more powerfully built, so that they seem to have usurped the
proper functions of the male, and probably engage in combats with one
another over his person.
[Illustration:
FIG. 106.—Nauplius larva of _Peneus_, sp. × 25. (From Balfour, after
F. Müller).
]
As a general rule the Shrimps and Prawns occur in large shoals in the
shallow waters of the littoral zone, and they have a remarkable power of
adapting their colours to the surroundings in which they happen to be at
any particular moment.[133] This is brought about by the variously
coloured chromatophores, which contract and expand in obedience to a
stimulus transmitted through the eyes of the animal. A number of the
Palaemonidae go up rivers into fresh water, while one family, the
_Atyidae_, live in the completely fresh water of rivers and inland
lakes. The Peneidea undergo a very complete metamorphosis which is
primitive in respect to the order of formation of the segments from
before backwards. The larva hatches out as a Nauplius (Fig. 106), which
by the orderly addition of segments behind is converted into the
Protozoaea (Fig. 107), possessing two pairs of biramous maxillipedes. It
should be noted that the maxillae, which are foliaceous in the adult,
are laid down in this condition in the larva, and this principle holds
good throughout Crustacean metamorphosis, viz. that when a limb is
foliaceous in the adult it is foliaceous in the larva, and when biramous
in the adult it is biramous in the larva. Whilst the rest of the
thoracic limbs are still rudimentary, the sixth pair of pleopods are
being precociously developed (Fig. 108), being the only precociously
formed limbs in the Peneidea, though the abdominal segments are fully
marked off before the thoracic segments, and so must be considered as
precocious in development. When the biramous thoracic limbs are
completed the abdominal biramous pleopods are added, beginning from in
front backwards. Thus the Mysis stage (Fig. 109) is reached, which
resembles in all particulars the adult condition of the Schizopoda. The
adult Prawn develops from this stage by the loss of some or all of the
exopodites on the thoracic pereiopods.
[Illustration:
FIG. 107.—Protozoea larva of _Peneus_, sp. × 25. (From Balfour, after
F. Müller.)
]
[Illustration:
FIG. 108.—Zoaea larva of _Peneus_, sp. × 25. _A_, _A′_, 1st and 2nd
antennae; _Ab.6_, 6th abdominal appendage; _Mxp_, 2nd maxillipede;
_T_, 4th–8th thoracic appendages (future walking legs). (After F.
Müller.)
]
Some of the Peneid larvae take on very peculiar forms, _e.g._ the Zoaeae
of the Sergestidae,[134] which often develop the most wonderful spines
all over the body.
[Illustration:
FIG. 109.—Mysis stage in the development of _Peneus_, sp. _A.2_, 2nd
antenna; _Ab.6_, 6th abdominal appendage; _T_, telson; _Th_, the
biramous thoracic appendages. (After Claus.)
]
The Caridea have a greatly abbreviated metamorphosis, the larva hatching
out at a late Zoaea stage with all three pairs of maxillipedes fully
formed and with a fully segmented abdomen. The succeeding thoracic limbs
are added in order from before backwards, though the sixth pair of
pleopods appear precociously as in the Peneidea. The other swimmerets do
not begin to develop until the thoracic limbs are complete. Some Caridea
show a yet more abbreviated metamorphosis, _e.g._ the fresh-water
_Palaemonetes varians_ of S. Europe, which hatches out at the Mysis
stage.
We see, therefore, in the metamorphosis of the Macrura several
apparently primitive features. In the first place, a free swimming
Nauplius stage is preserved in certain forms, identical in all respects
with the Nauplius of the Entomostraca. Secondly, the thoracic limbs when
they are first developed are biramous, thus giving rise to the
characteristic Mysis stage which links the Macrura on to the
“Schizopoda.” Thirdly, the order of differentiation of the segments is
typically from in front backwards, the only precociously developed
appendage being the sixth abdominal. None of these characters are
reproduced in the higher Decapoda in which there is never a free-living
Nauplius, the first larval stage being the Zoaea; a number of the
thoracic pereiopods, and usually all of them, are uniramous from the
start; and the whole of the abdominal segments with their limbs tend to
be precociously developed before the hinder thoracic segments make a
distinct appearance.
=Tribe 3. Peneidea.=[135]
The third legs are chelate except in genera in which the legs are much
reduced. The third maxillipedes are seven-jointed, the second
maxillipedes have normal end-joints, and the first maxillipedes are
without a lobe on the base of the exopodite. The pleura of the first
abdominal segment are not overlapped by those of the second. The abdomen
is without a sharp bend. The branchiae are usually not phyllobranchs.
=Fam 1. Peneidae.=—The last two pairs of legs are well developed, and
there is a nearly complete series of gills. _Cerataspis_,[136] a pelagic
form. _Parapeneus_, _Peneus_, _Aristaeus_, etc.
=Fam. 2. Sergestidae.=—The last or last two pairs of legs are reduced or
lost. The gill-series is incomplete or wanting. _Sergestes_ possesses
gills, and the front end of the thorax is not greatly elongated.
_Lucifer_ has no gills, and the front of the thorax is greatly
elongated, giving a very anomalous appearance to the animal. All the
members of this family are pelagic in habit.
=Fam. 3. Stenopodidae.=—One or both legs of the third pair are longer
and much stouter than those of the first two pairs. On a number of small
anatomical points this family, including the littoral genus _Stenopus_
from the Mediterranean and other warmer seas and _Spongicola_ commensal
with Hexactinellid sponges from Japan, is separated by some authors in a
Tribe by itself.
=Tribe 4. Caridea.=
The third legs are not chelate. The third maxillipedes are 4–6 jointed,
the end-joint of the second maxillipede nearly always lies as a strip
along the end of the joint before it, and the first maxillipedes have a
lobe on the base of the exopodites. The pleura of the second abdominal
segment overlap those of the first. The abdomen has a sharp bend; the
branchiae are phyllobranchs.
=Fam. 1. Pasiphaeidae.=—In this family the end-joint of the second
maxillipedes is normally formed, and exopodites are usually present on
all the thoracic limbs. Rostrum small or wanting. Rather numerous genera
are known, most of which inhabit the deep sea, though a few come into
the littoral zone. _Pasiphaea_ chiefly in the deep sea, _Leptochela_ in
the tropical littoral zone.
=Fam. 2. Acanthephyridae.=—The end-joint of the second maxillipede is
modified as in other Caridea, and the rostrum is very strong and
serrate, but in the presence of exopodites, and in the form of the
mouth-parts, this family agrees with the preceding. It is also a
characteristic deep-sea family. _Acanthephyra_, _Hymenodora_,
_Nematocarcinus_, etc.
=Fam. 3. Atyidae.=—This is an entirely fresh-water family, especially
characteristic of the rivers and lakes of the tropics, some of the forms
being exceedingly large and taking the place of the Crayfishes in these
waters. Characteristic of this family is the fact that the fingers of
the chelae are spoon-shaped, and carry peculiar tufts of bristles.
Exopodites are present on the thoracic limbs of some of the genera
(_Troglocaris_, _Xiphocaris_ from Australia and the Malay Islands,
_Atyephyra_ from S. and W. Europe), but are absent in others.
_Caridina_, widely spread and common in Indo-Malay and Africa; _Atya_
from West Indies, West Africa, and Pacific Islands.
=Fam. 4. Alpheidae.=[137]—The exopodites are absent, and the rostrum is
absent or very feeble. The chelae are powerful, and usually very
asymmetrically developed. _Alpheus_ has an enormous number of species
which live chiefly in the tropical seas, where they haunt especially the
coral-reefs, making their homes among the coral or in sponges, etc.
Although occurring in the Mediterranean they penetrate very rarely into
colder seas.
=Fam. 5 Psalidopodidae.=—This family, characterised by the absence of
chelae on the second thoracic limbs, which carry instead a terminal
brush of hairs, and by the rudimentary condition of the eyes, is
represented by the genus _Psalidopus_ from the deep waters of the Indian
Ocean.
=Fam. 6. Pandalidae.=—The first thoracic limb is without chelae, only
six-jointed. The rostrum is large and toothed. The genus _Pandalus_ has
numerous representatives in the northern littoral, _P. annulicornis_
being one of the prawns most commonly met with in the fish-markets.
=Fam. 7. Hippolytidae.=—The first and second thoracic limbs bear chelae,
the carpus of the second being divided into two or more segments. The
first pair of chelae are not distinctly stronger than the second.
_Virbius_ has many species in the littoral zone of all seas, and one
species, _V. acuminatus_, is pelagic. _Hippolyte_ also has numerous
littoral forms distributed all over the world, but chiefly in the arctic
or subarctic seas. _H. varians_, common on the English coasts, shows
interesting colour-reactions to its surroundings.[138]
[Illustration:
FIG. 110.—_Glyphocrangon spinulosa_, from the right side, × 1. (From
an original drawing prepared for Professor Weldon.)
]
=Fam. 8. Palaemonidae.=—The first two pairs of legs are chelate, the
carpus of the second not being subdivided. _Palaemon serratus_, a very
common prawn in the British littoral. _Palaemonetes_ in the brackish and
fresh waters of Europe and N. America.
=Fam. 9. Glyphocrangonidae.=—The first pair of legs are subchelate, the
carpus of the second pair is subdivided, and the rostrum is long.
_Glyphocrangon_ (Fig. 110) with numerous species entirely confined to
deep water.
=Fam. 10. Crangonidae.=—The first pair of legs are subchelate, the
carpus of the second pair is not subdivided, and the rostrum is short.
_Crangon vulgaris_ is the common Shrimp of the North Sea.
=Tribe 5. Loricata.=
[Illustration:
FIG. 111.—Dorsal view of _Scyllarus arctus_, × ½. (From an original
figure prepared for Professor Weldon.)
]
[Illustration:
FIG. 112.—Embryonic area of developing _Palinurus quadricornis._
_Ab.1_, 1st abdominal segment; _E_, compound eye; _E′_, median
simple eye; _L_, upper lip; _L′_, lower lip; _M_, mandible; _Mx.1_,
_Mx.2_, 1st and 2nd maxillae; _Mxp.1_, 1st maxillipede; _T_, 6th
(antepenultimate) thoracic appendage. (After Claus.)
]
The Loricata include the Langouste (_Palinurus_) of the Mediterranean
coasts, which replaces there the Lobster of the North Sea as an article
of food, and the peculiarly shaped _Scyllarus arctus_ (Fig. 111), which
is also prized in the Mediterranean as a delicacy. The bright red
“Crayfishes,” _Panulirus_ and _Iasus_, of the Australian coasts are also
largely used as food. Besides its peculiarity in shape, _S. arctus_ has
remarkable scales on the second antennae in place of flagella. The larva
hatches out as the so-called Phyllosoma, which must be regarded as a
greatly flattened and modified[139] Mysis stage.
[Illustration:
FIG. 113.—Phyllosoma larva of _Palinurus_, sp. × 5. _Ab_, Abdomen;
_Mxp_, 3rd maxillipede; _T_, antepenultimate (6th) thoracic
appendage. (After Claus.)
]
In the embryo of _Palinurus_ just before hatching (Fig. 112) we can
recognise the limbs of the head and thorax normally developed in order.
There are present three thoracic limbs, besides the maxillipedes. When
the Phyllosoma hatches out the first maxillipedes have become quite
rudimentary, and the second much reduced, while the second antennae and
second maxillae are also reduced in size. The metamorphosis is completed
by the re-development of the limbs and segments that have been
secondarily suppressed during larval life, and by the appearance of the
pleopods.
This process is again met with in the Squillidae (p. 143), but it
resembles the suppression, in so many Decapodan metamorphoses, of
anterior limbs and the precocious development of segments and limbs
lying posteriorly. In the ordinary Decapoda, however, the suppressed
limbs are merely not formed till later; while in the Loricata the limbs
develop in the correct order, and subsequently degenerate. It is natural
to wonder whether the condition of affairs in the Loricata represents
the primitive process, and whether the precocious development of
segments in the other Decapoda owes its origin to these animals having
once had the direct mode of development when the segments were formed in
the proper order, and to their having subsequently acquired the larval
stages first of all by the degeneration, and then by the suppression of
certain segments which were not of use during larval life. The complete
metamorphosis, however, of the Peneidea, in which the segments and limbs
appear in the right order, rather goes to show that this is the
primitive mode of development in the Decapoda, and that the
disarrangement in the order of appearance of the segments, both in the
Squillidae and in the Loricata and other Decapods, has been
independently acquired in the two cases to meet the needs of the larval
existence.
=Fam. 1. Palinuridae.=—The cephalothorax is subcylindrical, the eyes are
not enclosed in separate orbits formed by the edge of the carapace, and
the second antennae possess flagella. _Palinurus_, with _P. elephas_,
the European Rock Lobster or Langouste. _Iasus_ with two species in the
Antarctic littoral; _Panulirus_ in the tropical littoral.
=Fam. 2. Scyllaridae.=—The cephalothorax is depressed, the eyes are
enclosed in separate orbits formed by the edge of the carapace, and the
second antennae have flat scales in the place of flagella. _Scyllarus_
(Fig. 111), with the European _S. arctus_; _Ibacus_ in rather deep water
with several species, chiefly found in the southern hemisphere.
=Tribe 6. Thalassinidea.=
This tribe is included by some authors in the Anomura, and held to be
closely related to the Galatheidea, but the unreduced abdomen is carried
straight and unflexed, and gives a very Macrurous appearance to the
animal. The Anomurous characters are the frequent reduction or absence
of the antennal scale, the fact that only the first two pairs of
pereiopods are ever chelate, and the reduced series of gills. The body
is symmetrical, but the first pair of chelae is always highly
asymmetrical. The posterior pairs of pereiopods, although small, are not
characteristically reduced as in the Anomura. The animals belonging to
this Tribe attain two or three inches in length, and generally burrow in
sand or mud either in the littoral zone or in deeper waters; at the same
time they can swim with considerable activity by means of the pleopods.
=Fam. Callianassidae.=—_Callianassa subterranea_ is common at Naples,
_Gebia littoralis_ in the North Sea.
=Sub-Order 2. Anomura=
In this division are included the so-called Hermit-lobsters and
Hermit-crabs, in which the condition of the abdomen is roughly
intermediate between that of the Macrura and that of the Brachyura. It
is not much reduced in size, and the pleopods of the sixth pair are
fairly well developed, but it is usually carried flexed towards the
thorax, and is never a powerful locomotory organ as in the Macrura. The
antennal scale, if present at all, is a mere spine, not the large
leaf-like structure of the Macrura; and there is never a partition
between the two first antennae as in the Brachyura.
The last or last two pairs of pereiopods are reduced, and are turned on
to the dorsal surface or carried inside the branchial chamber; but this
curious character is met with again in certain Brachyura (Dromiacea and
Oxystomata).
=Tribe 1. Galatheidea.=[140]
[Illustration:
FIG. 114.—Dorsal view of _Munidopsis hamata_, × ½. (From an original
figure prepared for Professor Weldon.)
]
These are symmetrical crabs with a long carapace; the abdomen, which is
as broad as the carapace, is always carried flexed under the thorax, and
the sixth pair of pleopods are expanded to form with the telson a
fan-like tail. The most anterior pereiopods are always much elongated
and chelate; while the last pair are much reduced, and either turned up
on to the dorsal surface, or else carried in the branchial chamber. The
exact meaning of this last characteristic in these forms is doubtful;
some of the species are said to carry shells temporarily upon their
backs, a proceeding probably assisted by the last pair of thoracic
limbs, while in others their limbs may be used for cleaning out the
branchial chamber. Most of the Galatheidea, for instance, the common
_Porcellana_ and _Galathea_, are littoral animals, and may be found
hiding under stones and in crevices on the shore; but a number occur in
deep water, e.g. _Munida_ and _Munidopsis_.
[Illustration:
FIG. 115.—Zoaea of _Porcellana_, × 20. _T_, Telson. (After Claus.)
]
The shallow-water species have ordinarily developed eyes; the various
species of _Munida_, which occur in fairly deep but by no means abyssal
regions, have usually very large and highly pigmented eyes; while in
_Munidopsis_, which is characteristic of very deep water, the eyes are
degenerate and colourless, as shown in Fig. 114.
The Zoaeae, or young larval stages of the Galatheidea, are characterised
by the immense length of the spines upon the carapace (Fig. 115). The
young Zoaea which hatches out from the egg resembles in other respects
that of the Brachyura. The Metazoaea, however, differs from that of the
Brachyura in the fact that the third maxillipede is first present as a
biramous swimming organ, and at its first appearance is not developed in
its definitive form. The other thoracic limbs are not schizopodous when
they appear, and indeed in nearly all respects the development proceeds
as in the Brachyura.
=Fam. 1. Aegleidae.=—The gills are trichobranchiae, and there are eight
arthrobranchs. There are no limbs on the second abdominal segment of the
male. The abdomen is not carried folded on to the thorax. The first two
characteristics separate this family from all the other Galatheidea.
_Aeglea laevis_, a fresh-water species from the rivers of temperate S.
America, is the sole representative.
=Fam. 2. Galatheidae.=—The abdomen is not folded against the thorax. The
members of this family are often littoral in habit (_Galathea_, Fig.
116), but often go down into great depths (_Munidopsis_, Fig. 114).
[Illustration:
FIG. 116.—Dorsal view of _Galathea strigosa_, × ½. (From an original
figure prepared for Professor Weldon.)
]
=Fam. 3. Porcellanidae.=—The abdomen is folded against the thorax, and
the body has a crab-like form. These are always littoral in habit, never
descending into the depths. _Pachycheles_ in the tropics, _Porcellana_
with numerous species in all seas, _P. platycheles_ being a common
British species.
=Tribe 2. Hippidea.=
The Mole-crabs have the habit of burrowing in sand, and their limbs are
peculiarly modified into digging organs for this purpose (see Fig. 117).
In other respects they are seen to be closely related to the Galatheidea
by the form of the carapace, the condition of the abdomen, and the
reduced last thoracic limbs.
In _Albunea_, which is found in the Mediterranean, the first
antennae[141] are greatly lengthened and apposed to one another, and by
means of a system of interlocking hairs they form a tube down which the
water is sucked for respiration. The object of this arrangement is to
ensure a supply of clear water, filtered from particles of sand, when
the crab is buried beneath the surface, on these occasions the tip of
the antennal tube being protruded above the surface of the sand. An
exactly similar tube is used by the true Crab _Corystes cassivelaunus_,
which has similar burrowing habits, but here the tube is formed from the
second antennae and not from the first, so that the tubes in the two
cases afford beautiful instances of analogous or homoplastic structures
between which there is no homology (see p. 189).
=Fam. 1. Albuneidae.=—The first legs are subchelate; the carapace is
flattened, without expansions covering the legs. _Albunea_ with several
species in the Mediterranean, West Indies, and Indo-Pacific.
[Illustration:
FIG. 117.—_Remipes scutellatus_, dorsal and ventral views, × 1. (From
original drawings prepared for Professor Weldon.)
]
=Fam. 2. Hippidae.=—The first legs are simple, the carapace is
subcylindrical with expansions covering the legs. _Remipes_ (Fig. 117)
and _Hippa_ in tropical or sub-tropical seas.
=Tribe 3. Paguridea.=[142]
The ordinary Hermit-crabs, common on the English as on every coast, are
characterised by the fleshy asymmetrical abdomen from which all the hard
matter has disappeared, and which is carried tucked away in an empty
Gasteropod shell. The abdomen is spirally wound in accordance with the
shape of the shell, and a firm attachment is effected by means of the
sixth pair of pleopods, especially that of the left side, which is
fashioned into the form of a hook and is curled round the columella of
the shell; this attachment is so secure that in trying to pull a
Hermit-crab out of its shell the body is torn apart before the hold
gives way. The other pleopods are in a much reduced condition, being
generally altogether absent from the right side of the abdomen, and
often greatly reduced on the left side, especially in the male, though
in the female they are still used for the attachment of the eggs.
The last two pereiopods are much reduced and are concealed inside the
shell, which they help to carry. The great chelae are usually
asymmetrically developed, that on the right side being much larger than
that on the left, and often serving the purpose of shutting the entrance
to the shell when the crab is withdrawn inside.
The constant association of a large group of animals like the
Hermit-crabs with the appropriated empty houses of another group is
sufficiently curious, but it does not stop there. In almost every case
there are present one or more Sea-anemones growing on the outside of the
shell, and each kind of Hermit-crab generally carries a special kind of
Anemone. Thus at Plymouth, _Eupagurus bernhardus_ is generally symbiotic
with _Sagartia parasitica_, or else with a colony of _Hydractinia
echinata_, while _E. prideauxii_ is usually associated with _Adamsia
palliata_. In the latter case the shell is frequently absorbed, so that
the Anemone comes to envelop the crab like a blanket. Instead of
Anemones carried turret-like and imposing aloft, or enveloping the
inmate of the shell like a blanket, some of the Hermits have Sponges, an
unexpected association; and it is a common sight at Naples to find the
little red round Sponge, _Suberites_, running around animated by its
Hermit within. It is held that Anemone and crab mutually assist one
another, that the Anemone stings the crab’s enemies, and that the
Hermit-crab carries the Anemone to new feeding-grounds. It is also said
that when a crab grows too big for its shell, and is forced to seek
another, it persuades the Anemone to loosen its attachment to the
deserted shell and to be transplanted to the new one, and that there is
something mesmeric in its power, because nobody else can pull an Anemone
off a shell without either cutting it off at the base or tearing it to
pieces. Other animals as well sometimes enter into this partnership. At
Plymouth a Polychaet worm, _Nereis fucata_, frequently inhabits the
Whelk’s shell, together with _Eupagurus bernhardus_, and puts out its
head for a share of each meal; and at Naples the Amphipod _Lysianax
punctatus_ is almost always present in the shells of _Eupagurus
prideauxii_.
[Illustration:
FIG. 118.—_Pylocheles miersii_, × 1. =A=, End view of a piece of
mangrove or bamboo, the opening of which is closed by the great
chelae (_c_) of the Pagurid; =B=, the animal removed from its house.
(After Alcock.)
]
Besides the ordinary twisted Pagurids which inhabit Gasteropod shells,
there are a few which preserve the symmetry of the body. The interesting
_Pylocheles miersii_[143] (Fig. 118), taken by the _Investigator_ in the
Andaman Sea at 185 fathoms, inhabits pieces of bamboo; it is perfectly
symmetrical, with well-developed pleopods and symmetrical chelae, which,
when the animal is withdrawn, completely shut up the entrance to its
house (Fig. 118, A).
It is doubtful whether this animal ever inhabited a spiral shell or not
in its past history; but there is no doubt that a number of peculiar
crabs, which caused the older systematists much trouble, are Pagurids,
derived from asymmetrical shell-haunting ancestors that have secondarily
taken to a different mode of life, and lost, or partially lost those
characteristics of ordinary Hermit-crabs which are associated with life
in a spiral shell. These are the Lithodidae and the “Robber-crab,”
_Birgus latro_, of tropical coral islands.
Although the Robber-crab and the Lithodidae bear a certain superficial
resemblance to one another in that they lead a free existence, and have
reacquired to a great extent their symmetry, yet it is clear that they
have been independently derived from different groups of asymmetrical
Hermit-crabs, and that their resemblance to one another is due to
convergence.
_Birgus latro_ (Fig. 119), a gigantic crab, frequently over a foot in
length, lives on land, and inhabits the coasts of coral islands in the
Indian and Pacific Oceans where cocoa-nut trees grow. It feeds on the
pulp of the cocoa-nut, which it extracts by hammering with its heavy
chela on the “eye-hole” until room is made for the small chela to enter
and extract the pulp. There is not the slightest doubt that the animal
often ascends the cocoa-nut trees for the purpose of picking the nuts, a
fact illustrated by a fine photograph by Dr. Andrews, exhibited in the
Crustacean Gallery in the Natural History Departments of the British
Museum. It uses the husk of the nut to line its burrow, and it is said
to have the habit of putting its abdomen into the nut-shell for
protection and carrying it about with it. Owing to its terrestrial mode
of life, the branchial chamber is highly modified, being divided into
two portions—a dorsal space, the lining of which is thrown into vascular
ridges and folds for aerial respiration, and a lower portion where the
rudimentary branchiae are situated. Although the Robber-crab lives
ordinarily on land, it must be supposed that these branchiae are of some
service; the young are hatched out as ordinary Zoaeas in the sea, and go
through a pelagic existence before seeking the land. At the present time
the Robber-crab is confined to the Pacific and the islands of the Indian
Ocean, wherever the cocoa-nut grows. It seems, however, that its
association with the cocoa-nut is a comparatively modern one. Mr. C.
Hedley, of Sydney, who has had great experience of the Pacific Islands,
informs me that the cocoa-nut is not, as is usually supposed, a native
of these coral islands, but has been introduced, probably from Mexico,
by the Polynesian mariners before the discovery of America by Columbus.
Before the introduction of the cocoa-nut the Robber-crab must have fed
on some other tree, possibly the Screw Pine, _Pandanus_.
The abdomen is full of oil, and is much prized as a delicacy by the
natives, who tell many strange legends about the creature, but the
philosopher may well find its structure more strange than fiction, and
the consideration of its morphology an intellectual feast.
The appearance of the thorax and of the thoracic limbs is thoroughly
Pagurid; the structure of the abdomen is highly peculiar.
From the ventral surface (Fig. 119) we can see at the tip of the tail
three small calcified plates, which represent the fifth and sixth terga
and the telson. Attached to the sixth segment are the much reduced and
rudimentary pleopods of that segment, and on the left hand side of the
body in the female are three well-developed pleopods of the first,
second, and third segments, which are used for carrying the eggs. The
extraordinary asymmetry of these limbs compared with the complete
symmetry of the abdomen itself is only explicable on the hypothesis that
these animals are descended from Hermit-crabs which had lost the
pleopods on the right side.
[Illustration:
FIG. 119.—_Birgus latro_, ♀, × ⅙, ventral view. _Ab_, First pleopod;
_T_, last pereiopod.
]
These appendages are entirely absent in the male. The ventral surface of
the abdomen is curiously warty and rugose, and is very soft and pulpy
owing to the immense store of oil which it contains.
[Illustration:
FIG. 120.—Dorsal view of abdomen, =A=, of _Cenobita_, sp.; =B=, of
_Birgus latro_. _T_, Telson; 1–6, 1st–6th abdominal segments.
]
If we look at the dorsal surface of the abdomen we find that, unlike
that of the Hermit-crabs, it is completely protected by a number of hard
plates (Fig. 120, B). Beneath the carapace can be seen a number of small
plates belonging to the last thoracic segment; following these there are
four large plates (1–4) representing the terga of the first four
abdominal segments; the fifth, sixth, and the telson are, as has been
stated, carried on the under side of the abdomen, but they are
represented diagrammatically (5, 6, _T_) in the dorsal view. Besides the
large terga, there are a number of small plates laterally, usually two
to each segment, but they show a tendency to subdivide and increase in
the largest specimens. This condition of affairs is very different to
that in the naked fleshy abdomen of an ordinary Pagurid, but it can
easily be deduced from that of the genus _Cenobita_, ordinary
Hermit-crabs found in the Indo-Pacific Oceans, from which the
Robber-crab has evidently descended. In _Cenobita_ (Fig. 120, A) we see
the same system of plates upon the dorsal surface of the abdomen, but
they are much smaller, and the lateral plates are not so numerous;
indeed, the greater part of the abdomen remains fleshy and uncalcified.
The under surface of the abdomen shows the same rugosity as is found in
_Birgus_, and from a number of other anatomical characters it is evident
that the Robber-crab is a highly modified _Cenobita_ that has deserted
its shell and developed a symmetrical abdomen protected by expanded and
hardened plates which represent those found in a reduced condition in
_Cenobita_. The species of _Cenobita_ although they inhabit shells and
have normal branchiae, live on the shore, and have not been seen to
descend actually into the sea.
The Lithodidae, which are found in temperate seas, especially on the
Northern Pacific coasts (though _Lithodes maia_ occurs in the North Sea,
and certain species inhabit deep water in the Indian Ocean), have a
deceptively Brachyuran appearance, the thorax being much shortened and
the abdomen being much reduced and carried tightly flexed on to the
ventral surface of the thorax. They live a free, unprotected existence,
and are highly calcified. They are, however, certainly Pagurids, as is
evidenced by a number of anatomical characters, but most clearly by the
asymmetry of the abdomen, especially in the female, which is not only
markedly asymmetrical in the arrangement of its dorsal plates (Fig.
121), but also in the presence of three pleopods upon the left side
only, as in _Birgus_. The male is without these appendages, and the
sixth pair of pleopods is absent in both sexes. The remarkable calcified
plates upon the abdomen bear a superficial resemblance to those in
_Birgus_, but their evolution is traced, not from a Cenobite, but from
an Eupagurine stock.[144]
[Illustration:
FIG. 121.—_Lithodes maia_, ♀, in ventral view, × ¼. The abdomen is
flexed on the thorax, so that its dorsal surface is seen. _l.3_,
Lateral plates of third abdominal segment; _l.5_, left lateral plate
of fifth abdominal segment; _m_, marginal plate; _T_, brush-like
last pereiopod; _Te.6_, telson and sixth abdominal segment.
]
In some of the Eupagurinae, e.g. _Pylopagurus_, feebly calcified plates
are present upon the segments of the abdomen (Fig. 122, A).
In the most primitive of the Lithodidae we witness the reduction (Fig.
122, B) and disappearance (C) of these original plates, their place
being taken first by a number of irregularly situated small spines and
warts, which, however, subsequently fuse up to form definite segmental
plates. In _Lithodes maia_, ♂ (D), there are a series of lateral and
marginal plates, while in _Acantholithus_ (E) a number of median plates
appear, presumably by the fusion of the small spines present in the
median line in _Lithodes maia_; finally, a fusion of the marginal and
lateral plates may take place, so that each abdominal segment is covered
by a median and two paired lateral plates.
[Illustration:
FIG. 122.—Diagrams of abdomen: =A=, of _Pylopagurus_, sp.; =B=, of
_Hapalogaster cavicauda_; =C=, of _Dermaturus hispidus_; =D=, of
_Lithodes maia_, ♂; =E=, of _Acantholithus hystrix_. _c_, Central
plates; _l_, lateral plates; _m_, marginal plates; _T_, telson; 1–6,
1st–6th abdominal segments. (After Bouvier.)
]
It is to be noted that the males and females of the various species do
not follow a parallel course of development, the plates in the male
being symmetrical, while those of the female are often highly
asymmetrical (compare Figs. 122, D, and 121), thus giving the strongest
evidence of a Pagurid ancestry.
_Birgus_ and the Lithodidae, then, are Pagurids which have given up
living in shells, and have become adapted to a free existence,
protecting their soft parts by the development of hard plates, and
re-acquiring, to a greater or less degree, a secondary symmetry of form.
But the story of Pagurid evolution does not apparently stop here. The
genus _Paralomis_, from the West Coast of America, superficially
resembles _Porcellana_, and is held to be descended from such forms as
_Pylocheles_, while isolated species are known (though not well known),
such as _Tylaspis_, described in the _Challenger Reports_,[145] which
appear to be Pagurids that have deserted their shells.
[Illustration:
FIG. 123.—Four stages in the development of _Eupagurus longicarpus_ or
_E. annulipes_, × 20. =A=, Ventral view of Zoaea; =B=, lateral view
of Metazoaea; =C=, dorsal view of Glaucothoe; =D=, dorsal view of
adolescent stage. _Ab.6_, 6th abdominal appendage; _Mxp.1_, _Mxp.3_,
1st and 3rd maxillipedes. (After M. T. Thompson.)
]
The metamorphosis of the Hermit-crabs has recently been studied by M. T.
Thompson.[146]
The Zoaea (Fig. 123, A) differs from that of the Galatheidea mainly in
the absence of the long spines. It possesses the usual appendages
characteristic of the Zoaea, namely, the first and second antennae,
mandibles, first and second maxillae, and two pairs of biramous swimming
maxillipedes and small third maxillipedes. In the Metazoaea (B), as in
the Anomura generally, the third maxillipedes develop into biramous
swimming organs, a thing they never do in the Brachyura, and the
rudiments of the thoracic segments put in a first appearance. The
abdominal segments are already fully formed in the Zoaea stage, so that
here as in all other Zoaeas, the order of development from in front
backwards is disturbed by the precocious differentiation of the
abdominal segments. The next stage is the “Glaucothoe” (Fig. 123, C),
which corresponds to the Megalopa of Brachyura (Fig. 125, p. 183). It
differs from the adult Hermit-crab in the perfect symmetry of its body,
the segmented abdomen, and the presence of five pairs of normal biramous
pleopods. At this stage, which lasts four or five days, it resembles
closely a little Galatheid. The asymmetry of the adult (Fig. 123, D) is
now imposed upon this larva by the migration of the liver, gonads, and
green glands into the abdomen, and by the shifting of the posterior
lobes of the liver on to the left side of the intestine, which is
displaced dorsally and to the right. The gonad lies entirely on the left
side. The pleopods of the right side now degenerate, more completely in
the male than in the female, and this degeneration is not completed
until the little crab has found a shell and lived in it for some time.
If a shell is withheld from it, the degeneration of the pleopods is much
retarded, so that although the Hermit-crab assumes its asymmetry without
the stimulus of the spiral shell, yet this stimulus is necessary for the
normal completion of the later stages.
=Fam. 1. Pylochelidae.=—The abdomen is macrurous and symmetrical, with
all the limbs present. _Pylocheles_ (Fig. 118, p. 173).
=Fam. 2. Paguridae.=—The abdomen is asymmetrical, with some of the limbs
lost. The antennal scale is well developed, and the flagella of the
first antennae end in a filament.
=Sub-Fam. 1. Eupagurinae.=—The third maxillipedes are wide apart at the
base, and the right chelipedes are much larger than the left.
_Parapagurus_ from deep-sea, _Eupagurus_ from temperate, especially
north temperate seas. _Pylopagurus._
=Sub-Fam. 2. Pagurinae.=—The third maxillipedes are approximated at the
base; the chelipedes are equal or subequal, or the left is much larger.
Chiefly in the warm and tropical seas, but _Clibanarius_ and _Diogenes_
also in the Mediterranean.
=Fam. 3. Cenobitidae.=—The abdomen is as in Paguridae. The antennal
scale is reduced, the flagella of the first antennae end bluntly. The
members of this family are characteristic of tropical beaches, where
they live on the land. _Cenobita_, with about six species, in the West
Indies and Indo-Pacific, living in Mollusc shells; _Birgus_ (Fig. 119)
on Indo-Pacific coral islands.
=Fam. 4. Lithodidae.=—The abdomen is bent under the thorax, and the body
is crab-like and calcified. The rostrum is spiniform, and the sixth
abdominal appendages are lost.
=Sub-Fam. 1. Hapalogasterinae.=—Abdomen not fully calcified, and without
complicated plates. _Hapalogaster_ and _Dermaturus_ in the North Pacific
littoral.
=Sub-Fam. 2. Lithodinae.=—Abdomen fully calcified, with a complicated
arrangement of plates. _Lithodes_ (Fig. 121) practically universal
distribution, littoral and deep sea. _Acantholithus_, deep littoral of
Japan; _Paralomis_, west coast of America. This last genus should
probably be placed in a separate family.
=Sub-Order 3. Brachyura.=[147]
The abdomen is much reduced, especially in the male, and is carried
completely flexed on to the ventral face of the thorax so as to be
invisible from the dorsal surface. The pleopods in the male are only
present on the two anterior segments, and are highly modified as
copulatory organs; the pleopods in the female are four in number and are
used simply for carrying the eggs; the pleopods of the sixth pair are
always absent in both sexes. The first antennae and the stalked eyes can
be retracted into special pits excavated in the carapace.
[Illustration:
FIG. 124.—=A=, Zoaea, × 24, and =B=, Metazoaea, × 13, of _Corystes
cassivelaunus_. _Ab_, 3rd abdominal segment; _An_, 1st antenna; _E_,
eye; _G_, gills; _M_, 1st maxillipede; _T.8_, last thoracic
appendage. (After Gurney.)
]
[Illustration:
FIG. 125.—Later stage (Megalopa) in the development of _Corystes
cassivelaunus_, × 10. _A_, Antenna; _Ab_, 3rd abdominal segment;
_C_, great chela; _T.8_, last thoracic appendage. (After Gurney.)
]
The larva hatches out as a Zoaea[148] (Fig. 124, A) very similar to that
of the Anomura; it is furnished with an anterior and posterior spine on
the carapace. It is characteristic of the Brachyuran Zoaea that the
third maxillipede is fashioned from the beginning in its definitive
expanded form, and is never a biramous swimming organ as in the Anomura.
The only exception to this rule is found in the Dromiacea, the most
primitive of the Brachyura, to be soon considered, in which not only the
third maxillipede, but also the first pair of pereiopods may be
developed as biramous oars, a condition taking one back to the Mysis
stage of the Macrura. The Metazoaea (Fig. 124, B) has the rudiments of
the thoracic limbs developed and crowded together at the back of the
carapace; they are all laid down in their definitive forms, and the
abdomen has the pleopods precociously developed. These Zoaeal stages are
of course pelagic, but the Metazoaea next passes into the Megalopa stage
(Fig. 125), in which the little crab forsakes its pelagic life and
assumes the ground-habits of the adult; the Megalopa, which corresponds
exactly to the Glaucothoe of the Pagurids, resembles a small _Galathea_
or _Porcellana_, the abdomen being still large and unflexed and
furnished with normal pleopods. From this stage the adult structure is
soon achieved, though, owing to the continued growth of the Crustacea
even after maturity is reached, there is often a slight progressive
change in structure, especially in the male, at each successive moult of
the individual. The Megalopa of _Corystes cassivelaunus_ is peculiar in
the immense production of the second antennae, which act as a
respiratory tube (Fig. 125).
The Brachyura must be considered under the following subdivisions:—
=Tribe 1. Dromiacea.=
All authorities are agreed that these[149] are the most primitive of the
Brachyura. In them the abdomen is much less reduced in both sexes than
in other Brachyura; there is a common orbitoantennary fossa, into which
eyes and antennae are withdrawn, instead of a separate one on each side
for each organ; the carapace is often much elongated as in the Macrura
and Anomura, and a number of other anatomical characters might be
mentioned which characterise the Dromiacea as intermediate between the
true Brachyura and the lower forms. There are, however, two views as to
the relationship of the Dromiacea; Claus held that they proceeded from a
Galatheid stock, and hence that the development of the Brachyura ran
through an Anomurous strain; but Huxley, and latterly Bouvier,[150]
adopt the view that the Dromiacea are descended, not from the
Galatheidae, but direct from the Macrura, and especially from the
Nephropsidea. Special resemblances are found between the Jurassic
Nephropsidae and certain present day Dromiacea, _e.g._ _Homolodromia
paradoxa_, the detailed form of the carapace in the two cases being very
similar. It is, however, a little strange that in the Dromiacea we meet
with the same reduction and dorsal position of the last, or last two
pairs of thoracic limbs which we saw to be such a characteristic feature
of the Anomura, especially of the Galatheidae. In the Dromiacea these
limbs may be chelate, and they are used for attaching shells and other
bodies temporarily to the back. Must we suppose that this resemblance to
the Anomura is due to convergence, or that the Nephropsidae, which gave
rise to perhaps both Galatheidae and Dromiacea, had this character, and
that it has been subsequently lost in the Macruran stock? We have
already mentioned that the Metazoaea of _Dromia_ has not only a
well-developed swimming third maxillipede, but also a biramous first
pereiopod, a character which speaks strongly for Macruran affinities.
[Illustration:
FIG. 126.—_Dromia vulgaris_, × 1. (After Milne Edwards and Bouvier.)
]
=Fam. 1. Dromiidae.=—The eyes and antennules are retractile into orbits.
The last two pairs of thoracic limbs are small, and held dorsally. The
sixth pair of pleopods are rudimentary or absent. _Homolodromia_ from
West Indies, deep-sea. _Dromia_, widely dispersed. _D. vulgaris_ (Fig.
126) occurs on the English coasts.
=Fam. 2. Dynomenidae.=—Similar to the preceding family, but only the
last pair of thoracic limbs is small, and held dorsally. The sixth pair
of pleopods are reduced, but always present. _Dynomene_ in the
Indo-Pacific.
=Fam. 3. Homolidae.=—The eyes and antennules are not retractile into
orbits. Only the last pair of thoracic limbs are reduced, the sixth pair
of pleopods altogether absent. _Homola_ and _Latreillia_, widely
distributed, occur in the Mediterranean. _Latreillopsis_ from the
Pacific. _L. petterdi_,[151] a magnificent species, with the carapace
nearly a foot long, and with very long legs like a Spider-crab, has been
dredged from 800 fathoms east of Sydney, New South Wales.
=Tribe 2. Oxystomata.=
This group comprises Crabs whose carapace is more or less circular,
while the mouth, instead of being square as in the remaining Brachyura,
is triangular with the apex pointing forward, and the third maxillipedes
are not expanded into the flattened, lid-like structures found in other
Crabs. There is the same tendency in some of the genera for the
posterior thoracic limbs to be reduced and carried dorsally, as in the
Galatheidae and Dromiacea. The well-known _Dorippe_ from the
Mediterranean has this feature, and frequently carries an empty shell
upon its back, and _Cymonomus_[152] presents the same peculiarity.
[Illustration:
FIG. 127.—_Cymonomus granulatus_, × 1. _A.1_, _A.2_, 1st and 2nd
antennae; _E_, eye-stalk; _S_, extra-orbital spine of carapace.
(After Lankester.)
]
_Cymonomus granulatus_ (Fig. 127) is an abyssal form that has been
dredged from the Mediterranean and North Atlantic, in which the
eye-stalks are curiously tuberculated, and the ommatidia of the eye are
entirely unpigmented and degenerate, though a few corneal facets are
still recognisable. This species is replaced by _C. quadratus_ in the
Caribbean Sea and by _C. normani_ on the East African coast, in which
the alteration of the eye-stalks into thorny, beak-like projections
becomes progressively marked, and all traces even of the corneal facets
disappear. This remarkable genus was mentioned in the excursus on
Crustacean eyes on p. 149.
[Illustration:
FIG. 128.—_Calappa granulata_, from in front, × ½. _C_, Hand of
chelipede; _T_, walking legs. (After Garstang.)
]
The Oxystomata, like the Cyclometopa, to be considered later, live in
sandy and gravelly regions, and burrow to a greater or less extent, and
we find in both groups admirable adaptations for securing a pure stream
of water, uncontaminated by particles of sand, for flushing the gills.
Perhaps the most remarkable of these adaptations is afforded by
_Calappa_.[153] This animal has the chelipedes wonderfully modified in
structure, and when it is reposing in the sand it holds them apposed to
the front of the carapace, as shown in Fig. 128, so that the spines upon
their edges, together with the hairy margin of the carapace, form a most
efficient filter for straining off sand and grit from the stream of
water which is sucked down between the closely-fitting chelipedes and
carapace, to enter the branchial chambers at their sides. The exhaled
current of water passes out anteriorly through a tube formed by a
prolongation of the endopodites of the first maxillipedes. The exhalant
aperture is shown in Fig. 128 by the two black cavities below the snout
in the middle line.
A similar method is pursued by the related _Matuta banksii_[153] (Fig.
129), a swimming and fossorial Crab found in the Indo-Pacific. In this
Crab the chelipedes also fit against the carapace to form a strainer,
and their function is assisted by the enlargement of the posterior
spine, which acts as a kind of elbow-rest to keep the chelipedes
properly in position. The inhalant openings are situated just in front
of the chelipedes. It is a most remarkable fact that among the
Cyclometopa, _Lupa hastata_ (Fig. 131) has an exactly similar
arrangement. Apparently we have here another instance of convergence,
similar to that of _Corystes_ and _Albunea_, but the case is complicated
by the fact that some of the Oxystomata, and among them _Matuta_, show a
certain amount of relationship to the Cyclometopous Portunids, so that
it is just conceivable that the resemblances in the respiratory
arrangement are due to a common descent and not to convergence.
[Illustration:
FIG. 129.—Dorsal view of _Matuta banksii_, × 1. (From an original
drawing prepared for Professor Weldon.)
]
In the Leucosiidae, of which the Mediterranean _Ilia nucleus_ (Fig. 130)
is an example, the inhalant aperture is situated between the orbits, and
leads into gutters excavated in the “pterygostomial plates” flanking the
mouth, which are furnished with filtering hairs and are converted into
closed canals by expansions of the exopodites of the third maxillipedes.
Thus these Crabs possess a filtering apparatus independent of the
chelipedes and of the margin of the carapace.
=Fam. 1. Calappidae.=—Cephalothorax rounded and crab-like. The abdomen
is hidden under the thorax, the antennae are small, and the legs normal
in position. The afferent openings to the gill-chambers lie in front of
the chelipedes. Male openings on coxae of last pair of legs. _Calappa_
(Fig. 128) circumtropical, and extending into the warmer temperate seas.
_Matuta_ (Fig. 129) from the Indo-Pacific.
[Illustration:
FIG. 130.—Dorsal view of _Ilia nucleus_, × 1. (From an original
drawing prepared for Professor Weldon.)
]
=Fam. 2. Leucosiidae.=—Similar to the above, but the afferent openings
to the gill-chambers lie at the bases of the third maxillipedes. Male
openings on the sternum. This family contains a great number of forms,
with headquarters in the tropical littoral, but extending into the
temperate seas. _Ilia_ in the European seas. _I. nucleus_ (Fig. 130)
common in the Mediterranean. _Ebalia_ in the Atlantic, North Sea, and
Indo-Pacific. _Leucosia_ in Indo-Pacific.
=Fam. 3. Dorippidae.=—Cephalothorax short and square. The abdomen is not
hidden under the thorax; the antennae are large, and the last two pairs
of legs are held dorsally, and have terminal hooked claws. _Dorippe_,
littoral in Mediterranean and Indo-Pacific. _Cymonomus_ (Fig. 127) from
deep-sea of Atlantic and Mediterranean.
=Fam. 4. Raninidae.=—Similar to Dorippidae, but the cephalothorax is
elongated, and the legs usually have the last two joints very broad.
Several genera, chiefly in the deeper littoral zone. _Ranina dentata_ in
the Indo-Pacific.
=Tribe 3. Cyclometopa.=
In these Crabs the carapace is circular rather than square; its frontal
and lateral margins are produced into spines and there is no pointed
rostrum. The mouth is square, and the third maxillipedes are greatly
flattened and form a lid-like expansion over the other oral appendages.
This group includes the common Shore-crab of our coasts (_Carcinus
maenas_), the swimming Crabs with expanded pereiopods (_Portunus_,
_Lupa_, etc.), the Edible Crab (_Cancer pagurus_), and many others.
_Corystes cassivelaunus_ is a Crab of doubtful affinities. It is
sometimes placed among the Oxyrhyncha, but, as Gurney[154] has pointed
out, the Megalopa shows Portunid characters, and the resemblance to the
Oxystomata in the front of the carapace and in the mouth may be
secondary. The respiratory arrangement of this Crab has already been
mentioned in comparing its structure with that of the Mole-crab
_Albunea_. The form of the antennal tube can be gathered from the figure
of the Megalopa stage (Fig. 125, p. 183). It should be noted that when
the Crab is buried in the sand with only the tip of the antennal tube
projecting, the water is sucked down and enters the branchial cavities
anteriorly, the antennal tube being continued by a tube formed from the
third maxillipedes and the forehead; the water is exhaled at the sides
of the branchial cavities beneath the branchiostegites. Thus in
_Corystes_ the normal direction of the current is reversed, but when the
Crab is not buried, and is moving over the surface, it breathes in the
usual manner, taking in the water at the sides of the branchiostegites
and exhaling it anteriorly by the tube. The related _Atelecyclus_, found
like _Corystes_ very commonly at Plymouth, uses two methods of
breathing: when it is in the surface-layers of sand it makes use of its
antennal tube, which is, however, much shorter than in _Corystes_; but
when it burrows deeper, where the antennal tube is no use, it folds its
chelipedes and also its other legs, which are densely covered with
bristles, so as to form a reservoir of pure water underneath it free
from sand, which it passes through the gill-chambers in the usual manner
(see Garstang, _loc. cit._ p. 186).
The respiratory adaptations in _Lupa hastata_ and their convergence
towards those of the Oxystomatous _Matuta_ have been already touched
upon (pp. 186, 187).
In this connexion must be mentioned the interesting experiments of W. F.
R. Weldon[155] upon the respiratory functions of _Carcinus maenas_ at
Plymouth, since these were the first noteworthy observations directed
towards the exact measurement of the action of natural selection upon
any animal, a field of observation in which Weldon will always be looked
upon as a pioneer. An extended series of measurements by Weldon and
Thompson on male specimens of _Carcinus maenas_ of various sizes between
the years 1893 and 1898 showed a steady decrease in the ratio of
carapace breadth to length; the Crabs appeared to be becoming steadily
narrower across the frontal margin, and the same thing, though not to
the same extent, was happening in female Crabs. Weldon supposed that
this change might be correlated with the silting up of Plymouth Sound
and the consequent fouling of the water. To test this hypothesis he kept
a very large number of male Crabs in water to which fine porcelain clay
was added and kept in continual motion. In the course of the experiments
the survivors and the dead were measured, and it was found that the mean
carapace breadth of the survivors was less than that of those that
succumbed. The experiment was repeated with the fine sand that is
deposited and left at low water upon the stones on Plymouth beach, and
the same result was observed. It was also noticed that the individuals
which died had their gills clogged with the sand, while those that
survived had not. As a further confirmation, a great many young male
Crabs were isolated and kept in pure filtered water, and they were
measured before and after moulting; these measurements, when compared
with measurements of the frontal breadth in Crabs of the same size taken
at random upon the beach, were found to show a greater breadth than the
wild Crabs, thus indicating that a selection of narrow Crabs was taking
place in Nature which did not take place when the Crabs were protected
from the effects of fine sand in the water.
The whole chain of evidence goes to show that the carapace breadth in
_Carcinus maenas_ in Plymouth Sound is being influenced by the rapid
change of conditions occurring in the locality. Various objections have
been urged against this conclusion, but, though they merit further
investigation, they do not appear very weighty.
The fresh-water Crab, _Thelphusa fluviatilis_, common in the South of
Europe and on the North coast of Africa, belongs to the Cyclometopa, and
is interesting from its direct mode of development without
metamorphosis.
=Fam. 1. Corystidae.=—The orbits are formed, but, unlike all the other
families of the Cyclometopa, are incomplete. The body is elongate and
oval, and the rostrum and front edge of the mouth rather as in the
Oxyrhyncha, in which Tribe they are sometimes included. _Corystes_, with
a few species in European seas. _C. cassivelaunus_ at Plymouth.
=Fam. 2. Atelecyclidae.=—Perhaps related to the foregoing. The carapace
is sub-circular, and the rostrum short and toothed. _Atelecyclus_,
European seas.
=Fam. 3. Cancridae.=—The carapace is broadly oval or hexagonal, and the
flagella of the second antennae are short and not hairy as in the
foregoing. The first antennae fold lengthwise. _Carcinus maenas_ on
English and North European coasts. This crab has become naturalised in
some unexplained manner in Port Phillip, Melbourne. _Cancer_ in North
Atlantic, North Pacific, and along the west coast of America into the
Antarctic regions. _C. pagurus_ is the British Edible Crab.
[Illustration:
FIG. 131.—Dorsal view of _Lupa hastata_, × 1. (From an original
drawing prepared for Professor Weldon.)
]
=Fam. 4. Portunidae.=—The legs are flattened and adapted for swimming.
The first antennae fold back transversely. _Portunus_, Atlantic and
Mediterranean. _Neptunus_, Indo-Pacific. _Callinectes_, _C. sapidus_,
the edible blue Crab of the Atlantic coasts of America. _Lupa_ (Fig.
131).
=Fam. 5. Xanthidae.=—The first antennae fold transversely, but the legs
are not adapted for swimming; the body is usually transversely oval.
This family is especially characteristic of the tropical littoral, where
it is very widely represented. _Xantho_, _Actaea_, _Chlorodius_,
_Pilumnus_, _Eriphia_, with _E. spinifrons_, common in the
Mediterranean.
=Fam. 6. Thelphusidae (Potamonidae).=—Fresh-water crabs, with the
branchial region very much swollen. _Thelphusa_ (or _Potamon_) has
nearly a hundred species distributed from North Australia, through Asia,
Japan, the Mediterranean region, and throughout Africa. _Potamocarcinus_
in tropical America.
=Tribe 4. Oxyrhyncha.=
This section includes the Spider-crabs and related genera, in which the
carapace is triangular, with the apex in front formed by a
sharply-pointed rostrum. There are two chief series, the one comprising
the Spider-crabs, with much elongated walking legs, _e.g._ the huge
_Maia squinado_ of European seas, the yet more enormous _Macrocheira
kämpferi_ from Japan, supposed to be the largest Crustacean in
existence, and sometimes spanning from outstretched chela to chela as
much as eleven feet, and the smaller forms, such as _Inachus_, _Hyas_,
and _Stenorhynchus_, which are so common in moderate depths off the
English coasts. The other series is represented by genera like _Lambrus_
(Fig. 133), in which the legs are not much elongated, but the chelipedes
are enormous.
The Spider-crabs do not burrow, and their respiratory mechanism is
simple; but since they are forms that clamber about among weeds, etc.,
upon the sea-bottom, they often show remarkable protective resemblances
to their surroundings, which are not found in the burrowing Cyclometopa.
Alcock[156] gives a good account and figure of _Parthenope
investigatoris_, one of the short-legged Oxyrhyncha, the whole of whose
dorsal surface is wonderfully sculptured to resemble a piece of the old
corroded coral among which it lives.
But besides this, the long-legged forms, such as _Inachus_, _Hyas_,
etc., have the habit of planting out Zoophytes, Sponges, and Algae upon
their spiny carapaces, so that they literally become part and parcel of
the organic surroundings among which they live. It may, perhaps, be
wondered what are the enemies which these armoured Crustacea fear.
Predaceous fish, such as the Cod, devour large quantities of Crabs,
which are often found in their stomachs; and Octopuses of all sorts live
specially upon Crabs, which they first of all paralyse by injecting them
with the secretion of poison-glands situated in their mouth. The poison
has been recently found by Dr. Martin Henze at Naples to be an alkaloid,
minute quantities of which, when injected into a Crab, completely
paralyse it. When the Crab is rendered helpless the Octopus cuts out a
hole in the carapace with its beak, and sucks all the internal organs,
and then leaves the empty shell.
Many of the Oxyrhyncha are found in the abysses; among them are
_Encephaloides armstrongi_ (Fig. 132), dredged by Alcock from below the
100–fathom line in the Indian Ocean, which has the gill-chambers (G)
greatly swollen and enlarged to make up for the scarcity of oxygen in
these deep regions.
[Illustration:
FIG. 132.—_Encephaloides armstrongi_, × 1. The long walking legs are
omitted. _C_, Great chela; _G_, one of the greatly swollen
gill-chambers. (After Alcock.)
]
=Fam. 1. Maiidae.=—The chelipedes are not much larger than the other
legs, but are very mobile. Orbits incomplete. A very large family,
including all the true Spider-crabs, very common in the Atlantic and
Mediterranean littoral. _Inachus_, _Pisa_, _Hyas_, _Stenorhynchus_,
_Maia_, _Encephaloides_ (Fig. 132).
=Fam. 2. Parthenopidae.=—The chelipedes are much larger than the other
legs. Orbits complete. _Lambrus_ (Fig. 133), _Parthenope_.
[Illustration:
FIG. 133.—_Lambrus miersi_, × 1. (After Milne Edwards and Bouvier.)
]
=Fam. 3. Hymenosomatidae.= The carapace is thin and flat; the chelipedes
are neither very long nor especially mobile. There are no orbits, and
the male openings are on the sternum. Characteristic of the Antarctic
seas. _Hymenosoma_, _Trigonoplax_.
=Tribe 5. Catometopa.=
These Crabs resemble the Cyclometopa in general appearance, but the
carapace is very square in outline, and its margins are never so well
provided with spines as in the Cyclometopa. The position of the male
genital openings is peculiar, since they lie upon the sternum, and are
connected with the copulatory appendages upon the abdomen by means of
furrows excavated in the sternum. The Catometopa are either littoral or
shallow water forms, or else they live entirely on land. The Grapsidae
are marine Crabs, _Pachygrapsus marmoratus_ (Fig. 134) at Naples being
exceedingly common on rocks at high-water mark, over which it scuttles
at a great rate; in the Mediterranean it takes the place of our common
_Garcinus maenas_, which is not found there.
[Illustration:
FIG. 134.—Dorsal view of _Pachygrapsus marmoratus_, × ⅓. (From an
original drawing prepared for Professor Weldon.)
]
Among the land genera are _Ocypoda_, _Gelasimus_, and _Gecarcinus_ of
tropical lagoons and coastal swamps. _Ocypoda_ often occurs in vast
crowds in these regions, and digs burrows in the sand.
[Illustration:
FIG. 135.—_Gelasimus annulipes_, × 1. =A=, Female; =B=, male. (After
Alcock.)
]
_Gelasimus_ (Fig. 135) is remarkable for the enormous size of one of the
chelipedes, generally the right, in the male, which may actually exceed
in size the rest of the body. It is not known what purpose this organ
serves in the various species. In _Gelasimus_ it is supposed that the
male stops up the mouth of the burrow with it when he and the female are
safely inside. It is also used as a weapon in sexual combats with other
males; but Alcock, from observations made in the Indian Ocean, believes
that the males use it for exciting the admiration of the females in
courtship, as the huge chela is bright red in colour, and the males
brandish it about before the females as if displaying its florid beauty.
The species of _Ocypoda_ are exclusively terrestrial, and cannot live
for a day in water. The gills have entirely disappeared, and the
branchial chambers are converted into air-breathing lungs with highly
vascular walls, the entrances into which are situated as round holes
between the bases of the third and fourth pairs of walking legs. As
their name implies, they can run with astonishing rapidity, and they
seem to be always on the alert, directing their eyes, which are placed
on exceedingly long stalks, in all directions.
Some of the Grapsidae, _e.g._ _Aratus pisonii_, are partially adapted
for life on land. Fritz Müller, in his _Facts for Darwin_, alludes to
this creature as “a charming lively crab which ascends mangrove bushes
and gnaws their leaves.” The carapace can be elevated and depressed
posteriorly, apparently by means of a membranous sac, which can be
inflated by the body-fluids. This Crab retains its gills and can breathe
under water in the ordinary way.
A great many other Catometopa are land-crabs; but we may specially
mention the genus _Gecarcinus_, related to the marine Grapsidae, which
has representatives in the West Indies and West Africa. The Crabs of
this genus may live in sheltered situations several miles from the sea,
but in spring the whole adult population rushes down in immense troops
to the shore, where breeding and spawning take place; and when this is
completed they migrate back again to the land. The young pass through
the normal larval stages in the sea and then migrate inland.[157]
=Fam. 1. Carcinoplacidae.=—The carapace is rounded and broader than
long, usually with toothed front margin. The orbits and eyes are normal,
and not much enlarged. _Geryon_, in the deep littoral of the northern
hemisphere. _Euryplax_, _Panoplax_, etc., in the American coastal
waters. _Typhlocarcinus_, etc., in the Indo-Pacific.
=Fam. 2. Gonoplacidae.=—The carapace is square, with the antero-lateral
corners produced into spines. The orbits are transversely widened, and
the eye-stalks long. _Gonoplax_, widely distributed in the littoral
zone. _G. rhomboides_ in British and European seas.
=Fam. 3. Pinnotheridae.=—Carapace round, with indistinct frontal margin.
Orbits and eyes very small, often rudimentary. The members of this
family live symbiotically or parasitically in the shells of living
Bivalve Molluscs, corals, and wormtubes in all seas except the Arctic.
_Pinnotheres pisum_ is fairly commonly met with off the English coasts
in the mantle-cavity of _Cardium norwegicum_.
=Fam. 4. Grapsidae.=[158]—Carapace square, the lateral margins either
strictly parallel or slightly arched. The orbits and eyes are moderately
large, but the eye-stalks are not much lengthened. Littoral,
fresh-water, and land. _Pachygrapsus marmoratus_ (Fig. 134), the common
shore-crab of the Mediterranean. _Sesarma_, with fresh-water and land
representatives in the tropics of both hemispheres. _Cyclograpsus_,
marine in the tropical littoral.
=Fam. 5. Gecarcinidae.=—Carapace square, but much swollen in the
branchial region. Orbits and eyes moderately large. Typically land
forms, which only occasionally visit the sea or fresh water. _Cardisoma_
is a completely circumtropical genus, with species in tropical America,
West and East Africa, and throughout the Indo-Pacific. _Gecarcinus_ in
West Indies and West Africa.
=Fam. 6. Ocypodidae.=—Carapace square or rounded, generally without
teeth on the lateral margins. The orbits transversely lengthened,
eye-stalks usually very long. The members of this family generally
inhabit the mud-flats and sands of tropical coasts; in the southern
hemisphere they extend far into the temperate regions. _Macrophthalmus_,
with numerous species, in Indo-Pacific. _Gelasimus_ (Fig. 135), in the
tropics of both hemispheres. _Ocypoda_, with similar distribution.