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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.
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.
Gelasimus (Fig. 135) is
remarkable for the enormous size
of one of the chelipedes, generally
Fig. 134.—Dorsal view of Pachygrapsus
the right, in the male, which may
marmoratus, × ⅓. (From an original actually exceed in size the rest of
drawing prepared for Professor the body. It is not known what
Weldon.) 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 Fig. 135.—Gelasimus annulipes, × 1. A,
marine Grapsidae, which has Female; B, male. (After Alcock.)
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.
CHAPTER VII
REMARKS ON THE DISTRIBUTION OF
MARINE AND FRESH-WATER CRUSTACEA
A. Marine.
B. Fresh-Water.[164]