194 
CRUSTACEA—EUCARIDA—DECAPODA 
CHAP. 
the place of our common Carcinus maenas, which is not found 
Fic. 134.—Dorsal view of Pachygrapsus marmoratus, 
x 4. (From an original drawing prepared for Professor 
Weldon.) 
size of one of the chelipedes, generally the 
which may actually exceed in size the rest 
of the body. It is not known for what 
purpose this organ serves in the various 
species. In Grelasimus 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, be- 
leves 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 
there. 
Among the 
land genera are 
Ocypoda,  CGelasi- 
mus, and Gecar- 
cinus of tropical 
lagoons and coastal 
swamps. Ocypoda 
often occurs im 
vast crowds in 
these regions, and 
digs burrows in 
the sand. 
Gelasimus (Fig. 
135)is remarkable 
for the enormous 
right, in the male, 
Fre. 135. — Gelasimus 
annulipes, x 1. Ay 
Female ; B. male. 
(After Alcock.) 
as round holes between the bases of the third and fourth pairs 
of walking legs. 
As their name implies, they can run with 
