much larger in the male than in the female, is not known. As I hear
from Mr. Bate, the chelae are sometimes of such length and size that
they cannot possibly be used for carrying food to the mouth. In the
males of certain fresh-water prawns (Palaemon) the right leg is
actually longer than the whole body. (10. See a paper by Mr. C. Spence
Bate, with figures, in Proceedings, Zoological Society, 1868, p. 363;
and on the nomenclature of the genus, ibid. p. 585. I am greatly
indebted to Mr. Spence Bate for nearly all the above statements with
respect to the chelae of the higher crustaceans.) The great size of the
one leg with its chelae may aid the male in fighting with his rivals;
but this will not account for their inequality in the female on the
opposite sides of the body. In Gelasimus, according to a statement
quoted by Milne Edwards (11. Hist. Nat. des Crust. tom. ii. 1837, p.
50.), the male and the female live in the same burrow, and this shews
that they pair; the male closes the mouth of the burrow with one of its
chelae, which is enormously developed; so that here it indirectly
serves as a means of defence. Their main use, however, is probably to
seize and to secure the female, and this in some instances, as with
Gammarus, is known to be the case. The male of the hermit or soldier
crab (Pagurus) for weeks together, carries about the shell inhabited by
the female. (12. Mr. C. Spence Bate, British Association, Fourth
Report on the Fauna of S. Devon. ) The sexes, however, of the common
shore-crab (Carcinus maenas), as Mr. Bate informs me, unite directly
after the female has moulted her hard shell, when she is so soft that
she would be injured if seized by the strong pincers of the male; but
as she is caught and carried about by the male before moulting, she
could then be seized with impunity.
[Fig.8. Orchestia Darwinii (from Fritz Muller), showing the
differently-constructed chelae of the two male forms.]
Fritz Muller states that certain species of Melita are distinguished
from all other amphipods by the females having the coxal lamellae of
the penultimate pair of feet produced into hook-like processes, of
which the males lay hold with the hands of the first pair. The
development of these hook-like processes has probably followed from
those females which were the most securely held during the act of
reproduction, having left the largest number of offspring. Another
Brazilian amphipod (see Orchestia darwinii, Fig. 8) presents a case of
dimorphism, like that of Tanais; for there are two male forms, which
298