DARWIN’S THEORY TESTED 97 
Miiller was prepared to regard it as a very damaging blow 
to the theory of evolution. The result was just the oppo- 
site, as will be seen by a comparison of his observations on 
several species and genera. Those on Ocypode have been 
already quoted. 
In the family Grapsidee he describes, under the nane 
Aratus Pisoni, the species which Milne-Edwards calls 
Sesarma Pisoni, a sweet little vivacious crab, which climbs 
the mangrove-bushes and feeds upon their leaves. Its short 
sharp claws are well fitted for climbing, but they prick like 
pins when the creature runs over a bare hand. Once, 
when he had one of these seated on his hand, Fritz Miiller 
noticed that it raised up the hinder part of its carapace, 
and that by this means a wide slit was opened upon each side 
over the last pair of feet, affording a view into the bran- 
chial cavity. When studying this phenomenon in another 
species, which he took to be a true Grapsus, he observed 
that with the formation of the slit behind, the anterior part 
of the carapace seems to sink so as partly or entirely to 
close the anterior afferent opening. As the lifting of the 
carapace never takes place under water, he infers that the 
animal opens its branchial cavity in front or behind accord- 
ing as it requires to breathe water or air. He had noticed 
the elevation of the carapace, also, in species of Sesarma 
and Cyclograpsus, which burrow deep in swampy ground, 
and often scamper about on the wet mud, or sit watch- 
fully before their burrows. But to observe the action in 
these is a work of patience, since they can continue to 
breathe water long after they have quittcd the source of 
supply. 
That reticulation of the shell between the afferent and 
efferent branchial orifices, which has been mentioned in 
the character of the genus Sesarma, has a special purpose. 
The squared meshes of network are due partly to fine 
tuberculation and partly to curious geniculate hairs form- 
ing over the surface a sort of fine hair-sieve. When the 
water issues from the branchial cavity it spreads through 
this network, and can take up fresh oxygen, whereupon 
the appendages ot the third maxillipeds, working in the 
