DARWIN'S THEOEY 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 Grapsidffi he describes, under the name 

 Aratus Pisonii, the species which Milne-Edwards calls 

 Sesarma Pisonii, a sweet little vivacious crab, which climbs 

 the mangrove-bushes and feeds upon their leaves. Its short 

 shai-p 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 Gh-apsus, 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 Gyclograpsus, 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 quitted the source of 

 supply. 



That reticulation of the shell between the afferent and 

 efierent 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 ih the 



