ON CRABS WHICH CLING. 183 



To revert to platycheles, it is important to notice that the 

 animal brings np the two pairs of legs next behind the great 

 claw very close up behind that limb. If now a Httle Indian ink 

 be added to the water just near the base of the great claw the 

 stain will be at once drawn in and will issue forth again 

 almost immediately at the si ies of the mouth. The inference 

 is that the stream of water used for breathing finds its way in 

 between the base of the great claw and the limbs behind. 

 This channel must not be too broad, whence the pressing 

 forward of limbs to reduce the free way. These limbs, too, 

 have a great many hairs which are almost feather-like under 

 the microscope and must act as a strainer to the breathing 

 current, as Grarstang has shown the spines of many crabs 

 to do. 



Like other clinging forms, Porcellana sometimes, often in 

 fact, drops away from its stone, and its behaviour on such 

 0(!casions is characteristic. It will usually drop on its back 

 into a little water. If so, the fourth pair of legs, the third 

 pair that is behind the great claws, stretch out backwards and 

 endeavour to grip some firm support. They are often, how- 

 ever, unable to reach down to one, and then the animal will 

 flap its abdomen vigorously until the whole body SAvays up 

 and so allows these legs to stretch down far enough. Once 

 they grip, the animal pulls itself round again on them, 

 using the first part of the abdomen (the part just behind 

 the carapace) as a pivoting line. The flapping of the 

 abdomen is an imdoubted trace of the swimming habit of the 

 Galathea-like ancestor, and if Porcellana be dropped into a 

 faiily deep tank it will probablj flap the abdomen as it falls. 

 Still the swimming habit is almost gone and the abdomen is 

 very weak, though it retains the terminal fin-like expansion 

 which is so much better seen in Galathea and is still far 

 stronger in a lobster. 



P. jjlutychelea resets itself much more adroitly than does 

 P. hngicoriiis^ limbs and abdomen working Avell together. It 

 seems possible to correlate this with the difl'erences in their 

 nervous systems. P. hugicoriiis is like Galathea in this 

 respect, but P. jjlattjclwles has the ganglia (nerve-centres) of 

 the tail region fused into one mass with those of the walking 

 legs. 



One character Porcellana retains which it has inherited 

 from Galathea and the lobsters, and that i> its long feelers or 

 antennae (the second pair). Those of ordinary crabs are short, 

 these forms push their w^ay through mud and obstacles, but Por- 

 cellana needs feelers which will reach all around the body as it 



