HABIT AND HABITAT IN THE GALATHEIDEA. 85 



tide marks. The form of body presents many peculiarities, both super- 

 ficial and in detail, comparable with those which are regarded as charac- 

 teristic of the Brachyura, furnishing an excellent example of " con- 

 vergence." . . . The abdomen is greatly reduced in size, the length of 

 the carapace scarcely exceeds its breadth, and the habit of clinging to 

 stones is developed to a marked degree. 



C. The Munida rugosa type. 



Characteristic of fairly deep waters (found at depths varying from 

 10 to 600 fathoms). The general form of body closely resembles that 

 of type A, but obvious specializations for deep-sea life exist. 



Thus, within the group G-alatheidea, a few species (those of group C) 

 have been driven by stress of competition in the shallower waters to seek 

 shelter in the deeper and more sparsely populated regions of the sea ; 

 but the main line of specialization in the group is in the direction A to B 

 — towards adaptation for life higher up the shore. It is the aim of this 

 memoir, by describing details of the structure of the branchial and other 

 organs of the Galatheidea, to show what peculiarities have arisen in con- 

 nection with the need for special precautions : 



(a) For ensuring steadiness of balance when the animal is in pro- 

 gression or at rest within the wave-washed region of the shore ; 



(6) For guarding against the choking of the branchial cavity and 

 clogging of the branchial organs by the mud of the low shore. 



In doing this, an endeavour is made to arrange these details in logical 

 and progressive order, as indicating for the more crab-like members of 

 the group " the base degrees whereby they did ascend." 



I. ]\10DIFICATI0NS WHICH PEOMOTE EFFICIEXCY IN RESISTANCE TO 

 "WAVE-WASH AND IN PROGRESSION ON THE SHORE. 



It is convenient to use the type-species A {Galathea squamifera) as 

 a starting-point from which to trace various lines of specialization 

 within the group, this species being probably among the nearest to the 

 Macrurous ancestor. We find throughout the genus Galathea few special- 

 izations of a definitely Anomurous character : the long, rather narrow 

 carapace and powerful abdominal swimming " tail " with broad tail-fan 

 obviously mark the active swimmer. In those members of the group 

 which frequent the higher coast-zones — notably the Porcellanids — and 

 which thus assume the creeping rather than the swimming habit, we note 

 a reduction in importance of the Macrurous characters — a diminution 



