1916-17.] Experiments and Observations on Crustacea. 261 
The forward direction and mode of articulation of the basipodites, as 
well as the absence of hooks or special curvature on the dactylopodites, 
suggest that in the evolution of the animal clinging power has been sacri- 
ficed. Dr Bruce informs me that in spite of its size the living Glyptonotus 
can be handled with impunity, which is more than can be said of some 
smaller isopods — see Stebbing (1893, p. 343). The power of clinging or of 
clasping implies the existence (“ to every action there is an equal and 
opposite reaction ”) of some oppositely directed (paired) mechanism, trans- 
verse or antero-posterior as the case may be.* The limbs of Glyptonotus 
show such obvious arrangement neither in the transverse nor in the antero- 
posterior direction ; indeed the predominantly unidirectional orientation 
of the resting peraeopods might suggest that in the natural economy the 
animal is given either to hanging head downwards from a support or, on a 
level surface, to depressing its head and tilting its pleon upwards. 
When the functional employment of the walking limbs of Ligia comes 
to be described it will be shown that the creature’s “ internal world,” as 
von Uexkiill (1909) puts it, is in large degree pieced together of impressions 
of surfaces touched by the limbs ; the content of its psyche largely centres 
around the ventral part of the body with its cluster of limbs. Without 
something to cling to, Ligia is deprived of its main source of sensory 
communication with the external world ; its power of orientation (and 
probably its perception of orientation) is related not so much to gravity as 
to a fixed surface of some kind. The animal is just as much at home on 
the under surface or on the vertical face of a rock as on the upper surface 
thereof (“ he clasps the crag with crooked hands ”), which peculiarity in 
turn is undoubtedly connected with the large number of limbs it possesses. 
In Glyptonotus the walking limbs are reduced in number and the dacty- 
lopodites straightened out. It would be of much interest to determine just 
in what proportion this animal’s power of progression and of body orienta- 
tion is dependent on gravity on the one hand and on contact with an 
adequate surface on the other. Can it walk upside-down adhering to 
under-surfaces like Ligia or like the water-encompassed Idoteinae with 
their numerous limbs and curved dactylopodites ? 
Doflein (1910) has drawn attention to a probable function of the hairs 
or setse which clothe the appendages and other movable parts of Crustacea. 
Species of Leander resting upon rough bottom seek to obtain contact with 
surrounding objects through a maximal number of these hairs; on the 
limbs the setse are especially numerous in the vicinity of the articulations, 
and Doflein’s view is that a sense equivalent to the muscular sense of 
* In this connection compare the feet of different birds. 
