Xll] ASTACUS 199 



particles of grit or sand which will roll into different positions and 

 stimulate different sense-cells for each new position which the 

 animal assumes. When however the animal undergoes ecdysis the 

 cuticle lining the ear-pit is shed and with it these grains of grit. 

 After each ecdysis therefore the animal must obtain fresh grains 

 of sand from outside. Now the experimenter kept specimens of 

 Palinurus in a tank till they shed their shells which he immedi- 

 ately removed. The animals were then placed in filtered sea-water 

 to which some grains of magnetic oxide of iron were added. In a 

 day or two when the new shell had hardened and the crayfish 

 regained their activity the experimenter approached the tank with 

 a powerful magnet. The animals then set themselves at right 

 angles to the lines of magnetic force; they behaved as if the 

 magnetic attraction were the force of gravity. This could only be 

 ^ explained on the assumption that they regulated the positions of 

 their bodies in accordance with the position of the particles of grit 

 in the ears in other words the balancing function of the ear was 

 proved. 



The sense of smell is also situated in the antennule and is no 

 doubt one of the most important senses the animal has, but the 

 only organs which can be associated with it are the small flattened 

 setae on the exopodite already described. The sense of touch is 

 probably associated with the small setae scattered all over the body. 

 Each of these is inserted by means of a tiny arthrodial membrane 

 in the shell and is consequently moveable, and sense cells are 

 situated at the bases of many of them which must be stimulated 

 when anything moves these tiny levers. 



The muscles of the crayfish practically all consist of bundles of 

 longitudinal fibres which must be regarded as remnants of the walls 

 of the vanished coelomic sacs. There are no circular muscles, 

 except those surrounding heart and arteries, for the comparatively 

 rigid shell in which the animal is enclosed would make circular 

 muscles useless. The muscles may be divided into trunk muscles 

 and appendage muscles, and we may treat of the latter first. The 

 skeleton of each segment of each appendage articulates with each 

 succeeding and preceding segment, so that it can move with respect 

 to each of them in only one plane. To effect this movement two 

 muscles are necessary, an extensor, or straighterier, and a flexor, 

 or bender. Since the skeleton is outside the muscles, the flexor 

 muscle is placed on the opposite side of the limb from that towards 

 which it is bent, the reverse of the condition which prevails in the 



