64 HOUSE, GARDEN, AND FIELD 



will be seen to gape at very frequent intervals ; the slit 

 between them enlarges to an oval ; and then a kind of 

 hand, with six fingers on each side, is protruded for a 

 moment. The " hand " is really the middle part of the 

 body, that part which in a crayfish bears the walking legs, 

 and the " fingers " are legs, which get longer and longer 

 backwards ; each leg ends in a pair of slender, jointed, 

 curled, bristle-bearing filaments. At the moment when 

 the valves are parted, the " hand " is extended and the 

 filaments uncurl ; then they bend all together towards the 

 mouth, and disappear beneath the valves. By the con- 

 tinual repetition of this action minute floating particles 

 are drawn into the gullet. 



The conical shell, with its six outer plates and its four 

 valves, is firmly attached to the body of the barnacle as 

 well as cemented to the rock. It is at first very small, 

 and enlarges steadily as the animal within grows. The 

 growth of its rigid shell presents no great difficulty to a 

 mollusk, for there is always a free margin to which addi- 

 tions can be made ; nor is the difficulty insuperable to 

 any free crustacean which can change its shell periodically. 

 But how does the barnacle manage ? The only free 

 margin to the outer shell is at the apex of the cone, and 

 addition of new substance here, while not materially 

 increasing the cavity, would quickly close the aperture 

 on which both nutrition and respiration depend. The 

 shell is never cast. A soft-bodied barnacle deprived of its 

 external armour might dry up, or be dislodged by breakers, 

 or devoured by other animals. Somewhat similar diffi- 

 culties beset an Echinus within its globular calcareous 

 shell. The box must grow and retain its original shape 

 without being parted with for a moment. An Echinus- 

 shell is composed of a mosaic of small pieces, which, tightly 

 as they are compacted together, have free borders. By 

 additions to these borders the shell increases steadily, 

 preserving all the time its original shape. The shell of 

 the rock-barnacle is fixed beneath, which complicates the 



