9 6 



PROBLEMS OF RELATIVE GROWTH 



greatest, in the segments measured, in the fourth segment, 

 and decreasing distally. Without further measurements it is 

 impossible to state whether the high point in breadth-growth 

 predominance is really in the fourth segment, or further 

 basally ; but at least the orderly and graded relation between 

 the intensity of growth in the two planes of space is clearly 

 brought out. 



But a more complex, and perhaps more interesting problem 

 is afforded by the two chelae of markedly heterochelous Crus- 

 tacea of which the lobster (Homarus) is the most familiar, 

 and the pistol-crab (Alpheus) the most extreme example. In 

 all of these, the heavier (crusher) claw is broader and alto- 

 gether bulkier in build ; and the lighter (nipper) claw always 

 has a relatively and often an absolutely longer dactylus 

 (Huxley, unpublished : see figures in Przibram, 1930). It 

 would seem to mere inspection that the fundamental difference 

 between the two claws lies in the difference of their growth- 

 gradients in the three planes of space, the growth-coefficients 

 for breadth and depth being relatively as well as absolutely 

 much higher in the crusher, and the growth-centre as regards 

 length being more distal in the nipper. 



A few measurements have been made on the chelae of 

 lobster (Homarus) to check this (Huxley, unpublished). So 

 far as they go, they confirm the impression made by inspec- 

 tion. The (only very approximate) value of the growth- 

 coefficients are as follows (relative to carpus length as standard) : 



TABLE X 



Approximate Growth-coefficients (k), relative to carpus-length, 

 of the linear dimensions of the segments of the chelae of 

 the European Lobster 



It will be seen that the chief difference between the two 

 claws is in the distal region. In the crusher dactylus, the 



