THE FORM OF GROWTH-GRADIENTS 91 



relative to the body be moderate or high. The growth- 

 gradient of the female is not quite so steep as that of the 

 male, a fact also brought out by Tazelaar on P. carcinus 

 (p. 92) ; but the male and female chela-gradients are much more 

 like each other than they are to the gradients of any of the 

 pereiopods. 



Still further proof of the radical difference of the growth- 

 gradients leading to pereiopod and to large chela is afforded by 

 the male hermit-crab Eupagurus (Bush, 1930 ; Bush and 

 Huxley, 1930). Here the right chela during early life is not 

 much enlarged, and its growth-coefficient is no greater than 

 that of the pereiopods ; only later does it begin the marked 



1,6 



1,5 



1,4 



1*. 



1,0 



I TTL C 



dJstaJ - 





Fig. 50. — Change in form of growth-gradient with increase of growth-rate in 

 large (right) male claw of the hermit-crab, Eupagurus. 



i, ischium ; m, merus ; c, carpus ; p. propus ; d, dactylus. A — A, juvenile phase ; the growth- 

 gradient resembles that of a pereiopod. B — B, phase of heterogony of right chela ; the main growth- 

 centre shifts distally. 



heterogony which provides its definitive enlargement. And 

 during the earlier period its growth-gradient is similar to that 

 of a pereiopod, with centre in the merus ; while so soon as 

 the final heterogony becomes marked, the main growth-centre 

 shifts to the propus (Fig. 50). 



Tazelaar (unpublished) has also collected facts bearing on 

 this subject. In Palaemon carcinus, there is a change in the 

 growth-coefficient of the chela in both sexes at about 4-5 cm. 

 carapace length. In the female, before this, the chela has 

 been growing less rapidly than the neighbouring pereiopods ; 

 after this it exhibits a considerable heterogony. During the 

 first of these phases its growth-gradient is almost flat, like 

 those of the pereiopods, but with a slight growth-centre in 



