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Part III. — Eighteenth Annual Report 



with the rate of growth of the plaice has been applied to a considerable 

 number of crab measurements. The method consists in regarding the 

 peaks which appear in the curve plotted from the measurements of a 

 large number of individuals as marking the modal size, at intervals of 

 one year. If, then, in a curve made from the measurements of crabs, 

 distinct peaks were found, we should probably be entitled to look upon 

 such intervals as approximately representing the average increase in the 

 size of the crab following casting. The increase in size varies with the 

 size oi the crab. All the crabs have been measured across the greatest 

 breadth of the carapace. Crabs were got from two sources — (1) 2336 

 crabs (1153 males and 1183 females) collected between tide-marks on 

 the beach ; (2) 3029 crabs (1507 males and 1522 females) caught in the 

 creels of the fishermen. The beach crabs * were measured in centimetres 

 (Table VIII), and the curves plotted from the measurements are to be 

 found on Plate II. The curve for the males is black ; that for the females 

 red. It is at once seen that the curves of the two sexes, although each 

 very irregular, follow one another closely. The irregularity of the 

 curves indicates great variation in growth. Between the sizes 1 Tcm. and 

 7cm., male and female crabs were found at every half -millimetre, except 

 in the following cases. Females were not obtained at the following- 

 sizes — 1-85, 5-45, 6 05, 6 25cm., and males were not observed at 5-55, 

 5*95, 6*05, and 6 , 65cm. Since the crab does not increase in size 

 gradually, but by leaps, the irregularity in growth is by the preceding 

 facts rendered very apparent. Until the rate of increase in size at each 

 moult is determined, it is not possible to interpret curves such as we have 

 in the present case. The curves of the creel crabs (Plate III.) also show 

 great variation in growth. These crabs were measured in eighths of an 

 inch ; their curves are shown as dotted black and red lines. In order 

 to admit of a comparison between the beach and creel crabs, the curves 

 of the former have been converted to a one-eighth-inch basis, and are 

 shown alongside the curves of the adult crabs. On reference to the 

 former curves it is seen that the great majority of the beach crabs are 

 between gin. and 2^in. in breadth. There are very few crabs below 

 gin. We, however, know that they must exist in even greater numbers 

 than the larger crabs, and the reason for their non-appearance in the 

 lists is probably simply their absence from that region. It is no doubt 

 the case that these small crabs are more likely to be overlooked during 

 their collection on the beach, but a crab of even half-an-inch in breadth 

 is by no means inconspicuous. I am of the opinion that the majority 

 of the crabs in the stages less than gin. do not make the beach their 

 habitat. The beach crabs were almost without exception found 

 beneath stones which were left dry by the ebb ; they were usually 

 buried as deep as the edge of the carapace in a mixture of mud and 

 sand. Very rarely were any found where the bottom consisted of 

 rough gravel, into which they could not burrow. A look-out was kept 

 specially for the very small crabs, and the non-success in finding them 

 is no doubt due to the fact that they do not come within tide-marks. 

 While we may occasionally get there crabs about Sin. across, still from 

 gin. to 2|in. may be taken as the description of the beach group. With 

 regard to the creel crabs, the measurements of which are detailed in 

 Table IX., it is seen from the curves that very few measure less than 

 4in. across. The dates of the ten catches which were measured are 

 —December 20, 1897; January 28, February 25, I larch 31, May 3, 

 May 27, November 5, 1898 ; June 3, July 12, August 4, 1899. Of "the 

 3029 crabs, 2887, that is 95 per cent., are above 4in. in breadth. The 

 smallest crabs in the catches are two measuring respectively 2|in. and 3in. 

 * For the dates of collection see Table IV., p. 101. 



