172 



Part HI. — Ttuentij-first Annual Report 



tions, and this variation shows how much the growth of the crab may- 

 be affected by its environment. It is, moreover, to be noted that in the 

 case of No. 20 the rate of increase at each moult was, to begin with, 

 very small, and so it happened that No. 19 gained a larger size in four 

 months than Waddington's (No. 20) reached in five months. 



The size at the end of the first twelve months is obtained from 

 crabs Nos. 7, 10, 11, and 21, the ages of which are known exactly. 

 They measured respectively 22 : 12-75 : 8 : 11*6; these figures give an 

 average of 13-5 mm. In the four sizes given we have probably the 

 upper and lower limits of size after one year's growth, viz. 22 and 

 8 mm. The crab will not, it is believed, grow to a greater size than 

 22 mm. in the first year, and no crab is probably less than 8 mm. when 

 it is a year old. 



Crabs Nos. 12, 13, 14, 15, and 23 are about one year old, while Nos. 

 9, 17, and 22 are in their second year. Crab No. 23 appears to have 

 been in the Megalops stage later in the year than August, probably in 

 September. 



From the size of the crab when captured, we can tell, by considering 

 the month when it was found, whether the crab is or is not a year 

 old. The size alone without the date is in the case of the small crabs 

 of little value. For example, a crab that measured 13 mm. in May 

 would probably be over a year old, whereas one of that size captured in 

 September would be six or seven months old. 



The crabs then have been arranged in Table III. according to the 

 age which their size and date of capture seem to indicate. The crabs 

 of known age are taken as a standard of comparison. 



Column V. contains one series about which there might be some 

 doubt as to its being properly included there. It is No. 22. In August 

 when captured it measured 14*5 mm. — that is, very little greater than 

 No. 2, which was regarded in August as being only a few months old. 

 The former has, however, been placed in the second year, mainly on 

 account of the large size which it reached (56 mm.) at the end of one 

 year in captivity. 



During the second year the casts were still numerous in certain 

 cases ; that was especially so in the case of crabs which commenced the 

 second year at a small size. The more frequent casting of the latter 

 then enabled them to overtake the larger specimens. And so at the 

 end of the second year the differences in size between the crabs tend 

 to get eliminated or altered in a difierent sense, the smaller of the 

 first year becoming the larger at the end of the second year. This 

 is well shown by Nos. 10 and 11. No. 11 commenced the second year 

 at a size of 8 mm., while No. 10 measured 12'75. The former moulted 

 eight times to the latter's five, and at the end of the second year it 

 measured 53, while No. 10 had reached only 39 mm. The difierent rate 

 of growth in the second year may have been in some measure due to 

 sex; No. 11 was a male and No. 10 a female. A good example is, 

 however, shown by Nos. 7 and 11. They are both males. At the 

 beginning of their second year they measured respectively 22 and 8 mm. : 

 during the second year No. 7 moulted four times, No. 1 1 eight times, 

 and at the end of the period they had reached 52 and 53 mm. in 

 breadth. The number of moults during the first two years was then 

 15 to 17. The sizes of the crabs which are known to be two years 

 old are— Male (Nos. 7, 11), 52 and 53 mm. : female (Nos. 3, 10), 48*5 : 

 39 : and the arrangement of the Table gives the following in cases 

 where the crab was not reared from a very small size : — Male, 56 : 45 (?) ; 

 female : 35 ; sex not noted, 33 : 46*5 : 62. The average sizes for 

 those of definitely known age are, at the end of two years — Male, 52*5 ; 



