Crustacea. 835 



any partiality for particular theories, or preconceived opinions ; 

 though they may be, and frequently are, erroneous, from considering 

 isolated facts as general laws. The foregoing observations of Mr. 

 E. Chirgvvin are valuable, therefore, as the experience of one who is 

 practically acquainted with all the fisheries of Mount's Bay, and 

 without any theoretical predilections. Yet there are a few points 

 that require a little explanation. 



The experience on the exuviation of the craw-fish, given above, 

 only proves that the shedding of the external calcareous covering 

 takes place during the months of July and August, and not that it is 

 confined exclusively to these periods. For the craw-fish, like other 

 crustaceans, undergoes this process very irregularly, and at different 

 periods of the year. When the young first escapes from the egg, the 

 first shedding is effected within three days, and sometimes within the 

 day ; the second shedding does not occur for three weeks or a month, 

 and then at irregular intervals, increasing in length as the creature 

 increases in age. The young being born in a form entirely unlike 

 the adult, the first and second changes are not merely a shedding of 

 the shell, as it afterwards becomes, for the purposes of growth, but 

 also the means of undergoing the metamorphosis necessary to its 

 taking on the adult form. But, even after the creatures have under- 

 gone those changes, which may be deemed metamorphic, they do not 

 immediately assume the adult form, though they have taken on the 

 type of the adult development. Thus, the first shedding occurs so 

 soon after birth, that the second state is the one generally first seen ; 

 this lasts for a few weeks, and another change and exuviation occurs, 

 and again, in about a month, another shedding and change of form 

 occurs, which is the last of the metamorphoses. At this stage, the 

 permanent form makes its appearances. But the different species are 

 so much alike in this young state, that it is almost impossible to dis- 

 criminate between them. But at each shedding the form alters, and 

 the specific characters become gradually developed. For the crus- 

 taceans appear to be formed on a common type, and during growth 

 they become removed from this by the development of some parts, 

 and the obliteration of others, thus assuming their generic and spe- 

 cific characters. Thus, in their early changes, the exuviation is fre- 

 quent, and even when half grown, they shed their shells several times 

 during the year, but the full-grown animal very rarely does it, that 

 is, only after long intervals ; so that the exuviation in the above 

 cases, during July and August, are only proofs that it occurs during 

 these months, and not that it is confined to them. The mode in which 



