Asvnuuelrv, Eff'cct of Parasites. "23 



of about lo inches long, ohtaineJ on 16th Septoniber, 1904. Wall- 

 Between that date and 31st July, 1909, when the lobster died, it ^Jf^"^, ., 



•' . Nos. 1-3. 



moulted seven times and grew to a length of 9 inches. 



Asymmetry. — A point on which information is often asked, 

 the unlikeness in size and shape of the great claws of the Lobster 

 and other Crustacea, is illustrated by specimens in Wall-case No. 1. 

 In the preparations of the male and female Lobster (Fig. 5), for 

 instance, or in the pair of claws from a verj^ large Lobster in the 

 lower part of the case, it will be seen that one of the claws is 

 more massive than the other and that the fingers are armed with 

 blunt knobs. It is, in fact, used for crushing the shells of animals 

 on which the Lobster may be feeding, and is known as the 

 " crushing-claw." The other is more lightly built, with sharp saw- 

 like edges to the tingers, and is known as the " cutting-claw." 

 There is no rule as to the side of the body on which either form 

 of claw is found, " right-handed " and " left-handed " specimens 

 being about equally common. In others of the higher Crustacea 

 the disparity in size of the two claws is much greater than in the 

 case of the Lobster. This is shown by the claws of the large 

 Tasmanian Crab (^Pseudocarciniis gigas), of which a pair is 

 exhibited in the lower part of Wall-case No. 1, and other examples 

 will be found in the table-cases. In some crabs the larger claw 

 is more or less constantly on the same side of the body ; that 

 is to say, right-handed (or, more rarely, left-handed) individuals 

 predominate. 



Occasionally, in the Lobster, specimens with similar claws 

 occur. Most commonly, in these, both claws are of the cutting 

 type, but, very rarely, specimens like that shown in the lower 

 part of Wall-case No. 1, are found in which both claws ai-e of the 

 crushing type. The mode of production of such abnormalities is 

 not fully understood, but it seems probable that in most cases it is 

 associated with the regeneration of limbs removed by accident or 

 thrown off after injury. 



MODIFICATIONS CAUSED BY PAEASITES. 



A series of specimens, exhibited in Wall-case No. 2, illustrate the 

 changes of structure produced in certain crabs which are infested 

 by the degenerate Crustacean parasite Sacciilina. It is a curious 

 and significant fact that these changes affect almost exclusively 



