THE CRAYFISH 



151 



grows out from the stump of the liml), develops into the shape 



of a small limb, and grows larger, with each successive molt, 



until the normal size is reached. 



The antennae, too, are much exposed 



to injury, but with them autotomy 



is not practised. They begin to 



grow out at the place of injury, and 



at least one molt is necessary for 



their complete restoration. 



Abnormalities in the claws of the 

 lobster are not uncommon. The 

 tlmmb-like protuljcrances of the 

 next to the last of the joints of the 

 great claw is sometimes bifid, or 

 carries a large wart. The finger 

 which opposes the thumb is also 

 sometimes forked (Fig. 1 52) . Many 

 of these abnormalities are probably 

 due to injury of the claw ; but others 

 cannot be explained in this way. 

 For instance, cases have been ob- 

 served of lobsters having crushing 

 claws of equal size on the two sides 

 of the body. Similar abnormalities 

 are found in other arthropods. 

 They are always instructive be- 

 cause, being natural phenomena, 

 they must have a cause, and this lies in the altered conditions 

 of development. 



Physiological Division of Labor. — The difference between a 

 " highly developed " animal and a lowly organized one is not 



Fig. 152. — Two abnormal claws. 

 Upper figure shows a double 

 outgrowth to the finger. In 

 lower figure there i.s an e.xtra 

 finger. From Hernck. 



