274 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



animal within the molluscan shell which it habitually 

 inhabits. 



"When the limbs are examined, the same rule will 

 be found to hold good, viz. : that the movable part is 

 pulled into the fixed part. A modification of this is 

 well illustrated in the evolution of the large chelae. In 

 some forms, take for example Ibacus, the first pair, 

 and in fact all of the thoracic limbs end in a sharp- 

 pointed segment, there being not the slightest sugges- 

 tion of a chela. In Crangon, on the other hand, the 



terminal segment is pulled 

 against the broad face of the 

 penultimate one thus making 

 a shift for a chela. In the 

 Stomatopoda this step has 

 been developed, for the last 

 segment can be drawn against 

 the whole length of the pen- 

 ultimate one (which is some- 

 times grooved to protect the 

 points of the spines of the 

 latter) and forms with it a 

 very effective grasping or- 

 gan. The continual use of 

 the terminal segment, the 

 increase of the muscular power will tend to draw this 

 terminal segment backward (into) on the penultimate 

 which enlarges with the increase of bulk of muscle, so 

 that a well-developed chela, as in the lobster is found, 

 where the ultimate segment is pulled backward to 

 about the middle of the penultimate segment." 



Fig. 65. — Diagrams of, <j, hand 

 of a form of Crangon ; /', hand of a 

 form of Astacus. 



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