SEGMENTATION 



263 



The body of a King-crab can be analysed into twenty-one 

 segments, but these do not all persist to the adult stage. They are 

 grouped together in higher aggregates, or " tagmata " as Lankester 

 calls them, and most of the segments bear paired appendages. 



The segments with their respective appendages and their 

 grouping into tagmata are shown in the following scheme : 



^ Prosoma 



Metasoma 



We have followed Carpenter l in inserting the rostral segment. 

 This corresponds with the segment that in Insects and Crustacea 

 bears the antennae or first antennae respectively, the absence of 

 these organs being one of the characteristic but negative features 

 of all Arachnids. The evidence for the existence of this evanes- 

 cent segment rests partly upon the observation of von Jawor- 

 owski ~ on the vestigial feelers in an embrye Spider, Trochosa, 

 and perhaps more securely on the fact that, according to Korschelt 

 and Heider, there is a distinct neuromere for this segment, 

 between the proto-cerebral neuromere which supplies the eyes 

 and the trito-cerebral neuromere which supplies the chelicerae. 

 According to Brauer 3 the chelicerae of Scorpions are also supplied 

 by the third neuromere. 



The bases of the chelicerae do not limit the mouth, but 

 between and behind them is a ridge or tubercle which has the 



1 Quart. J. Micr. Sci. xlix., 1906, p. 469. 



2 Zool. An~. xiv., 1891, pp. 164, 173. 

 * Zeitschr. v:iss. ZooL lix., 1895, p. 351. 



