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 : — 



I. 



11. 



III. 



IV. 



V. 



VI. 



VII. 



VIII. 



IX. 



X. 



XI. 



XII. 



XIII. 



XIV. 



XV. 



XVI. 



XVII. 



XVIII. 



XIX. 



XX. 



XXI. 



Segment 



Appendages. 

 Median eyes 

 Rostrum 

 Chelicerae 

 Pedipalpi 

 1st "Walking Legs 

 2nd Walking Legs 

 3rd Walking Legs 

 4th Walking Legs 

 Chilaria 



Genital operculum 

 1st Gill-books 

 2nd Gill-books 

 3rd Gill-books 

 4tli Gill-books 

 5th Gill-books 



No appendages 



Preoral 



Lateral to mouth 

 Postoral 



Prosoma 



- Mesosoma 



Metasoma 



We have followed Carpenter ^ 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 embryo Spider, Trocliosa, 

 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 ^ 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. Anz. xiv., 1891, pp. 164, 173. 

 ^ Zeitschr. wiss. Zool. lix., 1895, p. 351. 



