1024 



CRUSTACEA. 



The following table exhibits the relations of the segments to the 

 normal annuli, showing those that are coalesced in the different 

 varieties of cephalothorax. The normal annuli are each named by 

 the pair of organs it bears, and we add also the normal number of the 

 annulus, the first antennae pertaining to the second normal annulus. 



Cephalo- 

 thorax 

 7-jointed. 



II. 1st Antennae, 



III. 2d Antennae, 



IV. Mandibles, 

 Y. Maxillae, 



VI. Maxillipeds, 



VII. 1st Feet, 



VIII. 2d Feet (1st Nat.), 



IX. 3d Feet (2d Nat.), 



X. 4th Feet (3d Nat.), 



XI. 5th Feet (4th Nat.), 



XII. 6th Feet (5th Nat .*) 



h. 



I. 



II. 



Cephalothorax 

 6-jointed. 

 A. B. 



I. u. 



III. 



IV. 



d. 



e. 

 | VII. 



II. 



>II 



III. 



iv. Jul 

 }y. }iv. 

 }vi. }v. 



} 



Cephalothorax 

 5-jointed. 

 A. B. C. 



Cephalothorax 

 4-jointed. 



I. 



0. 



VI. 



VII. 



I. 



fl- 



)'■■ 



JUL | II. JUL 



jiv. }in. }iv. 



jv. }iv. jv. 



o. jv. 0. 



}n. 



III. 



IV. 



The last column represents the most common form, numbering but 

 four segments, and the first the seven-jointed cephalothorax. The 

 number seven, it is seen, is reduced to six in two ways : — 



1. By the last segment becoming obsolete. 



2. By the obsolescence of the articulation b (fig. 4). 



There is a farther reduction to five segments in three ways :— 



1. By the obsolescence of the articulation b and the last segment 

 becoming obsolete. 



2. By the obsolescence of the articulations a, b (fig. 3). 



* Usually either obsolete or adapted for prehension. 



