THE ARTHROGASTRA. 323 



sal joints of the first two (v', Vl') lie behind the mouth, the 

 posterior and inferior boundary of which they form, and are 

 directed forward. The basal joints of the last two (vii', vm'), 

 on the other hand, directed inward, are firmly united together, 

 and are altogether excluded from the mouth. 



Thus the mouth is situated between the labrum in front, 

 the bases of the pedipalpi and those of the first two pairs of 

 ambulatory limbs, at the sides and behind ; just as, in Limulns, 

 the mouth lies between the labrum and the basal joints of the 

 third, fourth, and fifth limbs, which answer to the mandibles 

 and first and second maxillag of the higher Crustacea. If this 

 comparison is just, there is one pair of prae-oral appendages, 

 which exist in Limulus, w r anting in the Scorpion ; and the 

 difference between the two may be represented thus : 



Limulus. Antennule. Antenna. Mandible. Maxilla 1. Maxilla 2. 



Scorpio. Chelicera. Pedipalpus. Leg 1. Leg 2. 



Again, if the eye-bearing part of the head may be regarded 

 as a somite, then the body of the Scorpion, like that of a mala- 

 costracous crustacean, will consist of twenty somites and a 

 telson. We may regard the six posterior somites (xv-xx) as 

 the homologues of those which constitute the abdomen in the 

 crustacean ; while the eight middle somites (yii-xiv) will 



Fig. 86.— Scorpio.— Vertical section of the cephalo-thorax: At, chelicera ; lb, labrum; 

 M, month; a, pharyngeal sac; N, N', supra and infra-oesophageal ganglia; b, 

 oesophagus; d, opening of the salivary ducts; e, intestine ; H, heart. 



answer to those which enter into the thorax of the latter ; 

 and the head will resemble that of an Edriophthalmian with 



