THE ARTHROGASTRA. 



323 



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

 posterior and inferior boundary of which they form, and are 

 directed forward. The basal joints of the last two (vn', Tin'), 

 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 Limiilus, 

 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 maxillae of the higher Crustacea. If this 

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

 which exist in Limuhis, wanting in the Scorpion ; and the 

 difi"erence between the two maj' be represented thus : 



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



ScoT'pio, 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 w^iich constitute the abdomen in the 

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



j>r 2^ 



Fig. 86.— 5^rorp?o.— Vertical f?ection of thp cephalo-thorax: At, chelicera ; », labmm, 

 Jf, month; a. pharyngeal sac; iV, K', supra and infra-oesophageal gangJia ; 0, 

 cesophaiTue ; <?, opening of the salivary ducts ; c, intestine ; E, heart. 



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

 and the head will resemble that of an Edriophthalmian with 



