324 PHYLUM ARTHROPOD A. 



paired mass in front of it to form the supraoesophageal ganglion, 

 the pedipalpi being innervated from the suboesophageal mass.* 



The absence of resemblance between arachnid appendages 

 and those of the other groups of Arthropods renders recogni- 

 tion of the homology of the segments exceedingly dubious. 

 We are reduced to the order of the segments, reckoning as 'first 

 the pre-cheliceral in Arachnida and the preantennal in Trachea ta 

 and Crustacea ; though when it is remembered how slender the 

 evidence is of the existence of these segments, the insecurity of 

 the proceeding is apparent. 



It remains to consider the segmentation of Peripatus which 

 is perhaps better marked than that of any other Arthropod. 

 As shown by Sedgwick the coelomic sacks are formed in pairs, 

 and the members of the anterior pair, though postoral at their 

 origin move forward and come to lie in apposition in front of 

 the mouth, forming the first segment. The antennae of the 

 adult are the appendages of the first, the mandibles of the 

 second, the oral papillae of the third segment, and in relation 

 with each of these appendages, as with their successors, a pair of 

 nephridia, opening into the corresponding coelomic sacks is 

 formed. 



At no stage of development is the central nervous system of 

 Peripatus divided up into distinct ganglia, the lateral thickenings 

 of the ectoderm on either side of the ventral median line " are 

 from their origin continuous from somite to somite," and they 

 are continuous with one another in front of the mouth. The 

 brain of the adult consists of two simple swellings produced 

 behind into posterior lobes. They are in apposition with one 

 another, and there are slight swellings of the lateral cords op- 

 posite each of the postoral appendages. The antennary and optic 

 nerves spring from the anterior part of the brain, the nerves to 

 the jaws from the ventral cords as they leave the brain, those 

 to the oral papillae from the ventral cords posterior to the 

 oesophagus. 



The relation between the segments of the head (in the case 

 of the Arachnida of the cephalothorax) in the main groups of 

 the Arthropoda, which the evidence at our disposal appears to 

 indicate as probable, are set forth in the following table, though 



* It is stated however that in the Phalangidae and the Acarina (Gama- 

 sidae) the innervation of the chelicerae is suboesophageal. 



