STRIJCTUEE AND CLASSIFICATION OF THE ARTHROPODA. 547 



What has become of the nerve-ganglion of the prostoniial 

 lobe of the Cha)topod in Peripatus is not clearly ascertained, 

 nor is its fate indicated by the study of the embryonic head 

 of other Arthropods so far. Probably it is fused with the 

 protocerebrum, and may also be concerned in the history of 

 the very peculiar paired eyes of Peripatus, which are like 

 those of Chastopods in structure — viz. vesicles with an intra- 



ant 



Fig. 3. — Diagram of the liead and adjacent region of Peripatus. 

 Monoprosthonierous. m, Moutli ; I, ccelom of t.lie first somite 

 which carries the antennae, and is in front of the mouth; II, ccelom 

 of the second somite, which carries the mandibles (lience deutero- 

 gnathous) ; III and IV, ccelom of the third and fourth somites ; 

 P.P., rudimentary frontal processes, perhaps representing the pro- 

 stoniial tentacles of Poljchseta; ^?i^, antenna or tactile tentacle; Md, 

 mandible; Oj?, oral i)apilla; P, protocerebruni or foremost cerebral 

 mass belonging to the first somite; D, deulerocerebrum, consisting 

 of ganglion cells belonging to the second or mandibular somite. 

 (After Goodrich.) 



vesicular lens, whereas the eyes of all other Arthropods have 

 essentially another structure, being "cups" of the epidermis, 

 in which a knob-like or rod-like thickening of the cuticle is 

 fitted as refractive medium. 



In Diplopoda (Julus, etc.) the results of embryological 

 study point to a composition of the front part of the head 

 exactly similar to that which we find in Onychophora. They 

 are deuterognathous. 



The Arachuida present the first stage of progress. Here 

 embryology shows that there are two prosthomeres (fig. 4), 



