568 E. RAY LANKESTER. 



The more primitive forms are anomomeristic ; tlie liiglier 

 forms nomomeristic, showing typical!}^ three groups or tag- 

 mata of six somites each. 



The genital apertures are placed on the first somite of the 

 second tagma or mesosoma. Their position is unknown in 

 the more primitive forms. The more primitive forms have 

 branchial respiratory processes developed on a ramus of each 

 of the post-oral appendages. In higher specialised forms 

 these branchial processes become first of all limited to five 

 segments of the mesosoma, then sunk beneath the surface as 

 pulmonary organs, and finally atrophied, their place being 

 taken by a well-developed tracheal system. 



A character of great diagnostic value in the more primitive 

 Arachnida is the tendency of the chitinous investment of the 

 tergal surface of the telson to unite during growth with that 

 of the free somites in front of it, so as to form a pygidial 

 shield or posterior carapace, often comprising as many as 

 fifteen somites (Trilobites, Limulus). 



A pair of central monomeniscous diplostichous eyes is often 

 present on the head. Lateral eyes also are often present, 

 which are monostichous with aggregated lenses (Limulus) or 

 with isolated lenses (Scorpio), or are diplostichous with 

 simple lens (Pedipalpi, Araneas, etc.). 



Class 3 (of the Euarthropoda). — Crustacea. 



Head tetartognathous and triprosthomerous, — that is to say, 

 with three prosthomeres : the first bearing typical eyes, the 

 second a pair of antenniform appendages (often biramose), 

 the third a pair of appendages, usually antenniform, some- 

 times claw-like. The ancestral stock was (as in the Arach- 

 nida) pantognathobasic, — that is to say, had a gnathobase or 

 jaw-process on the base of every post-oral appendage. 



Besides the first post-oral or mandibular pair, at least two 

 succeeding pairs of appeiulagcs are modified as jaws. These 

 have small and insignificant rami, or none at all, — a feature in 

 which the Arachnida differ from them. The appendages of 



