574 E. RAY LANK ESTER. 



Grade oy (below the Artlii-opoda). — Agnatha Aprosthomp^ra. 



Without parapodial jaws; without the addition of originally 

 post-oral somites to the prasoral region, which is a simple 

 prostomial lobe of the first somite ; the first somite is per- 

 forated b}^ the mouth, and its parapodia are not modified as 

 jaws. = Ch^topoda. 



Grade 1 (of the Arthropoda). — Monognatha Monopros- 



thomera. 

 With a single pair of parapodial jaws carried by the somite 

 which is perforated by the mouth ; this is not the first somite, 

 but the second. The first somite has become a prosthomere, 

 and carries a pair of extensile anteunee. = Onychophora 

 (Peripatus, etc.). 



Grade 2 (of the Arthropoda). — Dignatha Monoprosthomera. 

 The third somite, as well as the second, develops a pair of 

 parapodial jaws; the first somite is a prostliomere carrying 

 jointed antennse. = Diplopoda. 



Grade 3 (of the Arthropoda). — Pantognatha Diprosthomera. 



A gnathobase is developed (in the primitive stock) on 

 every pair of post-oral appendages ; two prosthomeres pre- 

 sent, the second somite, as well as the first, having passed in 

 front of the mouth, but only the second has appendages. 

 = Arachnida. 



Grade 4 (of the Arthropoda). — Pantognatha Triprosthomera. 



The original stock, like that of the last grade, has a gnatho- 

 base on every post-oral appendage, but three prosthomeres 

 are now present, in consequence of the movement of the oral 

 aperture from the third to the fourth somite. The lateral eyes 

 are polymeniscons, with specialised vitrelhe and retimihie of a 

 definite type peculiar to this grade. = Crustacea, Chilo- 

 PODA, Hexapoda. 



