STRUCTURE AND CLASSIFICATION OF THE AETHROPOUA. 561 



Each such complex of cells underlying the lenticle of a 

 compound eye is called an ^' ommatidium ; " the entire mass 

 of cells underlying a monomeniscous eye is an "ommatasum.'^ 

 The ommata3um_, as already stated, tends to segregate into 

 retinulfe which correspond potentially each to an ommatidium 

 of the compound eye. The ommatidium is from the first 

 segregate, and consists of few cells. The compound eye of 

 the king-crab (Limulus) is the only recognised instance of 

 ommatidia in their simplest state. Each can be readily com- 

 pared with the single-layered lateral eye of the scorpion. In 

 Crustacea and Hexapoda of all grades we find compound 

 eyes with the more complicated ommatidia described above- 

 We do not find them in any Arachuida. 



It is difficult, in the absence of more detailed knowledge as 

 to the eyes of Chilopoda and Diplopoda, to give full value to 

 these facts in tracing the affinities of the various classes of 

 Arthropods. But they seem to point to a community of 

 origin of Hexapods and Crustacea in regard to the com- 

 plicated ommatidia of the compound eye, and to a certain 

 isolation of the Arachnida, which are, however, traceable, so 

 far as the eyes are concerned, to a distant common origin 

 with Crustacea and Hexapoda through the very simple com- 

 pound eyes (monostichous, polymeniscoua) of Limulus. 



The Trachea. — In regard to tracheas the very natural 

 tendency of zoologists has been until lately to consider them 

 as having once developed and once only, and therefore to hold 

 that a group " Tracheata " should be recognised, including 

 all tracheate Arthropods. We are driven by the conclusions 

 arrived at as to the derivation of the Arachnida from branchi- 

 ate ancestors, independently of the other tracheate Arthropods 

 (see article Aeachnida), to formulate the conclusion that 

 tracheae have been independently developed in the Arachnidan 

 class. We are also, by the isolation of Peripatus and the 

 impossibility of tracing to it all other tracheate Aithropoda, 

 or of regarding it as a degenerate offset from some one of the 

 tracheate classes, forced to the conclusion that the trachete of 

 the Onychophora have been independently acquired. Having 



