INTRODUCTION 11 



The compound eye consists essentially of a group of tiny 

 ocelli of a peculiar kind, known as ommatidia {6/j.fji.aTiSiov = ^i 

 diminutive eye), each of which is structurally complete in 

 itself and morphologically independent, though functionally 

 they all act together as a single organ. The number of 

 ommatidia in a compound eye varies ; it is said in the case of 

 the house-fly to be 4000, and in the case of a dragon-fly to be 

 20,000. Compound eyes are present by no means in all 

 Arthropoda, but are not found outside this phylum. 



The Arthropoda are usually divided into five great Classes 

 according to the style of their segmentation, the number 

 and general disposition of their appendages, and the nature 

 of their breathing-apparatus, namely : Crustacea, Proto- 

 tracheata, Myriapoda, Insecta, and Arachnida. 



The Crustacea are, for the most part, aquatic, and breathe 

 by gills. The segments of the body are usually arranged in 

 three regions — head, thorax, and abdomen — with the separa- 

 tion between the head and thorax quite obscure. There are 

 two pairs of antennae, and of the numerous other appendages 

 some are biramous. 



The Prototracheata (or Onychophord) are terrestrial Arthro- 

 pods breathing by tracheae (p. 27), which are diff"use. The 

 individual segments of the body are superficially obscure, the 

 body being soft and worm-like. The head is not demarcated 

 from the body, and its appendages are a pair of antennae, a 

 single pair of jaws, and a pair of oral papillae. The append- 

 ages behind the head are numerous and all alike stumpy. 

 This class includes one genus, Peripatus, the species of which 

 are found in parts of the Ethiopian, Oriental, Australian, and 

 Neotropical regions. They have some resemblance to a slug, 

 and also some resemblance to a caterpillar ; they live in 

 rotting timber or under stones, and avoid the light, so that 

 they are rarely seen. Though they are of the greatest interest 

 zoologically, as being intermediate between Arthropods and 

 Annelids, they are of no concern from a medical standpoint 

 and so cannot be noticed further here. 



The Myriapoda are terrestrial Arthropods breathing by 

 tracheae, which have a definite arrangement. The segments 

 and appendages behind the head are for the most part alike. 

 There is one pair of antennae. 



