204 



THE EYE IN EVOLUTION 



ARTHROPODA 



ARTHROPODS embrace more than three-quarters of the known 

 species of animals, and in view of their number and variety and the 

 diversity of their habits, it is not surprising that an extraordinary 

 variation occurs in their visual organs, while the intense and purposive 

 activity of many of them accounts for the complexity and efficiency of 

 their eyes. Arthropods are characterized by their bilateral symmetry, 

 their cegmental structure with jointed appendages, their chitinous 

 cuticle, a distinct head where the sense organs are aggregated, and a 

 nervous system consisting of a dorsal brain-ganglion connected by a 

 ring round the gullet with a double chain of ventral ganglia. From 

 the ocular point of view, although simple eyes often of quite a rudi- 

 mentary type are frequent, and may indeed be the sole visual organs 

 (as in Arachnids), the characteristic feature of the phylum is the 

 presence of compound eyes of elaborate structure and frequently with 

 highly developed functional abilities. 



The Arthropods may conveniently be divided into five sub-phyla : 



(1) the primitive worm-like onychophora, unique in having a soft, velvety 



skin, and provided with a separate head, one pair of antennae 



and 20 legs all alike ; 



(2) the CRUSTACEANS, comprising some 25,000 species, 

 with the head fused with the thorax, 2 pairs of antennae and 

 at least 5 dissimilar pairs of legs ; 



(3) the MYRiAPODS (centipedes, millipedes, etc.), of some 

 2,000 species, with a distinct head, one pair of antennae and 

 numerous legs all alike ; 



(4) the ARACHNIDS, of some 36,000 species, having 2 

 body -segments with a fused cephalothorax, without antennae 

 or wings, and 4 pairs of legs ; 



(5) the INSECTS, of which more than 577,000 species 

 have now been scientifically described and probably several 

 times as many await investigation, with a body divided 

 sharply into 3 segments, head, thorax and abdomen, bear- 

 ing one pair of antennae, 3 pairs of legs and (usually) one or 

 two pairs of wings in the adult. ^ 



Fig. 197. — The 

 Onychophore, 

 Peripatus. 



Note the two 

 simple eyes on top 

 of the head at the 

 base of the anten- 

 nae (Thomson's 

 Zoologij, James 

 Ritchie ; Oxford 

 Univer.^ity Press). 



ONYCHOPHORA 



The most primitive class of Arthropods, the 

 ONYCHOPHORA {Peripatus and its allies), inhabiting 

 the forests of the Southern Hemisphere, represent an 

 archaic type, differing widely from other members of 

 the phylum. Seeking out damj) places under leaves 



^ In oue acre of farm-land in England it has been estimated that there are from 

 700,000,or^:; to 800,000,000 Insects and as many Arachnids. They would usurp 

 Man's do. .11 it ion of the earth were their numbers not kept in check by voracious 

 predators ^ parasites of their own kind. 



