ARTHROPODA 15 



be expected, as in the annelid worms. In the Arthropoda, however, 

 there are differences due to fusion of the segments, in which case there 

 is also fusion of their ganglia. Such fusion is usually accompanied by 

 more or less shortening of the body, an example of which is afforded by 

 the spiders and crabs where the whole ventral chain unites in a single 

 ganglionic mass. From the most anterior of the ventral gangha there 

 spring two nerve cords which pass on either side of the esophagus to 

 unite above it with the paired cerebral ganglion or brain, l3ang in the 

 head. This ganglion remains distinct, its dorsal position preventing its 

 fusion with ganglia of the ventral chain. 



Of the sense organs the most highly developed are the eyes, which are 

 compound (Fig. 6), or appear as simple ocelli. In many arthropods 

 there are both of these forms, while others are provided only with ocelli, 

 and in some arthropods eyes are absent. In the compound eyes the 

 cuticle covering them is divided into hexagonal facets, the number of 

 which varies with different groups from a dozen to two thousand or 

 more, each of these areas corresponding to a small chitinous lens. The 

 compound eyes are two in number, while the number of ocelli varies. 

 The latter are very small and have their highest development in the 

 spiders. 



With rare exceptions the sexes are separate, and reproduction is 

 generally by fertilized eggs, though parthenogenesis occurs, in some 

 cases having a certain relationship to the life history. Usually the sexes 

 can be readily distinguished by the difference in size and by various 

 modifications of the appendages. 



Of the subgroups of the phylum Arthropoda only those containing 

 parasitic species of medical interest will be considered in this work. 

 These are included in the two classes Insecta and Arachnida, which, 

 with scarcely an exception, contain all of the external parasites. It is 

 not correct, however, to say that the arthropodal parasites are exclusively 

 external, as certain insects and arachnids pass a phase of their develop- 

 ment within the bodies of their hosts. 



Class I. Insecta 



Arthropoda (p. 13). — ^In number of species the insects constitute 

 the largest of all animal groups. The body is essentially segmented, and 

 is divided into three regions, — ^head, thorax, and abdomen, which are 

 distinctly marked off from each other (Fig. 1). 



The head is usually freely movable at its jimction with the thorax, 

 and typically bears on each side a compound eye (Figs. 1 and 7), be- 

 tween which there may be a varying number of simple ocelli. 



Arising from the head are a pair of antennae which consist of seg- 

 ments varying in size, shape, and number according to species. 



