CHAPTER III 

 PHYLUM I. AETHROPODA 



While there are advantages in arranging a description of parasites 

 according to their location, as those of the skin, those of the intestines, 

 those of the liver, those of the circulation, etc., the fact that so many 

 in their life histories pass certain stages ia different organs and different 

 species of hosts makes such an arrangement somewhat confus.ed. It 

 seems better, therefore, to treat of the natural history of each parasite 

 in the parasite's order, essentially including such anatomical and zoolog- 

 ical migrations as may be involved, while at the same time considering 

 its pathogenic influences in these varying locations. 



Aside from the phytoparasites, which are not included in this work, 

 the parasites infesting man and domestic animals are distributed among 

 four grand divisions or phyla of the animal kingdom, which, in the order 

 of their zoological grade, are Protozoa, Platyhelminthes, Coelhelminthes, 

 and Arthropoda. The last named group contains most all of the external 

 parasites and is the first to be considered in the pages to follow. 



As a foundation for the scientific control of parasitism and for the 

 recognition of adaptations to its various forms, at least an elementary 

 knowledge of the structure and habits pecuhar to the phylum and its 

 subdivisions to which the parasite belongs is of essential importance. 

 Only the more prominent structural features upon which the separation 

 of the different groups and their subgroups is based will be given here. 

 For more detailed study the student is referred to an advanced text-book 

 in zoology. 



The phylum Arthropoda includes such animals as the crayfish, crabs, 

 lobsters, spiders, centipedes, and insects. The body is provided with a 

 hard or leathery external chitinous skeleton divided into a number of 

 segments demarcated externally by constrictions, each segment in the 

 adult, or a certain nimiber of the segments, bearing jointed appendages 

 (Fig. 1). There are usually two or more body regions distinguished by a 

 special modification of the constituant segments. In order that move- 

 ments may take place between the segments of both the body proper 

 and of the appendages, the cuticle at these points is thin and delicate 

 (Fig. 9), forming joints which are protected by an overlapping of the 

 heavier chitinous armor. 



All arthropods periodically molt, the process consisting of the break- 

 ing and casting off of the chitinous cuticle after it has loosened from the 



