The class Sporozoa is divided into five small groups or orders. Nearly all of these contain forms that are of more or less importance, but the ones that live in the blood-cells (Hæmosporidiida) are of the most interest to us because the parasites that cause the malarial fevers and various other diseases belong here. These are dependent on two hosts for their existence, the sexual generation usually occuring in an insect or other invertebrate and the asexual generation in some vertebrate.
CHAPTER III
TICKS AND MITES
he other group or Phylum of animals with which we will be particularly concerned is known as the Arthropoda, which means "jointed-feet" and includes the crayfish, crabs, spiders, mites, ticks and insects. Of these only the last three are of interest to us now. It is customary to speak of spiders, mites and ticks as insects, but as they have four pairs of legs, instead of three pairs, in the adult stage, and as their bodies are not divided into three distinct regions as in the insects, they are placed in a different class.
GENERAL CHARACTERS OF TICKS
The ticks are all comparatively large, that is, they are all large enough to be seen with the unaided eye even in their younger stages and some grow to be half an inch long. When filled with blood the tough leathery skin becomes much distended often making the creature look more like a large seed than anything else ([Fig. 14]). This resemblance is responsible for some of the popular names, such as "castor-bean tick," etc.
The legs of most species are comparatively short, and the head is small so that they are often hardly noticeable when the body is distended. The sucking beak which is thrust into the host when the tick is feeding is furnished with many strong recurved teeth which hold on so firmly that when one attempts to pull the tick away the head is often torn from the body and left in the skin. Unless care is taken to remove this, serious sores often result.