FIG. 98. A MALARIAL MOSQUITO (_A. maculipennis_) FEMALE 109
FIG. 99. DIAGRAM TO ILLUSTRATE THE LIFE-HISTORY OF THE MALARIAL PARASITE 110
FIG. 100. MALARIAL MOSQUITO (_A. maculipennis_) ON THE WALL 111
FIG. 101. MALARIAL MOSQUITO (_A. maculipennis_) STANDING ON A TABLE 111
FIG. 102. SALT-MARSH MOSQUITO (_O. lativittatus_) STANDING ON A TABLE 118
FIG. 103. ANOPHELES HANGING FROM THE CEILING 118
FIG. 104. YELLOW FEVER MOSQUITO (_Stegomyia calopus_) 122
FIG. 105. RAT-FLEA (_Læmopsylla cheopis_); MALE 152
FIG. 106. RAT-FLEA (_L. cheopis_); FEMALE 152
FIG. 107. HEAD OF RAT-FLEA SHOWING MOUTH-PARTS 153
FIG. 108. HUMAN-FLEA (_Pulex irritans_); MALE 153
FIG. 109. HUMAN-FLEA (_P. irritans_); FEMALE 156
FIG. 110. MOUSE-FLEA (_Ctenopsyllus musculi_); FEMALE 156
The dictionary says that a parasite is a living organism, either animal or plant, that lives in or on some other organism from which it derives its nourishment for a whole or part of its existence. This definition will serve as well as any, as it seems to include all the forms that might be classed as parasites. As a general thing, however, we are accustomed to think of a parasite as working more or less injury to its host, or perhaps we had better say that if it does not cause any irritation or ill effects its presence is not noted and we do not think of it at all.
As a matter of fact the number of parasitic organisms that are actually detrimental to the welfare of their hosts is comparatively small while the number of forms both large and small that lead parasitic lives in or on various hosts, usually doing no appreciable harm, often perhaps without the host being aware of their presence, is very great indeed.
Few of the higher animals live parasitic lives. The nearest approach to a true parasite among the vertebrates is the lamprey-eel (Fig. 1) which attaches itself to the body of a fish and sucks the blood or eats the flesh. Among the Crustaceans, the group that includes the lobsters and crabs, we find many examples of parasites, the most extraordinary of which is the curious crab known as _Sacculina_ (Fig. 2). In its early stages this creature is free-swimming and looks not unlike other young crabs. But it soon attaches itself to another crab and begins to live at the expense of its host. Then it commences to undergo remarkable changes and finally becomes a mere sac-like organ with a number of long slender root-like processes penetrating and taking nourishment from the body of the unfortunate crab-host.