2 



Insects and Disease 



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 in- 

 deed. 



Few of the higher animals live parasitic lives. 

 The nearest approach to a true parasite among 

 the vertebrates is the lamprey-eel (Fig. i) which 

 attaches itself to the body of a fish and sucks 

 the blood or eats the flesh. Among the Crus- 

 taceans, 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 ex- 

 pense 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. 



The worms furnish many well-known examples 

 of parasites, whole groups of them being especially 

 adapted to parasitic life. The tapeworms, com- 

 mon in many animals and often occurring in man, 

 the roundworms of which the trichina (Fig. 3) that 

 causes " measly" pork is a representative, are 



