KINDS OF PARASITES / 13 



mittent parasites which visit and leave their hosts at intervals. 

 Some, as mosquitoes, visit their hosts only long enough to get a 

 meal, others, as certain lice, leave their hosts only for the purpose 

 of moulting and laying eggs, and still others, as the cattle tick, 

 Margaropus annulatus, never leave except to lay eggs. There 

 are parasites which pass only part of their life cycles as para- 

 sites; botflies, for instance, are parasitic only as larvae, hook- 

 worms only as adults. Some organisms live parasitically in two 

 or more different animals, often of widely different species, in 

 the course of their life histories. Such, for instance, are the 

 filarial worms and numerous protozoan parasites, which begin 

 life in a vertebrate animal, continue it in an insect, and finish it 

 in a vertebrate again; the tapeworms, which begin life in cer- 

 tain vertebrates and finish it in other individuals of the same or 

 different species; the flukes, which begin life as free-living em- 

 bryos, continue it through two or more asexual generations in 

 particular species of snails, become again free-living or else 

 parasitize second intermediate hosts such as crabs or fishes, and 

 finally gain admittance to their ultimate vertebrate hosts. 

 There are permanent parasites which live their whole lives, from 

 the time of hatching to death, in a single host, but in which the 

 eggs, or the corresponding cysts in the case of Protozoa, must be 

 transferred to a new host before a second generation can develop. 

 Such are many intestinal protozoans and round worms. The 

 final degree of parasitism is reached, perhaps, in those parasites 

 which live not only their whole lives, but generation after gener- 

 ation on a single host, becoming transferred from host to host 

 only by direct contact. Such are the scab mites and many 

 species of lice. There is every gradation among all the types of 

 parasites mentioned above, and a complete classification of para- 

 sites according to mode of life would contain almost as many types 

 as there are kinds of parasites. 



It is sometimes convenient to classify parasites according to 

 whether they are external or internal. External parasites, as 

 the name implies, are those which live on the surface of the body 

 of their hosts, sucking blood or feeding upon hair, feathers, skin 

 or secretions of the skin. Internal parasites live inside the body, 

 in the digestive tract or other cavities of the body, in the organs, 

 in the blood, in the tissues, or even within the cells. No sharp 

 line of demarcation can be drawn between external and internal 



