58 BOARD OP AGRICULTURE. 



and become mature. Some species seem to have a much 

 more simple history during development. There are but few 

 species, however, of which the complete history is known. 

 The Trichina spiralis is, perhaps, better understood in all 

 stages of its life than any other species. Important discov- 

 eries remain to be made, without doubt, concerning other 

 species that infest our own bodies, as well as those of our 

 domestic animals. At present the complete histories of the 

 two commonest human parasites, the round-worm and pin- 

 worm, are still unknown, although they have been most care- 

 fully investigated and experimented upon by many able nat- 

 uralists. 



Of this order about 1,000 species have been already described, 

 but it is probable that a considerable number of these will event- 

 ually prove to be only the young of others. They particu- 

 larly abound in mammals, birds, and fishes, inhabiting the in- 

 testine, stomach, windpipe, lungs, kidneys, muscles, brain, and 

 various other parts of the body. Twelve species or more are 

 liable to infest human beings. 



ACARINA. 



In addition to the internal parasitic worms, there are certain 

 parasites which belong to other groups, Among these are 

 certain forms of acarina, or mites, some of which are, perhaps, 

 only accidentally parasitic, like the Acarus dysenterice mentioned 

 in our previous lecture. But the genus Peritastoma includes 

 species which are true parasites and have nearly the same 

 habits as some of the parasitic worms, which, indeed, they 

 closely resemble in general appearance. Formerly these par- 

 asites were regarded as worms by all authors, but they are 

 now generally referred to the Acarina, although they differ 

 greatly from the more typical forms. One of these species 

 is represented in Figure 84. 



Protozoa. 

 Several genera and species of infusoria have been found liv- 

 ing in the human body, or in the excreta, especially of pa- 

 tients attacked with certain contagious diseases. The effects 

 of these parasites and their relations to diseases are not well 

 understood. 



