THE ANIMAL PARASITES OF MAN. 



MAN is one of those organisms in which a whole host of parasites 

 find conditions suitable for their existence : Protozoa, Plathelminthes, 

 Nematodes, Acanthocephala, Hirudinea, and a large number of Arthropoda, 

 Arachnida, as well as Insects, all include members which are parasites of man. 

 These creatures either live on the external surface of the body or within 

 the intestine and its appendages ; yet other organs and systems are not 

 quite free from foreign guests we are acquainted with parasites in the 

 bones, in the circulatory system, in the brain, in the muscles, in the excre- 

 tory and genital organs, and even in the organs of sense. 



Although it is possible in a x way, and perhaps might be advantageous, 

 to arrange and describe the parasites of man according to the situations 

 in which they are found (parasites of the skin, intestinal parasites, &c.), their 

 description in the various stages of development would doubtless be disturbed 

 when, as is generally the case, the different stages are passed in different 

 organs, and a work which treats more fully of the natural history of the 

 parasites than of the local disorders they give rise to would suffer thereby. 



I therefore prefer to describe the parasites of man in their systematic 

 order, and to mention their different anatomical stations in man in describing 

 each species. 



A. PROTOZOA (Primitive Creatures). 



All those animal organisms which throughout their entire life never 

 rise above the unicellular stage, or merely form simple, loose colonies 

 of similar unicellular creatures, are grouped under the term Protozoa, as 

 the simplest types of animal life All the vital functions of these, the lowest 

 forms of animals, are carried out by their bodily substance, the SARCODE 

 (protoplasm); often particular parts possess particular functions; but the 

 limits of a cell are never over-stepped thereby ; these special parts of the 

 cells are called "cell-organs" ; recently they have been termed "organelles." 



The living protoplasm has the appearance of a finely granular, viscid 

 substance which, as a rule, when not surrounded by dense investing mem- 

 branes or skeletons, exhibits a distinct kind of movement, which has been 

 termed AMOEBOID ; according to the species, processes of various forms and 

 various numbers called pseudopodia are projected and withdrawn, and with 

 their assistance these tiny creatures glide along, one might almost say flow 

 along, over the surface. In most protozoa two layers of plasma may be 

 recognised, and distinguished by their appearance and structure, namely, 



