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PART III. 

 ARTHROPODA. 



To the student of microscopic parasitology, the interest attaching 

 to the parasitic Arthropods centres chiefly in their external appear- 

 ances, as these have a diagnostic significance. For this reason the 

 descriptions here given are confined to details of the external body- 

 form and structure, though it is obvious that they may incidentally 

 reflect conditions governing the internal organization. 



The body of the Arthropod, together with its appendages, is 

 covered with a cuticle of varying thickness, which consists of a 

 chitinous substance of extreme resistance. Where the animal is 

 opaque, or nearly so, it is necessary to isolate what is known as the 

 " chitinous skeleton." 1 This is done by maceration in a weak (10 per 

 cent, or less) solution of potassium, and the action of the fluid may 

 be hastened by warming or, better still, boiling. The object should 

 first be killed in alcohol and then transferred by graduated alcohol 

 and water stages to pure water. 2 The maceration of the more fragile 

 varieties requires careful management, as boiling, especially if con- 

 tinued for any length of time, will not only cause the slender body 

 appendages to drop off, but may give rise to pronounced structural 

 changes and so render the specimen useless. The best method of 

 treating delicate objects is to allow them to macerate in potassium 

 solution at room temperature ; large varieties will take several days, 

 or even a week or more, smaller specimens proportionally less. If, 



1 According to G. Enderlein (Zoolog. Anz., vol. xxvii, 1904, p. 497), small dried objects 

 may be prepared for examination in the following manner : one part of a "moderately 

 strong " potassium solution is mixed with eight to ten parts water, and the objects 

 are allowed to soak in this mixture until they have approximately regained their 

 normal shape. This will take from ten minutes to several hours, according to the 

 size and delicacy of the objects. They should be rinsed in water and the larger air- 

 bubbles smoothed out with a fine camel's hair brush. They are returned to the 

 solution, again rinsed in water, and dehydrated by means of the alcohol stages. They 

 should be finished for the microscope by the methods described above. 



2 When dealing with the larger varieties, the action of the drug should be hastened 

 by pricking the specimens with a fine needle. The site selected should be without 

 special structural significance, as, for instance, a segmental margin in forms with 

 segmented abdomen. 



