INTRODUCTION. xiii 



of these produces the dreaded ' Gapes,' and another 

 is often too abundant in the intestines. 



Amongst the Arthropoda the chief culprits are the 

 Mites (Acarina) and the Lice (Mallophaga). The latter 

 are true degraded insects — insects devoid of wings 

 but, unfortunately for the Fowl, still possessed of a 

 very powerful biting-mouth. The Mites, on the other 

 hand, are often provided with a sucking- and piercing- 

 mouth, which they plunge into their unfortunate 

 victim and so feast off its blood. These latter can 

 be told from true insects by the presence of four pairs 

 of legs ; the Lice, on the other hand, will be seen to 

 possess six legs only — true insects. 



Parasitism may be either internal or external, 

 permanent or partial. Internal parasites live in all 

 parts of the vertebrate frame : many, such as the 

 Tapeworms, live in the alimentary canal ; others, as 

 the Gape-worm, in the air-passages ; even the eye and 

 kidney may be invaded by some unwonted visitant. 

 The Protozoa and Vermes are the chief internal 

 guests, although a few Mites penetrate some distance 

 into the skin and air-passages of birds. The ex- 

 ternal parasites are mainly Insects and Mites, living 

 upon the skin and amongst the epidermal pro- 

 ductions, hair and feathers. Sometimes these pests 

 live permanently on their host : the Lice of poultry, 

 for instance, seldom leave the birds unless the latter 



