THE EFFECT OF PARASITES ON THE HOST 3I 



layers of aragonite and conchiolin, thus forming the iridescent and 

 highly prized pearls. These rhythmical lines of growth are well 

 illustrated in the photograph of a section of pearl from the River Tay 

 in Plate Vila. Other intruders, such as mites or larval tapeworms or 

 inorganic matter, may also serve as the centre round which pearls are 

 formed. 



Pearls and tuberculosis are both manifestations of the host's 

 response to parasites. They represent two extremes. The single 

 lustrous sphere in which the duck fluke lies entombed is the mussel's 

 successful solution of a relatively simple problem. The extreme 

 emaciation, suppurating liver and other gruesome symptoms of an owl 

 or a rook suffering from tuberculosis are, on the other hand, the bird's 

 desperate and ineffectual reactions to bacterial toxins which eventually 

 prove fatal. 



The results of parasitism upon a bird are varied but generally the 

 harmful effects are brought about in a Hmited number of ways. The 

 parasites may consume the body tissues or body fluids of the bird, or 

 produce substances which are poisons or irritants. They may inflict 

 grievous bodily wounds or cause mechanical injury by pressure or 

 obstruction. They may bring about changes in both metabolism and 

 behaviour and, lastly, introduce other more deadly parasites into the 

 bird's body. 



Sometimes one parasite can, at different times, affect the host in 

 all these ways. Leeches, for example, eat the blood of their bird host. 

 Their saliva is poisonous and may inflame the body tissues and even 

 kill the bird. The wounds they inflict while feeding do not heal easily 

 and severe haemorrhages frequently ensue. They also occasionally 

 suffocate the bird by crowding together and obstructing the air 

 passages. They are carriers of spirochaetes and fowl-pox. 



Apart from the obvious consequences of these attacks by parasites, 

 obscure and indirect reactions are also involved. For example, the 

 herring-gull fluke, in its first larval stage, infests the common peri- 

 winkle (Plate XXVIIIa) and feeds upon its sex organs. It castrates the 

 host, thus first of all making further reproduction impossible for the 

 snail and also causing it to grow in size. The host is thus turned into a 

 giant eunuch. From a cursory glance at the outside of the shell a 

 practised eye can pick out any one of these sadly afflicted winkles. 

 They are specially favoured by fishmongers who, innocent of the reason 

 for their large size, display them prominently on their slabs and 



