42 FLEAS, FLUKES AND CUCKOOS 



Almost all the blood suckers — insects and worms, as well as leeches 

 and ticks — have developed a special sort of saliva which mixes with the 

 blood as it issues from the wounds they have inflicted and prevents it 

 clotting, both at its source and within the proboscis or gut of the 

 parasite. Many parasites also lack the sensory organs which normally 

 keep animals in touch with the external world. Eyes and ears would be 

 useless to a fluke in the liver of a bird, or to a feather mite in a curlew's 

 quill and, indeed, they possess neither. Instead they have developed 

 other senses or tropisms by which they are guided to small circumscribed 

 areas of the host's body or on long migrations through the tissues of the 

 host. 



The difficulty of finding their host has imposed upon many parasites 

 a fantastically complicated life-cycle. Moreover most endo-parasites, 

 in order to reach their goal, must pass from a highly specialised but 

 stable environment into the strikingly different and fluctuating condi- 

 tions of the outside world. No man can leave an air-conditioned 

 hotel, say in Toronto, in mid-winter without putting on an overcoat, 

 but a parasite must face even more violent changes without any artificial 

 protection. 



A few parasitic Protozoa pass from host to host during contact 

 between individuals — by licking, kissing, sexual intercourse or feeding 

 of young by the parents — but many form resistant cysts or spores which 

 pass into the outside world, where they are carried hither and thither 

 by the elements and possibly reach another host by the medium of 

 air or water or contaminated food. Some Protozoa are entirely 

 dependent on invertebrate vectors. The malarial parasites are perma- 

 nent prisoners in the circulatory system of their vertebrate host and 

 doomed to perish with it, unless they are rescued by a blood-sucking 

 insect. Only the sexual stage is passed in the mosquito and it seems 

 likely that the insect carrier has been secondarily interpolated in the 

 cycle. The opposite is probably true in the case of trypanosomes which 

 were primitively insect parasites. 



The majority of worms have become involved with a complicated 

 series of intermediate hosts. Many bird flukes, for instance, have seven 

 stages : egg, free-swimming miracidium, sporocyst and redia within a 

 snail host, a free-swimming cercaria, an encapsuled metacercaria in a 

 second intermediate host (which can belong to almost any group of ani- 

 mals ranging from mammals and frogs to leeches and jelly-fish) and final- 

 ly the sexually mature individual in the bird (Fig. 4) . Sometimes an extra 



