32 FLEAS, FLUKES AND CUCKOOS 



counters. Fortunately, in this stage both worms and winkles are 

 digested by the equally innocent customer. 



The second intermediate hosts of the herring-gull fluke are various 

 kinds of inshore fish. The larvae penetrate beneath the scales and into 

 the superficial layers of the skin and there become encapsuled. The 

 tissues of the host react by producing a concentration of pigment round 

 the cysts. The whole fish then appears to be covered in an unsightly 

 rash of black spots. The common goby photographed on Plate VI lb 

 has been heavily infected with these larval trematodes. Such a heavy 

 attack as this, which results from a chance meeting with a dense swarm 

 of larvae, frequently kills the fish. The cercariae pour out a secretion 

 from the penetration glands to facilitate their entry through the skin. 

 This secretion is highly toxic if injected by thousands of larvae 

 simultaneously. To get some idea of what the fish experiences we may 

 imagine walking into a swarm of flying ants on a hot summer afternoon 

 and suddenly, all too late, realising that the ants have settled, cast off 

 their wings and are quickly boring into the skin. When parasites 

 attack human beings in swarms they are generally microscopic organ- 

 isms which are drawn passively into the body with air or swallowed with 

 food or water. Even those trematodes which will attack a swimmer and 

 actively penetrate the skin are so small compared with their victim 

 that at the time of invasion he is unconscious of the event. Sometimes 

 he may experience a mild prickling sensation of the skin and a fiaint 

 rash appears and almost as quickly disappears. Fortunately we are 

 spared the knowledge of what we are calmly breathing in and out in the 

 bus or the theatre. Because we have a limited range of eyesight and 

 are lacking in imagination we do not lynch the man sitting beside us 

 who spits on the floor. 



The harmfulness of parasites largely depends on their numbers. 

 The adult stage of the herring-gull fluke which browses in the bird's 

 intestine does not seem to cause any inconvenience when only one or 

 two specimens are present. If the gull is unfortunate and catches a 

 very heavily infected fish similar to the specimen shown on Plate Vllb 

 and many thousands of flukes are liberated simultaneously in the 

 intestine, severe inflammatory conditions and even death may ensue. 

 It is true that single individuals of some parasites are dangerous. One 

 tick can provoke a mortal paralysis owing to its toxic saliva. On the 

 whole, however, such cases are rare. Broadly speaking, providing the 

 numbers of parasites are low the effects are slight. But accident or 



