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TREMATODA 



The old idea that worms were good for children has died out long 

 ago, and we know of no cases of mutualism, or benefit to parasite 

 and host, in the animal parasites of man. 



True parasitism is found in those cases in which the parasite 

 benefits and the host is injured. Chance parasites are animals 

 which accidentally and temporarily become parasites. 



The life-history of a parasite may be simple, being carried out in 

 one host, or it may be complex, with one or more hosts for its larval 

 stages and another for its adult condition. 



Generally these hosts bear a direct relationship to one another, 

 the intermediary host often being herbivorous, and thus becoming 

 infected through eating fsecally contaminated food, and the definitive 

 host being carnivorous or omnivorous, becoming infected through 

 eating the herbivorous host of the larva. 



Some parasites, particularly the Microfilariae in the blood, show 

 a remarkable periodicity in their habits, which appears to be asso- 

 ciated with the means of escape from the definitive host by some 

 intermediary host, such as a mosquito or a tick, whose habits agree 

 with the periodicity of the parasite. Such correlations are very 

 numerous in Nature— as, for example, the opening, or emitting of 

 strong odours, by certain flowers at definite times of the day or 

 night, which accords with the habits of insects which help on their 

 fertilization. 



Tropical countries are the home par excellence for parasites, as 

 the means of infection by bad sanitation, biting flies, etc., are easily 

 available. It is therefore obvious that, as there may be many 

 methods of infection by the mouth, the skin, and the nose, preven- 

 tion is not an easy matter. It depends upon two factors — personal 

 and public hygiene. The question of personal hygiene is the more 

 important and more easily applied, as only one person is concerned, 

 while that of public hygiene is more expensive and not so easy to 

 apply, as many people have to act in unison in order to produce any 

 effect. 



Personal hygiene includes such matters as personal and domestic 

 cleanliness; protection of the skin against infection, by the use of 

 mosquito curtains and boots; avoidance of infection from domestic 

 pets, such as dogs and cats, by not too close association with them ; 

 the protection of articles of food against flies and vermin ; and the 

 careful cooking of food and filtering of water. 



With regard to public rules, the first is the proper disposal of 

 waste, particularly of faecal matter; careful cattle and meat inspec- 

 tion in well-kept slaughter-houses ; and the destruction of ownerless 

 dogs by means of a lethal chamber, in which they are killed by gas. 



Classification.— The metazoan parasites of man can be arranged 

 into phyla as follows : — ■ 



Phylum I. Platyhelmia. 



Phylum n. Nemathelmia. 



Phylum III. Annulata. 



Phylum IV. Arthropoda. 



