132 NEMATODA [CH. VIII 



They are believed to be the cause of the strange tropical diseases 

 associated with the name elephantiasis. This disease is due to a 

 clogging of the lymph vessels (see p. 434), the lymph accumulates 

 behind the obstruction, and the cells floating in it give rise to a 

 loose gelatinous connective tissue which causes certain parts of the 

 body to be enormously distended and disfigured. The clogging is 

 due to the fact that under certain circumstances this worm lays 

 eggs instead of producing active wriggling larvae. The non-motile 

 eggs choke the smaller crevices of the lymphatic system. But it 

 would lead us too far to enumerate all the modifications of the 

 life-history produced by parasitism. Suffice it to say that the 

 Nematoda are perhaps the most successful of all parasites; there 

 is scarcely a phylum in the animal kingdom which they do not attack. 

 A smooth slippery body which as a general rule causes little incon- 

 venience to the host and a low grade of metabolism requiring small 

 supplies of oxygen seem to have been the leading features in 

 their success. 



