252 THE LIFE OF CRUSTACEA 



None of the Crustacea can be regarded as directly 

 harmful to man. They have not the power of 

 inflicting envenomed v^ounds which renders some 

 other Arthropods, such as Scorpions, some Spiders, 

 Centipedes, and Insects, formidable in spite of their 

 small size ; and although blood-curdling tales of the 

 ferocity of the Land Crabs are to be found in the 

 accounts of old voyages, even the largest of these is 

 hardly an antagonist to be dreaded. 



A considerable number of invertebrate animals, 

 not of themselves noxious, are now known to be 

 the indirect cause of much serious injury to human 

 life by harbouring and disseminating organisms 

 which produce disease. The progress of research is 

 adding, almost every day, to the number of species 

 known to be disease-carriers, and it is possible that 

 in the future some Crustacea as yet unsuspected 

 may be added to the list. 



At present, however, there is only one case in 

 which a Crustacean has been shown to be concerned 

 in the transmission of a parasite of man. The 

 " Guinea- worm," Filar ia (or Dracunculus) medinensis, 

 is a parasite belonging to the group of " Thread- 

 worms " (Nematoda) which causes dangerous ab- 

 scesses under the skin of the legs in many parts 

 of tropical Africa. It has been shown that the 

 embryos of the worm, which are discharged in vast 

 numbers on the bursting of the abscess, do not 

 develop unless they fall into water containing certain 



