84 ECONOMIC ZOOLOGY AND ENTOMOLOGY 



exert a baneful influence on the general health and longevity 

 and on the material welfare of the people. 



Porto Rico has suffered severely from this disease, as indeed 

 have nearly all tropical and sub-tropical countries. In the 

 old world the most common species of hookworm is 

 Uncinaria duodenalis. In the United States Necator 

 (Uncinaria) americanus is the most abundant. Both of these 

 species were formerly included in the genus Ankylostomum 

 and so the disease that they cause is frequently called 

 ankylostomiasis. 



Several of our domestic animals are infected with other 

 species of hookworms. Uncinariasis, or "salt sick," or hook- 

 worm disease of cattle, is a very serious disease in some of the 

 southern states. This disease can be partially controlled by 

 intelligent methods of handling the stock. As it occurs chiefly 

 on low wet lands the selection of pasture lands is of first 

 importance, or rather it is of second importance, for the most 

 important thing of all is to keep the disease out altogether by 

 not allowing infected cattle to come on the farms or into the 

 locality. 



A species of hookworm occurring in fur seals often causes a 

 loss of thousands of the young or pup seals each year on the 

 breeding grounds. 



Filaria and Elephantiasis. Another genus of very serious 

 Nematode parasites is known as Filaria. The filariae cause the 

 various forms of disease known as filariasis. Filaria bancrofti 

 is the name of a minute, transparent, little worm that occurs in 

 human blood and lymph in many tropical and sub-tropical 

 regions, extending often into temperate climates. The larval 

 forms, that occur in the blood, are but a little more than 

 one one-hundredth of an inch long and about as big around 

 as a blood corpuscle. During the day time but few of these 

 are to be found in the blood near the surface of the body, 

 but as evening comes on they may be found there in in- 

 creasing numbers. 



This night-swarming to the peripheral circulation has been 

 found to be a remarkable adaptation in the life history of the 

 parasite to the presence of night-flying mosquitos, for it has 

 been demonstrated that in order to go on with their develop- 



