538 Animal Biology 



Liver flukes (Figs. 180 and 181) frequently live in the bile ducts of 

 the liver of sheep^ pigs, cows, etc., and occasionally in man. This para- 

 sitism causes the organs to rot or become otherwise aflfected. The sheep 

 liver fluke (Fasciola hepatic a) (Figs. 180, 181, and 374) spends part of 

 its life history in the soil, on grass, and also in the body of a certain 

 species of snail of the genus Lyrnnaea, which acts as a secondary host for 

 the liver fluke. 



Hoohs 



.Juckcf 



Brain 



}nvaqinaied 

 ^ jcolex _ 



Coenuras ,xsr- 



Fig. 268. — Gid tapeworm [Multiceps multiceps) causes "staggers" or "gid" in 

 sheep by lodging in the nerv^ous system. A, Anterior region of adult; B, portion of 

 brain with a cyst. Such a cyst with several scoleces is known as a coenurus. 



PHYLUM 6— NEMATHELMINTHES (ROUNDWORMS) 



The number of parasitic roundworms is probably small in comparison 

 with the number living freely in water and soil. Dr. N. A. Cobb esti- 

 mates that the upper foot of arable soil contains thousands of miflions 

 per acre, where they constitute very important biologic and mechanical 

 factors. Cobb also estimates that there are many thousands of species 

 of roundworms which infest vertebrate animals, besides manv thousands 

 which infest such invertebrates as insects, worms, and Crustacea. Round- 

 worms are universally distributed, being present in the cold waters of the 

 Antarctic, in hot springs, in the depths of the sea, and at high mountain 

 altitudes. Geologically, the roundworms range from the Upper Pale- 

 ozoic era (250-330 million years ago) to the present (Figs. 320 to 322). 



Ascaris (Fig. 184) is a genus of roundworms which is parasitic in the 

 intestines of frogs, hogs, calves, man, etc. Ascaris lumbricoides infests 

 the small intestine of the hog; the stomach, causing nausea; the pancreas, 



