WORMS, ROTIFERS, LEECHES, POLYZOA 75 



they are called Cercaria. They burst forth, and, after a free- 

 swimming existence, penetrate the body of some animal. They 

 drop their tails and become encysted in the tissues. Finally they 

 assume the adult form and develop reproductive organs, out of 

 which pass the eggs. The Redia acts as the " nurse," and the 

 Trematode may pass through life by inhabiting two very different 

 animals after coming forth from that inhabited by the parent. 

 The stages vary in different genera, and, as a rule, the first are 

 passed in invertebrate and the last in vertebrate animals." 1 



The first sub-order of these Trematodes includes Distomum 

 (Fasciola) hepaticum, the Liver-fluke of sheep, which produces the 

 disastrous disease known as liver-rot. The appalling destruction for 

 which these worms are responsible may be gathered from the fact 

 that, in 1830, one and a half million sheep, representing something 

 like four millions of money, perished from liver-rot in England 

 alone ; while in 1879-80 three million sheep died of this disease. 

 The researches of Thomas and Leuckart have shown that the 

 intermediate host is a small water-snail, Limncea truncatula, and 

 that the disease has a distribution as wide as the snail, through- 

 out Europe, Northern Asia, North Africa, the Faroes, and the 

 Canaries ; while in Australia and the Sandwich Islands a variety 

 of the Limnaea is the intermediate host. Low-lying meadows 

 liable to flooding, and of damp, clayey soil, situations favourable 

 to and where the Limnc&a truncatula abounds, are naturally the 

 most dangerous feeding grounds for sheep. The early larval and 

 Redia stages are passed in the snail and a vast number of flukes 

 (Cercavice] developed, which, on leaving the snail, encyst upon the 

 grass and are eaten along with it by the sheep. 



Another closely allied Trematode, Distomum hepaticum, is para- 

 sitic in man, the horse, deer, goat, pig, rabbit, antelope, kangaroo, 

 and many other animals. Bilhania hcematobia is a formidable 

 parasitic Trematode, and a scourge among the natives of Egypt, the 

 east coast and inland countries of Africa, along the shores of Lake 

 Nyassa, the Zambesi River, and the Gold Coast ; and is responsible 

 for the very painful and serious disease called haematuria. 



The Tape -worms, or Cestoidea, are parasitic within many 

 vertebrate animals, including man, live in the intestinal canals of 

 their hosts, and are readily recognised by their long, flat, many- 



1 Dr. P. Martin Duncan, F.R.S. 



