INTRODUCTION 5 



of Africa, their attacks are not usually of a serious nature. Ecto- 

 parasitic Trematodes are Monogenetic ; that is, their larvae grow 

 up directly into mature forms. The Endoparasitic species, how- 

 ever, are usually Digenetic. Their larvae enter an Invertebrate 

 and produce a new generation of different larvae, and these 

 another. The last are immature flukes. They enter a second 

 host, which is swallowed by the final Vertebrate host in which 

 they become mature. 



The Cestodes or Tape-worms have undergone more profound 

 modifications both in structure and in mode of development. 

 They are all endoparasitic, and, with one exception (ArcJiigetes), 

 attain maturity solely within the alimentary canal of Vertebrates. 

 In length they range from a few millimetres to several metres, but 

 this great size is attained from the need for the rapid production 

 and accumulation of enormous numbers of eggs. The " head " 

 or " scolex " is attached to the mucous membrane of the host by 

 suckers or hooks, but there is no mouth nor any certain trace of a 

 digestive tract at any stage of the life-history of Cestodes. For 

 nourishment they absorb, through the skin, the previously-digested 

 food (of the host) that bathes them. In a few Cestodes the body 

 is simple and not divided into " proglottides " or generative seg- 

 ments, but in most cases it is jointed in such a way that the last 

 segment is the oldest, and each contains a set of reproductive 

 organs. The life-histories of Cestodes are most remarkable. The 

 proglottides containing the eggs pass out of the final host along 

 with the faeces and enter the intermediate host with the food. 

 The larvae hatch, and boring their way into the blood-vessels, are 

 carried by the circulation to various internal organs. Here they 

 usually become " bladder-worms," and develop the " head " of the 

 future sexual form. Then, if, as is usually the case, the inter- 

 mediate host is preyed upon by the final host, the larval Cestodes 

 enter the alimentary canal of the latter. The head of the larva 

 alone survives digestion, and from it the mature worm is formed. 



Of these three branches of the phylum Platyhelminthes, the 

 Turbellaria possess features of special interest and importance. Not 

 only do they furnish the explanation of the structure of the two 

 parasitic groups (which have probably arisen from Turbellai i;m- 

 like ancestors), but they occupy the lowest position in the whole 

 group of worms. There are reasons for thinking that this is the 

 simplest group of bilateral animals which adopt the habit of creep- 



