WOEMS. 233 



even for considerable distances, and finally ruptures, 

 so that the numerous eggs are liberated. Each joint 

 may therefore be regarded as an individual, though 

 the tapeworm, considered as a whole, also possesses 

 a certain individuality. The tapeworm is a species 

 of animal which exists in two forms. One of these is 

 the sexually reproducing joint, which leaves the 

 original host, and from the eggs of which are de- 

 veloped larvffi, which become, in another host, hladder- 

 worms, or measles. These develop a head, which is 

 the asexual second form of the tapeworm. This, 

 therefore, is a case of metagenesis (p. 16). It appears, 

 from what has been said, that the head can in no 

 sense be compared to the head of an ordinary animal ; 

 it has neither mouth nor jaws, for tapeworms do not 

 possess a gut. The head and joints take up osmoti- 

 cally through their walls the nutritive matter found 

 in the intestine of their host. There are suckers and, 

 in several species, a circlet of hooks on the head 

 (Fig. 136, a), by which the animal holds fast to the 

 intestinal wall. Thousands, or even tens of thousands, 

 of eggs may be produced by a single joint. The joint 

 is liberated when the eggs begin to ripen, often before. 

 In some species several adjacent joints are pinched 

 off at the same time. The tapeworm joints leave the 

 intestine of their host either alone or with the dung. 

 After falling to the ground they creep along like 

 snails, and attach themselves to various plants. It 

 is obvious that a tapeworm joint or a number of the- 

 eggs produced by it may easily be taken up by a 

 grazing animal. The eggs of a particular kind of 

 tapeworm are only able to develop further if they 

 reach the intestine of a particular kind of host. It 

 is undoubtedly true that, as a rule, only a relatively 

 small number of the eggs produced by a tapeworm 

 develop further, but the great powers of reproduction 

 prevent the species from dying out. If, now, either 

 a joint or the eggs from one are taken into the gut 



