TAPE-WORMS AS PARASITES 



203 



all directions. Those which lie near the surface of the head, and 

 penetrate into the tentacles, assume a worm-like form, and are 

 brightly coloured with rings of white and green, while the end of 

 each of them is marked with a red spot. These tints are easily 

 seen through the stretched and translucent skin of the snail. The 

 resemblance of these structures to worms is increased by the fact 

 that they expand and contract in a rhythmic 

 way, and they were at one time actually sup- 

 posed to be a kind of para- A 

 sitic worm, and received a 

 special name (Leucochlori- 

 dium paradoxum ). At- 

 tracted by the colours and 

 movement, small birds nip 

 off the tentacles of the 

 snail, the bright-hued tubes 

 of which contain numerous 

 tiny flukes that have de- 

 veloped within them. The 



fate Of these nOW trembleS Fig. 1151. Distomum macrostomum. A, A Land-Snail (Sue- 

 it-> i-Vi^ Kolon^ fr\r if f-"h^xr cinea P utris ^ infested by sporocyst, some of the banded worm-like 

 OaidnCC, lOr lliey branches of which can be seen through skin of head and ten- 

 are SWalloWed bv the adult tacleS " B ' A s P roc y st > enlarged, and showing two worm-like 



J branches. 



bird itself they are simply 



digested, but if, on the other hand, they are fed to its nestlings, 



they are able to develop into adult flukes. 



TAPE-WORMS (CESTODA) AS PARASITES 



Creatures resembling Planarian Worms (see vol. i, p. 445) 

 were probably ancestral to Flukes, and they, in their turn, stand 

 in a similar relation to the degenerate internal parasites known 

 as Tape- Worms. In these the peculiar mode of life has had a still 

 more far-reaching influence, for there are no digestive organs, and 

 the nutriment consists either of the fluids or the already digested 

 food of the animals which play the part of hosts. In either case 

 it is absorbed by the general surface of the parasite. 



The simplest kind of Tape- Worm known (Archigetes Sieboldi, 

 fig. 1152) is a minute creature, less than one-eighth of an inch 

 long, which lives within the body of the small Red River- Worm 

 (Tubifex), and there attains the egg-producing stage, which else- 



