204 ASSOCIATION OF ORGANISMS—THE WEB OF LIFE 
where is only reached within the digestive tube of a backboned 
animal. It may possibly be a specialized larva, like the Axolotl 
(see vol. i, p. 249), but our knowledge is too incomplete to justify 
such a conclusion. 
The best-known Tape-worms consist of a head, provided with 
organs of adhesion, and passing behind into a series of flat joints 
(proglottides), in which vast numbers of eggs are produced. The 
complex life-history of the Common Tape-Worm 
(Tenia sohun) has been briefly described else- 
where (see vol. i, p. 441). In the adult condition 
it lives in the intestine of man, sometimes attain- 
ing the length of 9 feet, while in an earlier stage 
it is found encapsuled in the muscles of the pig, 
, 
producing the disease known as “measles ”. 
B 
GUY 
A OS yea 
ORR 
Fig. 1152.—A Simple Fig. 1153.—A Fish Tape-Worm (Tetrarhynchus). a, Adult worm, 
Tape-Worm (Archigetes enlarged, showing the four proboscides; a, head of same still further 
Sieboldi), greatly en- enlarged, showing double suckers, and proboscides slightly protruded; 
larged Cc, part of a proboscis, very highly magnified, to show the hooks. 
In one small kind of Tape-Worm ( Ze¢rarhynchus, fig. 1153) the 
adhesive apparatus on the head is somewhat complex, consisting 
of two double suckers, and four tubes studded with numerous 
hooks. When adult it lives in the intestines of fishes of the shark 
and ray kind (Elasmobranchii). The life-history of one species 
has been worked out by Herdman and Hornell, and is of particular 
interest. In this case the host of the adult worm is a large Ray 
(Zygon), which is common in Indian seas. The eggs pass 
from the body of the fish, and hatch out into minute active 
larve, which perish unless they succeed in entering the shells of 
