THREAD-WORMS AS PARASITES 205 
the Pearl-Oyster (M/argaritifera vulgaris), a well-known bivalve 
on account of the pearls which it yields) The Gulf of Manaar, 
in particular, has been the seat of an important pearl-fishery for 
between two and three thousand years, Successful larve bore 
into the bodies of the oysters, and undergo further development, 
attaining the size of a small pin’s head. One of the mollusc’s 
enemies is the Trigger-Fish (Ba“stes), and if this swallows an 
infested oyster the tape-worm embryos bore through the wall of 
the stomach, and become encapsuled in the body of the fish. 
The trigger-fish in its turn may be devoured by a sting-ray, in 
which case the young tape-worms become adult. The life- 
history of this parasite is therefore passed within the bodies of 
three distinct kinds of animal, the final host being the most 
powerful, as usual in such cases. It remains to add that many 
of the tape-worm embryos die while still within the oysters, and, 
proving a source of irritation, are covered by successive layers 
of calcareous matter. It is in this way that the best or “orient” 
pearls are formed. 
THREAD-WORMS (NEMATHELMIA) AS PARASITES 
The members of this large group are cylindrical unsegmented 
worms, most of which are internal parasites in the bodies of 
animals or plants. They are 
less degenerate than tape-worms, 
and the very numerous species 
differ greatly in respect of the 
complexity of their life-history, 
and the hosts infested. One or 
two of them have already been 
briefly described (see vol. i, p. 
447), and something will be said 
about others in the section on 
ANIMAL FOES. 
Some curious internal para- 
. Fig. 1154. —Thorn-headed Worm (Gigantorhynchus 
sites, the Thorn-headed Worms gigas). 1, A worm attached to the lining (a) of a pig's 
intestine; (2), hooked proboscis of same, enlarged; 3, egg 
containing an embryo, greatly enlarged. 
(Echinorhynchide), are gener- 
ally regarded as related to the 
Thread-Worms, and in them the digestive organs are entirely 
absent. We may take as an example a form (Gzgantorhynchus 
