THREAD-WORMS AS PARASITES 



205 



the Pearl-Oyster (Margaritifera 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 larvae 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 (Batistes), 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- 



1 T^-I i i i ITT Fig. 1154. Thorn -headed Worm (Gigantorhynchus 



sites, the Thorn-headed Worms & X ; 4 A worm attached to the lining (a) of a pig - s 



( FrhiwnrhMnrhida>\ are fener- intestine ; ( 2 ) hooked proboscis of same, enlarged; 3 , egg 

 ( n cmnornyncmace ;, are geiier containing an embryo, greatly enlarged. 



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 (Gigantorhynchus 



