398 UTILITARIAN ZOOLOGY 



The explorations of the Phoenicians westward, which had no 

 small influence upon the course of history, were partly conducted 

 with the object of securing larger supplies of these molluscs. 



Some of the travellers of the seventeenth and eighteenth 

 centuries state that the natives of Ecuador and Costa Rica 

 obtained a purple fluid from a species of Purpura, and used it 

 to dye cotton. A kind of Sea- Slug (Aplysia camelus) is still 

 used by the Portuguese for a similar purpose. 



Bivalve Mollusca (Lamellibranckia). Pearls and mother-of- 

 pearl are chiefly derived from members of this group. Mother- 

 of-pearl or nacre is the iridescent internal layer of the shell, 



Fig. 1282. Shells of Pearl-Oyster (Margaritifera vulgaris], and pearl of same. 



while pearls consist of layers of similar material deposited round 

 various foreign bodies which, in the most valuable kinds, are 

 stages in the development of parasitic forms (see p. 204). The 

 most important pearl-fisheries are those of the Red Sea, India, 

 Ceylon, Queensland, some of the South Pacific archipelagoes, 

 lower California, and the Pacific coast of Central America. The 

 bivalve of greatest economic value in this connection is the 

 Pearl-Oyster {Margaritifera vulgaris], which in reality is more 

 of a mussel than an oyster, but belongs to a different family 

 from either (Aviculida). The once important pearl-fisheries of 

 Ceylon, after a record yield in 1891, benefiting the revenue of 

 the island to the extent of hard on a million rupees, completely 

 failed for an entire decade. As the outcome of this the pearl- 

 oyster question has recently been investigated by Professor 

 Herdman, assisted by Mr. James Hornell. These two experts 

 have fully worked out the life-history of the tape-worm which 

 leads to the formation of "orient pearls", and it is hoped that 



