i8 



ECONOMIC ZOOLOGY 



Occasionally, these reefs take a circular form and may form a ring-like 

 island or atoll, Fig. 12, with an enclosed lagoon; there have been several 

 theories to explain the formation of atolls. In many parts of the 

 Pacific Ocean, large groups of islands, have been formed through the 

 action of the coral polyps, and, in many places, large deposits of 

 limestone rock owe their origin to these small animals. 



The only corals that have any value as such, except for scientific 

 purposes, are the so-called "precious corals;" they are usually of a 



C.SS. 



FIG. ii. Ctenophora. Somewhat enlarged. A, Hormiphora plumosa; side 

 view, i, mouth; 2, aboral pole with sense organ; 3, funnel; 4, paragastric canal; 

 5, a ciliated band; 6, canal; 7, tentacular pouch; 8, tentacle; 9, gelatinous substance. 

 B, Pleurobranchia pileus; view of aboral aspect, showing central statocyst, polar 

 fields (Pf), and 8 ciliated bands (c.ss, c.tr). (From Hegner, College Zoology, A, 

 from Shipley and MacBride, after Chun; B, after Lankester.) 



bright red or pink color and are made into necklaces and other orna- 

 ments. These corals are of a delicate branching type and are collected 

 mainly in the Mediterranean Sea, especially along the coasts of Africa, 

 Italy, Sardinia, and elsewhere, even as far as the Cape Verde Islands. 

 They were formerly thought to have medicinal value and to be able to 

 ward off danger and are still worn by superstitious Italians to keep 

 off the "evil eye." 



Precious coral has been used in these ways since remote times; 



