226 



METAZOAN PHYLA 



The mouth opens near the ring of ciha and the anal opening is below at 

 the tip of the body. 



Since a trochophore larva appears in the development of bryozoans 

 and annelids, as well as mollusks, and since it resembles in a certain 

 degree some rotifers, the suggestion has been made that these groups 

 may have all descended from a common ancestral form called a trocho- 

 zoon. There are many strong arguments against this view as well as 

 others for it. 



Regeneration of lost parts and repair of injuries are general among the 

 mollusks, but they do not include replacement of ganglia, the loss of which 

 causes death. 



Ap/ca/ p/crf-e 



Ve/um 



A/toui-h 



Fooi- 



ce//s 



arrrar/ tre/Zs 



Fig. 132. — Trochophore of Patella, one of the limpets. A, viewed from the ventral 

 side. B, median section of a slightly older trochophore. {From Korschelt and Heider, 

 " Text-book of Embryology.'") 



266. Economic Importance. — Mollusks are of economic importance 

 from several points of view. Their greatest value is as human food. 

 The reports of the U. S. Fish Commission showed, in 1930, a total value 

 of canned oysters, clams, and other shellfish, and of by-products amount- 

 ing to over $17,000,000. The total for 1931 was nearly $14,000,000. 

 Snails are much eaten in some countries of Europe and to a certain 

 degree also in this country. Among the bivalve mollusks clams, oysters, 

 scallops, and other forms are regular articles of diet. The value of the 

 oyster industry alone along the Atlantic seaboard has been estimated at 

 between $30,000,000 and $40,000,000 annually. The arms of squids 

 are also cut off and used as food. The shells of many of the mollusks 

 are for one reason or another articles of commerce. Many of them are 

 sold as ornaments on account of their beauty. Others are used in the 

 production of pearl buttons, and the pearl-button industry has attained 

 great proportions in this country, especially in Ohio, Illinois, and Iowa. 



