230 



METAZOAN PHYLA 



The mouth opens near the ring of ciUa 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. 



Fig. 141. — Trochophore of Patella, one of the limpets. 

 side. B, median section of a slightly older trochophore. 

 "Text-book of Embryology.") 



A, viewed from the ventral 

 (From Korschelt and Heider, 



268. Economic Importance. — Mollusks are of economic importance 

 in a number of ways. Their greatest value is for human food. The 

 report of the U. S. Bureau of Fisheries of 1937 shows that the eastern 

 catch for oyster alone in 1936 amounted to over $7,000,000. Among the 

 bivalve mollusks, oysters, clams, scallops, and some other forms are 

 the chief articles of diet. These animals appear on the market in both the 

 fresh and canned condition, as well as in the form of soup and chowder. 

 The canned oysters for 1936 amounted to $181,301, while that for clam 

 chowder was $271,767. Snails are much eaten in some countries of 

 Europe and to a certain degree in this country. 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. The marine pearl-shell 

 buttons for 1936 were valued at over $4,669,000 while that for novelties 

 was about $825,000. The pearl buttons from fresh-water mussel shells 

 for the same year amounted to over $4,621,000, while the crushed-shell 



