270 METAZOAN PHYLA 



305. Economic Importance.— A large number of crustaceans have 

 been used as food, especially lobsters, shrimps, and crabs, and, in some 

 localities, crayfish. Reese states with reference to lobsters that in 

 Canada alone 109,000,000 have been caught in a single year. He also 

 says that the total catch in the United States in 1892 was about 23,250,000 

 pounds; in 1905 it was about 11,750,000 pounds, which sold for more 

 money than the catch of 1892. The catch in 1936 amounted to nearly 

 13,158,000 pounds. No more recent figures are available. The supply 

 has been seriously depleted and efforts are being made to replenish it by 

 the artificial rearing of young lobsters, which are liberated at places 

 favorable for their growth. The crab-fishing industry centers about 

 Chesapeake Bay, but there has been a serious diminution in the supply. 

 The Gulf States furnish most of the shrimps marketed in this country 

 and they are the most important one element in the fisheries of those 

 states. In 1931 the value of canned shrimps and crabs, and by-products 

 of these, amounted to over $4,000,000; and in 1936 the amount was over 

 $5,000,000. These figures are from the U. S. Fish Commission; they 

 do not include the value of the animals used in a fresh state. 



306. Biogenesis.— More than a century ago Von Baer directed 

 attention to the fact that there was a resemblance between the early 

 stages in the hfe of higher Metazoa and the adults of lower forms. With 

 the general acceptance of the concept of evolution a very natural expla- 

 nation of the fact was to assume that this resemblance was due to ances- 

 try, the lower forms having ceased to develop after reaching the condition 

 in which they now are, and the higher forms having continued to develop 

 but indicating in their early stages the characters of their ancestors. 

 This conception has been termed biogenesis and formulated in the 

 biogenetic law. Biogenesis is to be contrasted with abiogenesis, a term 

 synonymous with spontaneous generation, the idea of which is that each 

 individual form was the result of a separate creative act and when 

 created had the characteristics it now possesses. Strong arguments 

 for the biogenetic theory have been derived from the Crustacea. 



The shrimp, Penaeus (Fig. 175), is a good example of the appHcation 

 of the biogenetic law. This organism hatches as a larva known as a 

 nauplius (Fig. 174), exhibiting three pairs of appendages and a frontal 

 eye and resembling the larvae of Crustacea generally, including those of 

 the simpler forms. From the nauplius is produced the protozoaea, which 

 has 6 pairs of appendages and rudiments of segments. From this in 

 turn is derived the zoaea, which possesses 8 pairs of appendages, with 6 

 more developing, and has a distinct cephalothorax and abdomen. The 

 zoaea changes to a mysis, with 13 pairs of appendages on the cephalo- 

 thorax. Finally, from the mysis is produced the adult shrimp, which 

 has 19 pairs of appendages. Many crustaceans pass through a nauplius 

 stage, which may represent an ancestral type now extinct. The proto- 



