Chap. 29 



ARTHROPODS CRUSTACEANS 



575 



Fig. 29.4. Restoration of Silurian sea bottom, now the site of the city of Buflfalo, 

 New York. Made from a study of fossils found in the Niagara region, in that 

 period some 400 million years ago when it was overspread by ocean. A large 

 trilobite and several smaller ones creep upon the bottom showing the characteristic 

 furrows and triple sections of the body. A cephalopod, and two crinoids, the once 

 abundant stemmed echinoderms, are in the mid-ground of the scene. (Courtesy, 

 The Buffalo Museum of Science.) 



inch wide or less, to the American lobster that holds a record weight of 35 

 pounds, and the giant spider crabs of Japan that measure 20 feet from tip to 

 tip of the first pair of legs. 



Development. As in all higher animals, crustaceans pass through stages 

 that suggest either the adult or the immature stages of simpler animals. Like 

 most crustaceans, a shrimp (Penaeus) hatches into a larva called the nauplius 

 stage that has three pairs of appendages and a single eye (Fig. 29.7). The 

 nauplius transforms into a protozoea and the latter into a zoea in which the 

 cephalothorax appears. The zoea develops into the mysis, a stage named after 

 the common shrimp Mysis. 



Crayfishes and Lobsters 



Aristotle did well to call crayfishes "the small lobsters" of the rivers. Their 

 habits of living are remarkably similar considering the differences in their 



