508 The Animal Kingdom 



crabs and the marine shrimps. Many of the crabs and shrimps are 

 highly important as food items; thus they constitute a very important 

 group of invertebrates. 



Larval Stages of the Crustaceans. — While the crayfish which 

 has been considered in detail has no free-swimming larval stages, most 

 of the crustaceans do. The series of larval stages are quite distinct and 

 have been named nauplius, cypris, protozoea, zoea, mysis, and megalops. 

 The more primitive crustaceans such as the fairy shrimps, the ostracods, 

 and the copepods pass through only the nauplius stage. The barnacle 



Fig. 168. — Diagram of larval stages of crustaceans, illustrating recapitulation. 

 Those stages in ovals are gone through in the egg; all others are free-living. (By 

 permission from Natural History of Marine Animals by MacGinitie and MacGinitie. 

 1949. Copyright by McGraw-Hill Book Company, Inc.) 



passes through the nauplius stage and a cypris stage which resembles 

 the adult ostracod. The lobster passes through a nauplius in the tgg, 

 lacks the cypris stage, passes through a protozoea and zoea stage in the 

 tgg, and a free-swimming mysis which resembles the adult of the more 

 primitive Mysis. Thus many of the advanced crustaceans pass through 

 stages which resemble the adults of more primitive forms. This is used 

 as further evidence of Haeckel's biogenetic law or recapitulation theory. 



