ARTHROPODA. 289 



organs for attaching themselves, bivalve shells similar to those of 

 Mollusca, for protection ; they are often hermaphroditic, which is a very 

 uncommon thing in arthropods. The advantages gained by their special 

 habits are evident. The waters near the shore contain a great deal of 

 organic debris, and any organism which can attach itself here and yet 

 be protected from destruction by the waves is fortunate. Those attached 

 to floating objects are brought, without their effort, into constantly chang- 

 ing localities. 



Subclass 2. Malacostraca. Crustacea of larger size and more highly 

 organized. Segments, except in one order, twenty, and well differentiated. 

 Nineteen of these segments bear appendages. The first stage in the meta- 

 morphosis (the nauplius") is usually passed before hatching. The group 



FIG. 129. 



FIG. 129. Larva of Lobster (Homarus americanus) removed from egg shell. 

 From Herrick. 



Questions on the figure. Compare with the adult (Fig. 130) and note 

 similarities and differences? Examine Dr. Herrick's figures (Bull. U. S. 

 Fish Commission for 1895) and notice the gradual change to the adult 

 condition by successive moultings. What structures can you identify? 



embraces (i) the Decapoda, or the lobsters, crabs, cray-fishes and shrimps, 

 which agree in the possession of ten walking feet, eyes on movable stalks 

 and a carapace covering the thirteen fused segments of the cephalothorax ; 

 and (2) the Tetradecapoda, comprising numerous smaller types such as 

 beach-fleas, sow-bugs or wood-lice, in which head and thorax are not 

 fused, the eyes are not movable, and the walking appendages are fourteen. 

 The cray-fish and lobsters have well-developed abdominal segments, 

 whereas in the crabs the abdomen is reduced and bent under the thorax, 

 which becomes broad and massive (Figs. 127, 128). The larger Crustacea 

 are omnivorous, almost all organic matter, dead or living, being acceptable. 

 Lobsters are known to attack and devour fishes. The lobster (Homarus, 

 Figs. 129 and 130), of which there are two species, an American and a 

 European, is economically the most important member of the group, and 

 stands next the oyster as the most important invertebrate food species. 

 20 



