CRUSTACEA. 



211 



like all other Entomostraca, are found to have a free- 

 swimming nauplius stage. 



The only ones of the Entomostraca (aside from the fish- 

 parasites) which have received a com- 

 mon name are the barnacles, so familiar 

 at the seashore. In these the body 

 is enclosed in a hard calcareous shell, 

 which is either directly attached to 

 some solid support, as in the acorn- 

 barnacles, or there is a fleshy support, 

 as in the goose-barnacles. Inside the 



FIG. 93. Goose - barna- 

 cles (Lepas anatifera). 

 After Schmarda. 



shell is the animal, and a 

 cursory examination of its 

 two-branched feet and its 

 other features would con- 

 vince any one that these 

 forms are truly crustacean. 



Mention should be made 

 here of a large group of ex- 

 tinct animals, the Trilobites, 

 which recent investigations 

 have shown to be crusta- 

 ceans, but which cannot be 

 more definitely placed within 

 that group. They agree with 

 neither Entomostraca nor Malacostraca in their structure. 

 They have a flattened body, in which head, thorax, and 



FIG. 94. Restoration of the under 

 surface of a Trilobite, showing the 

 appendages. After Beecher. 



