CRUSTACEA 



269 



inch. A square meter of rock area observed in Puget Sound was 

 estimated to bear 41,500 individual barnacles, and since the rocks every- 

 where were covered with them, the total in the one locality must have 

 been enormous. A colony of such barnacles presents a very animated 

 spectacle when all of the individuals are kicking their legs at the same 

 time. 



303. Behavior. — The behavior of the crayfish has been described, 

 and that of forms which are like it in structure is similar. Brief references 

 to behavior have also been made as it is exhibited by barnacles, but 

 nothing has been stated with regard to that of swimming forms, par- 



A B 



Fig. 173. — Barnacles. A, Balanus hameri Darwin. The shell of the animal is closed 

 up, concealing the occupant. B, Balanus tintinnabulum Linnaeus, showing the internal 

 anatomy of the animal, also -wTth the shell closed. (From Bronn, "Klassenund Ordnungen 

 des Tierreichs," after Charles Darwin.) Natural size. 



ticularly entomostracans. These are exceedingly active, swimming 

 or darting here and there and seeking that light intensity to which they 

 are best adapted. Owing to the fact that some are adjusted to bright 

 light and others to dim, there is a vertical movement i'n bodies of water 

 of considerable depth which brings to the surface in the daytime certain 

 forms which are found at deeper levels at night. Others which remain 

 at these deeper levels in the daytime come to the surface at night. In 

 addition to light stimuli, crustaceans respond to contact and to chemical 

 stimulation. 



304. Reproduction. — Most crustaceans are diecious, though the 

 barnacles are not. The eggs are centrolecithal, undergo superficial 

 cleavage, and from them are produced larvae, which, as in the crayfish, 

 may be miniatures of the adults, or which, as in the shrimp, may be 

 quite different and pass through several larval stages, thus undergoing 

 complicated metamorphoses. The young of crabs have very prominent 

 eyes and for this reason have received the name of megalops. Many 

 crustaceans carry their eggs about attached to abdominal appendages 

 and in some cases contained in a brood pouch. The larvae may also be 

 so carried for a time. 



