THE PHYLA ARTHRDPODA AND ONYCHOPHORA 



are gradually acquired. Another branchiopod, Artemia salina, the "brine 

 shrimp," is interesting in that it thrives in waters of extremely high salt 

 concentrations. It is one of the relatively few animals able to tolerate con- 

 ditions in the Great Salt Lake, and it also occurs in coastal brine pools in 

 which sea water is evaporated to produce salt. The dried "eggs" of this 

 species are sold commercially to biological laboratories and breeders of 

 tropical fishes and will soon hatch to produce nauplii if placed in a strong 

 salt solution. 









i^L ^|r 





^ 



Fig. 15.11. Isopoda. A sow bug, ventral aspect. Note the relatively unspecialized nature of 

 the segmental appendages. (Photograph by A. Smith, courtesy Department of Zoologv, Cornell 

 University.) 



The Cirripedia, or barnacles, represent an extreme modification of the 

 crustacean form in which the animal is attached during its adult life, though 

 free-swimming as a larva. One type is the familiar rock barnacle, Balanus, 

 found encrusting on rocks and shells between tide marks in the ocean 

 (Fig. 15.10). Another type, Lepas, the "goose barnacle," is so called because 

 of its long "neck," and because it was supposed by early naturalists to be the 

 young of the wild goose. Goose barnacles are attached to the substratum by a 

 fleshy stalk and are commonly found growing on floating timbers or masses 

 of seaweed in deeper marine waters. During the change from larva to adult, 

 barnacles become attached by the anterior end; the carapace is represented 



439 



