The latter observation may support the belief that P. aztecus as 

 a species is hardier than P. setiferus. Further evidence may be adduced 

 from experiments in which both species were tested simultaneously. Brown 

 shrimp completed more successful molts (at least two, often three) and sur- 

 vived longer (up to 28 days) in artificial sea water than did white shrimp. 

 However, no significant growth was found in either species. In neither spe- 

 cies did percent survival approach that of animals fed the artificial diet but 

 held in recirculating natural sea water. Apparently some compounds that 

 may have some influence upon molting are still lacking from the artificial 

 medium, this being indicated by the lesion already described and by a sec- 

 ond lesion in which the most ventral portion of the carapace did not harden 

 nor apply closely to the body so that the animal appeared to have "wings. " 

 The second type, more common in the brown than in the white shrimp, was 

 not immediately fatal. Other apparent molting disorders, or indications of 

 abnormal stress, occurred. Many of the animals in molt were unable to shed 

 the carapace. Whether this was caused by general weakness or lack of a spe- 

 cific compound is unknown. It should be noted that the test animals lost ap- 

 petite and became somewhat sluggish after several days in the laboratory. It 

 may be concluded that at present we are xinable to raise shrimp satisfactorily 

 in artificial sea water, even when the animals are supplied with trace ele- 

 ments and essential vitamins. 



48 



