Studies on Arthropoda. I. 



In E. Krohnii Brandt no vestige. 



In Nematobrachion boopis Calm, and Nematoscelis megalops 

 G. O. Sars the gastric crest is at its highest point slightly angular 

 and shows a vestige of the organ ; seen from the side with trans- 

 mitted light the contents of that part of the keel is lighter than 

 the surrounding tissue. 



Larvae. Some larvae belonging to the genus Euphausia 

 have been examined, but it was not possible to discover any 

 rudiment of a dorsal organ. At a future occasion I will have to 

 examine the vast material of larval forms secured together with 

 adults in the North Atlantic by the Prince of Monaco, and then 

 there will be a good opportunity to look for the dorsal organ in 

 representatives of almost all genera. 



9. Decapoda. 



The dorsal organ is found in the adults of most species of the 

 Penseidae inspected by me, and in all genera and species of the 

 Sergestidae excepting Lucifer. Furthermore it was detected in 

 several genera of the Caridea. Besides it was discovered in cer- 

 tain larval stages of the genus Sergestes and in a few larvae of 

 Penaeidae. In the following an account shall be given, but as to 

 the genus Sergestes a brief abstract may be sufficient, because 

 in a monograph of the North-Atlantic species secured by the 

 Prince of Monaco and now in composition a more detailed report 

 will be embodied. In vain I have looked for the organ in a 

 species of the genus Cambarus, and it is scarcely to be found in 

 Astacidae, Palinuridae, Galatheidae, etc. or in the crabs. 



A. Penaeidae. 



Aristeus. A. Edwardsianus Johns, has a little behind the 

 first dorsal spine the median keel thickened with its upper 

 surface expanded ; in the middle of this part is found a somewhat 

 oblong area well marked off by a depression; this area has 



