74 Studies on Arthropoda. I. 



' In E. KroJinii Brandt no \'estige. 



In N ematohrachion boopis Calm, and Ncmatoscelis mcgalops 

 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. 



lyarvse. — Some larva? belonging to the genus Euphausia 

 have been examined, but it was not possible to discover ain- 

 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. 



q. Decapoda. 

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

 Penaeida? inspected by me, and in all genera and species of the 

 Sergestidse excepting Lucifer. Furthermore it was detected in 

 several genera of the Caridea. Besides it was disco\ered in cer- 

 tain larv^al stages of the genus Scrgcstcs and in a few larvae of 

 Penseida?. 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 Cambanis, and it is scarcely to be found in 

 Astacidse, Palinuridse, Galatheidse, etc. or in the crabs. 



A. Pena'idte. 



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



first dorsal spine the median keel thickened with its ui)per 



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



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



