The "dorsal organ" in Crustacea Malacostraca. 75 



slightly before the middle an oblong, dark-brown spot sharply 

 denned and with at least one minute pit. In A . semidentatus 

 Bate is found a moderately small but uncommonly deep de- 

 pression a little behind the first dorsal spine; the depression has 

 a small, circular, somewhat convex area behind the middle. 



Solenocera Agassizii Fax. A little behind the first dorsal 

 spine an oblong, rather depressed area with a small knot at the 

 middle. 



Penceus. P. setiferus L. has considerably behind the first 

 dorsal spine a proportionately somewhat long part of the median 

 keel thickened and flattened above or even longitudinally a 

 little excavated; the broadest part of that area has in one spec- 

 imen an oblong pit, in another specimen nothing. It must be 

 remarked that the area in question is situated more backwards 

 than in any other genus of the Malacostraca. In P. caramotc 

 Risso nothing could be discovered. 



Larvae. In a gigantic larva in the My sis-stage, measuring 

 20.5 mm. from the tip of rostrum to the end of telson and cap- 

 tured in the Pacific, a small, oblong and rather low protuberance 

 is easily seen a little behind the origin of the rostral keel. - 

 In a larva in an older Mysis-stage, belonging to a quite different, 

 somewhat small type, an oblong and somewhat high protuber- 

 ance is found a little in front of the gastro-hepatic groove; seen 

 from the side the protuberance is directed upwards and forwards ; 

 the larva is from lat. 4 2 / 3 N., long. 107* f z E. 



B. Sergestidas. 



Sergestes. The organ has been found in adult specimens of 

 everyone of the 15 species hitherto captured north of equator, 

 but it could not be seen in every specimen of some of the smaller 

 species, as 5. mollis Smith, 5. arcticus Kr., S. atlanticus M. Edw., 

 5. vigilax Stimps., and S. Edwardsii Kr. The organ is situated 

 a little or somewhat in front of the gastro-hepatic groove. It 



