The "dorsal organ" in Crustacea Malacostraca. 71 



Lophogaster. In L. typicus M. Sars no vestige is discover- 

 able. 



Boreomysis. B. scyphops G. O. S. has somewhat in front 

 of the gastric groove a moderately large and rather shallow 

 depression, at the middle of which is seen a somewhat low, a 

 little oval, rounded and very distinct protuberance with one 

 to three minute pits. B. nobilis G. O. S. has very near the 

 gastric groove a small but rather high, a little oval protuberance 

 without visible pits. B. microps G. O. S. has a little in front 

 of the gastric groove a conspicuous, nearly semiglobular, smooth 

 protuberance. 



Of the subfamily Mysinae two moderately large forms, viz. 

 Mysideis grandis Goes and My sis mixta lyilljeb., have been 

 inspected, but no vestige of any organ was found. 



4. Cumacea. 



Some few larger forms have been examined, but nothing 

 could be found. 



5. Tanaidacea. 



Some large specimens of Apseudes and Alaotanais, thus 

 representatives for both families of the order, have been in- 

 spected with negative result. 



6. Isopoda. 



Idothea. In /. balthica Pall, is frequently found near the 

 posterior margin of the head a very small area a little broader 

 than long, and with a few tiny pits or, in one specimen, with 

 impressed transverse lines; the area is generally a little or 

 somewhat depressed, but in one specimen it is larger than usual, 

 raised and shining. In a well-sized specimen from Iceland the 

 area is uncommonly conspicuous, moderately large, very slightly 

 depressed, shining, with a large, oblong pit in the middle, and 



