The "dorsal organ" in Crustacea Malacostraca. 73 



among the Oniscoidea Ligia oceanica L. and the gigantic Tylos 

 granulatus Krauss have been examined without finding any 

 vestige of the dorsal organ. 



7. Amphipoda. 



Large specimens of genera comprising forms of very consider- 

 able size, as Anonyx, Stegocephalus, Gammarus, Maera, have 

 been inspected, but the result was absolute negative. 



8. Euphausiacea. 



Thysanopoda.' In T. egregia H. J. H. the gastric keel 

 between the front margin and the gastric groove has somewhat 

 behind the middle a short part raised rather considerably; this 

 part is cut off horizontally so that its upper surface is flattened, 

 and this surface shows an aspect different from that of the 

 surrounding integument. (The raised part is shown in fig. 21 

 in my paper on Schizopoda in Bull. Mus. Ocean. Monaco, no. 

 30, 1905, p. 23) T. cornuta Illig shows a similar structure. 

 In T.cequalis H. J. H. nothing could be perceived. T. microph- 

 thalma G. O. S. has a feeble vestige on the gastric keel which 

 on its highest part is a little flattened above. 



In Bentheuphausia amblyops G. O. S. a vestige is scarcely 

 perceptible. 



In Meganyctiphanes norvegica M. Sars the gastric keel has 

 a somewhat high and rather short part thickened upwards with 

 the dorsal surface distinctly flattened; seen from the side with 

 transmitted light the contents of the raised part is generally 

 lighter than the surrounding tissue. 



Euphausia. In E. superba Dana the median keel is at some 

 distance in front of the gastric groove more or less distinctly 

 raised and thickened, and at each side of this part the surface 

 of the carapace is distinctly excavated, but the keel itself is 

 rounded above without vestige of any special structure. 



