The "dorsal organ" in Crustacea Malacostraca. 77 



found in the younger Mastigopus-stages of several and probably 

 all species, but it differs sometimes considerably in aspect, as 

 in S. pectinatiis Sund it is shaped as a thick, short spine directed 

 obliquel}' forwards. In the older lars'al stages it has apparently 

 disappeared, but 3'et it exists in all probability, because it is 

 found in adult specimens, though its aspect is quite dif- 

 ferent; I have not undertaken the certainl}^ difficult inve- 

 stigation of the dorsal integument in front of the gastro- 

 hepatic groove in older lar\'?e or very young specimens with 

 black eyes. 



Larvae of Lucifer. — In the youngest Mys?'s-stage — 

 without pleopods — of L. Faxonii Borr. a rather large and 

 considerably vaulted dorsal protuberance is found above the 

 base of the mandibles ; it is not marked off in any way from the 

 surrounding integument, and I am unable to decide whether it 

 is homologous with the very characteristic protuberance in the 

 Acanthosoma — or Mys/s-stages — of Sergestes. In older stages, 

 with pleopods, the protuberance is proportionately^ smaller. 



C. Caridea. 



Acanthephyra muUispina Cout. — A little behind the first 

 dorsal spine the median line has a small, circular depression sur- 

 rounding a more or less distinct protuberance. 



In Nematocarcinus exilis Bate a very smaU organ close at 

 the base of the first dorsal spine. 



In Ephyrina sp. from the North Atlantic the anterior crest 

 of the carapace is somewhat from its base flattened above with 

 a small but ven,^ distinct, circular, rounded protuberance, the 

 surface of which has two or three pits so tiny that they could 

 not be counted with certainty. 



Pasiphae. — In P. principalis Sund the dorsal keel is distinctly 

 thickened considerably behind the rostrum, but a protuberance 

 is wanting and no pit could be discovered. — A young P. sivado 



