76 studies on Arthropoda. I. 



is a small or very small area, in most cases distinctly or consider- 

 ably raised as a rounded, nearly circular or transYerse-o\'al 

 protuberance, sometimes, but far from always, marked off b}' 

 an depressed line; in a few cases the whole area is depressed. 

 Frequently the area has a ver>'' distinct granule or raised point 

 in front of the middle, in other cases a few tiny pits or no pit 

 could be made out. In a few cases the area is brown, but gener- 

 ally of the same colour as its surroundings. There is also some 

 individual variation in the area as to its height or the depth of 

 its surrounding impression. 



Petalidium. — In one specimen of P. obesuni Kr. is found a 

 moderately small, a little oblong and feebl}' elevated area with 

 a tiny and more raised point in front. In some specimens an 

 area was not discoverable with certainty, but the quality of the 

 integument makes the investigation difficult and uncertain. 



Sicyonella. — In one specimen of 5. maldivensis Borr. a 

 distinct, small group of fi\'e minute pits close behind the dorsal 

 crest; in another specimen the group has nearly vanished. 



Acetes. — In one specimen of A. vulgaris H. J. H. a group of 

 six minute pits close together and somewhat in front of the 

 rudimentary^ gastro-hepatic groove; in another specimen a 

 rudimentary^ protuberance with vestiges of a few pits. 



Lucijer. — No trace of any organ. 



Larvae of Sergestes. — As mentioned above (p. 66) Wasser- 

 loos discovered the dorsal organ — but did not inter])ret it as 

 such — in the second Protozoea-stage and the Acanthosoma- 

 stages of 5. arcticus Kr. ; the present writer had found the 

 protuberance in question in the youngest Mastigopus of the same 

 species. The dorsal organ is an erect or distinctly ])rocurved, 

 rounded protuberance in the Atlantic Acanthosoma-stag'^s known 

 to me and belonging to .S". arcticus Kr., S". tcniiiremis Kr., .S". ro- 

 bustus Smith, .S. c<)r)iicnlitni Kr., and .S". vigilax Stimps. ; the 

 protuberance is generally easily seen from llie side. It is also 



