76 Studies on Arthropoda. 



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

 ably raised as a rounded, nearly circular or transverse-oval 

 protuberance, sometimes, but far from always, marked off by 

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

 Frequently the area has a very 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. obesum Kr. is found a 

 moderately small, a little oblong and feebly 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 five 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. 



Lucifer. No trace of any organ. 



I^arvae of Sergestes. As mentioned above (p. 66) Wasser- 

 loos discovered the dorsal organ but did not interpret 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 procurved, 

 rounded protuberance in the Atlantic Acanthosoma-stages known 

 to me and belonging to S. arcticus Kr., 5. tenuiremis Kr., S. ro- 

 bustus Smith, S. corniculum Kr., and 5. vigilax Stimps. ; the 

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



