562 



of adult individuals. The youngest of these was at the six-pair-cirri 

 stage the shell measuring 0,6 mm in length; while the largest had a well 

 developed foot and was 4 mm in length. One could readily notice striking 

 differences between these young and those with four pairs of cirri as de- 

 scribed by Fritz Müller, Blochmann and myself. In the present 

 specimens the larval setae have been replaced by long definitive ones. 

 In the older ones (3,5 — 4 mm) setae much longer than the shell are 

 found along the anterior margin of the mantle. The tentacle of Disci- 

 nisca shortens much earlier than that of Lingida. Even at the six-pair- 

 cirri stage it has already diminished into a small elevation on the epistome. 



Fiff. 1. 



Fi^. 2. 





Fig. 1. Young Diseinisea at the eight-pair-cirri stage (0,8 mm) ; mantle and left half 

 of body omitted; one of the muscles not shown in this figure for the sake of clear- 

 ness. X 162,5. ot^ otocyst; neph, nephridium. 

 Fig. 2. A portion of a longitudinal section of a young Diseinisea of 4 mm ; shell not 

 represented. X 105. ot, otocyst; neph, nephridium. 



In Lingula the tentacle is absorbed at much later stages (1. c. p. 91) ; even 

 as late as at the ten-pair-cirri stage the tentacle is still of fairly large size. 

 In the young Diseinisea under consideration, i. e., from the six-pair- 

 cirri stage forward, one finds a pair of nephridia very similar to those I 

 found in the Lingula larvae (Fig. 1 and 2 neph). Anteriorly they open 

 near the lateral ganglia, and run along the ventral body wall. Posteriorly 

 they dilate into funnels, which are suspended by the ileo-parietal bands. 

 Judging from the fact, that Blochmann found no nephridia in his 

 specimens, it seems probable that these organs make their appearance 

 at about the five-pair-cirri stage. 



