636 



GERARDA STIASNY-WIJNHOFF 





7 



develop only in front of this region, at the side of or directly 

 behind the brain, a unique fact in Nemerteans. The Eeptantia, 

 containing the genera Drepanophorus and Uniporus, and, 

 as the Siboga material shows, quite a number of other genera, 

 that crawl about at the bottom of the sea and its coasts, 

 have as a rule eyes and metamerical blood-vessels, but always 

 they possess cerebral organs, nephridia, rhynchocoelomic 

 diverticula, and metamerically placed o^ gonads. Especially 

 the cerebral organs are different from those of the Monostilifera 

 and the development of a sac in this organ as well as the 

 presence of diverticula of the proboscis sheath show that the 

 Eeptantia are widely different from the Monoptilifera. Almost 

 all Polystilifera that the Siboga expedition brought home 

 belong to the Eeptantia. About one form only there can be 

 any doubt, as it is collected by the deep-sea trawl to the south 

 of Timor at a depth of 883 metres. This is the depth in which 

 most pelagic forms occur and, as occasionally pelagic Nemer- 

 teans can and have been caught by the trawl, we might be 

 in doubt as to the manner of life of this Nemertean. Moreover 

 the inner structure of Siboganemertes weberi reveals 

 such peculiarities that we cannot with certainty decide 

 anything. It has no eyes, but Uniporus, a genus of Eeptantia 

 of the Norwegian sea, living in the dis- and aphotic regions, 

 lacks them as well. It possesses cerebral organs, but they 

 are quite minute and of a much more primitive structure 

 than anything known. Nephridia are present, but metamerical 

 blood-vessels fail as in PolystilifefaT^ and Uniporus. Ehyn- 

 chocoelomic diverticula are present, but instead of lying 

 peripherically at the outside of all organs as in all Eeptantia 

 (Text-fig. 10) they lie inside between the proboscis sheath and 

 the digestive tract (Text-fig. 9). The testes are placed mefa- 

 merically, but they display features that we do not know in 

 other Polystilifera. The mouth lies under the brain, which 

 in its structure shows a great resemblance to the Pelagica and 

 differs greatly from the Eeptantia. The digestive tract, which 

 lacks an oesophagus in the pelagic forms and has a well- 

 developed one in the Eeptantia, has quite a short balloon-like 



