664 GKllARDA STIASNY-WIJNHOFF 



still is absent, to a large vessel, that communicates in the tail 

 with the lateral vessels. 



7. That the reproductive system shows quite a number of 

 characteristic features that are unknown in all other Nemerteans 

 and cannot have developed from stages known in Monostilifera 

 and Anopla, nor from those in the Reptantia ; that the influ- 

 ence of the pelagic habits, which caused reduction of the 

 number of eggs as these grew larger, and copulation, cannot 

 accoimt for all these facts, though the ventral position of the 

 gonopores may be due to it. 



Here I might call attention to another fact of importance. 

 If we look at the figures of the pelagic Nemerteans it must be 

 evident to everybody, especially on comparison with illustra- 

 tions that give the anatomy of the whole animals, that the 

 region we call the head in other Nemerteans, or the precerebral 

 region, is absolutely absent. As already stated above, the 

 insertion of the proboscis and the muscular septum before the 

 brain mark the place where originally the invagination of 

 the proboscidian system took place. This we see actualty in the 

 pelagic forms ; the rhynchodaeum is only very short and there 

 is no true head region (Text-fig. 26). Brinkmann states 

 several times that the rhynchodaeum may be extremely short, 

 and only in this way can we understand a dorsal migration of 

 the proboscis pore that comes to lie above the brain as in 

 Armaueria or Parabalaenanemertes, In the primitive genus 

 Siboganemertes the precerebral region is extremely short too, 

 and the broad line which demarcates the proximal end of the 

 animal reminds one of the same feature in the headless Pelagica. 

 The brain lies directly behind the septum, i. e. quite terminal^, 

 as seen in all the illustrations of B r i n k m a n n . A comparison 

 of Text-figs. 25 and 26 with Text-figs. 2, 3, 6, and 8, shows 

 this very clearly. In the armed Nemerteans a displacement 

 of the mouth goes hand in hand with the development of the 

 head, and in consequence of this the development of the 

 stomodaeum and oesophagus. If we understand the head 

 region of Nemerteans in this way the difference in the structure 

 of the body-wall before and behind the brain at once becomes 



