NEMERTEA ENOPLA 653 



place. What is the contmuation of the oesophagus m 

 A. marmoratus ( Joubin) is the ventral impaired diverti- 

 culum of Biboganemertes, and the true gastric cavity lies 

 enclosed between the pyloric tube and the ventral diverticulum. 

 All parts of the stomodaeum are unpaired in Siboganemertes 

 but the intestinal blind-gut shows at the side of ventral 

 unpaired pouches paired lateral diverticula that are longer 

 than the blind-gut itself, as in A. marmoratus (Joubin). 

 Nothing of this kind was ever found in Polystilifera, though 

 some very interesting features are known from Brinkmann's 

 studies on the bathypelagic species. Not only do they show the 

 absence of an oesophagus, but the whole stomodaeum is very 

 short and much less differentiated. In almost all his figures 

 the pylorus is already shown beneath the brain, and as a rule 

 the blind-gut extends till here. In fig. 9, PI. xv, a longitudinal 

 section shows the short and narrow gastric cavity of B a 1 a e n a - 

 nemertes musculocaudatus; fig. 13, PI. xii (Text- 

 fig. 26), gives the same features in Nectonemertes primi- 

 tiva, and Brink mann states that in N. minima the 

 epithelium of the gastric cavity is unfolded, the cavity still 

 narrower and shorter. Brinkmann takes these forms as 

 the most reduced ones. However, how can we explain these 

 differences in the same structure within a monophyletic group, 

 as the Polystilifera certainly are, the highly differentiated gastric 

 cavity of Siboganemertes, the quite differently but not less 

 highly developed structures of the Drepanophoridae and the 

 more or less simple stomodaeum of the Pelagica, if we do not 

 suppose these to be primitive ? 



The stomodaeum in armed Nemerteans is a structure different 

 from that in the Anopla, as is shown by its development and 

 by the presence of an entodermal blind-gut in the first. We 

 know it to he a simple structure, a mere narrow tube in Oto- 

 typhlonemertes, in Zygonemertes it is not much more ; we 

 know that the oesophagus is absent in Geonemertes, in Sticho- 

 stemma. Why then must the Pelagica, that have the same 

 peculiarities, have developed from highly differentiated forms 

 as Drepanophorus ? On the contrary we see here how the simple 



