NEMERTEA ENOPLA 649 



everywhere a very distinct border of the musculature (Text- 

 fig. 19). Pro- and Parabalaenanemertes, Balaenanemertes, 

 Nato-, Nectonemertes, and Chuniella show us this type of 

 rhynchocoelom, as found in all Monostilifera too, and so does 

 the greater part of the sheath of Pelagonemertes. The develop- 

 ment of a third layer, which might be indicated where the 

 proboscis is affixed, never took place, as I will now demonstrate. 

 The interlacing of circular and longitudinal muscle-fibres, 

 which in Pelagonemertes took place in the hinder part of the 

 rhynchocoelomic wall, is found in the genera Protopelago- 

 nemertes, Plotonemertes, Pendonemertes, Mergonemertes, 

 Paradino-, Phallo-, Crasso-, and Planktonemertes, in all 

 Reptantia and in Siboganemert es weberi. Whether 

 the new circular muscle-coat of Brinkmann is present or 

 not we cannot decide in these genera ; in Pelagonemertes it 

 certainly is absent, and in the Malayan species referred to 

 above the interlacing took place between the two original 

 layers, as I will describe in Siboganemert es. The proboscis 

 has a thin outer circular muscle-coat and a thick longitudinal 

 coat, but the new layer is absent. The precerebral septum 

 which connects the longitudinal musculature of the body with 

 the rhynchocoelomic wall and proboscis lies exactly in front 

 of the brain. Inner circular muscle-fibres between this septum 

 and the endothelial lining of the cavity of the sheath fail 

 absolutely ; a great quantity of fine mesenchymatic fibres, 

 which stain quite differently and are found at many places in 

 the body parenchyma too, lies inside the endothelium and 

 between these the first longitudinal fibres are embedded. 

 Outside of these the first circular fibres appear behind the 

 nerve-ring and they are very few. The whole muscular wall 

 is thin ancj, in the ventral part, disappears but for a few- 

 longitudinal fibres. It is, however, quite clear that in the 

 proximal part an interlacing of fibres takes place, and here 

 certainly the new circular layer has not developed. It is absent 

 in the proboscis too. From the results obtained in Siboga- 

 nemertes, and from similar facts in some Drepanophoridae and 

 in Pelagonemertes, I might conclude that the development of 



