NEMERTEA ENOPLA 



655 



is the continuation of the lateral vessel ; the course of the 

 blood-vessels seems too irregular to decide anything from this 

 single abnormality. 



Another interesting fact in Siboganemertes is the absence of 

 any connexion between the dorsal vessel and the rhynchocoelo- 

 mie cavity. This is known from one form of Polystilifera, 

 A r m a u e r i a f u s c a . The Monostilifera exhibit the same 

 feature in some Prostoma species (P . a m p h i p o r o i d e s , 

 d u b o i s i , a n t a r c t i c u m , g u 1 1 i v e r i (Biirger)), and it 

 is found in Malacobdella. Wherever a dorsal blood-vessel exists 



TEXT-FIC4. 21. 



CL6.V. 



'Tt.bL 



Blood-vascular system in the tail of Pendonemertes levin- 

 seni. ff, Schema of normal individual after Brinkmann 

 (4, p. 163, Text-fig. 29, II) ; h, abnormal individual (4, p. 20, 

 Text-fig. 4) ; c, schema of this abnormality (4, p. 163, Text-fig. 

 29, III). 



in the unarmed forms, it is in connexion with the rhyncho- 

 coelomic cavity (Text-fig. 20), though other special rhyncho- 

 coelomic vessels may be present. In most Palaeonemerteans, 

 however, the dorsal vessel is quite absent. This is a rare 

 case in Hoplonemertini, and, as far as I know, it has been 

 described in Pelagonemertes moseleyi, Balaena- 

 nemertes chuni, and Carcinonemertes carcino- 

 phila. Brinkmann, guided by the opinion that the 

 Pelagica are reduced Drepanophoridae, considers these forms 



