728 PROCEEDINGS OF THE ACADEMY OF [DeC, 



The blood system of Zygeupolia is of the Heteronemertean type, 

 Huhreclitia being the only Protonemertean that approaches it ia 

 any way, namely, in the presence of a dorsal blood vessel. 



In the structure of the nervous system and the cerebral sense 

 organs Zygeupolia is again a Heteronemertean ; but by no means 

 the highest tj'-pe of brain or sense organ is represented, Zygeupolia 

 having a very simple Heteronemertean brain and cerebral sense 

 organ. 



The presence of muscular crosses in the proboscis and its general 

 structure are further characters in common with certain Hetero- 

 uemerteans. 



The absence of lateral slits is a primitive character, and one 

 common to all the Protonemerteans and to the more primitive 

 Heteronemerteans. This character, as we know, is possessed 

 by Zygeupolia. The inner circular muscle layer of Zygeupolia is, 

 in my belief, a primitive character, and the short extent of the 

 layer is explicable on the grounds that the thickened region in 

 front of the middle intestine is only the remnant of a layer that 

 was once continuous throughout the body. 



The presence of this muscle layer in a limited region in Micrura 

 ccBca, and of a similar layer in M. alaskensis, Coe (1901), shows 

 that an inner circular muscle layer exists in two genera of the 

 Heteronemerteans. The dorso-ventral fibres of the Hetero- 

 nemerteans, regarded by Burger as derived from an inner circular 

 layer, and the so-called "oesophageal muscles" ("Darmrauscu- 

 latur") which, according to my view, are derived from the deflec- 

 tion and bending around of dorso-ventral fibres, are other evi- 

 dences of the remains of an inner circular muscle among the 

 Heteronemerteans. 



The lateral grooves of Zygeupolia, if they are sense organs, may 

 possibly be homologized with the side organs of Carinella. Their 

 position in the median lateral line of the body, and their charac- 

 ter as epithelial grooves, both agree with the side organs; but 

 until their undoubted sensory character is proved, the comparison 

 should not be emphasized. 



A brief summary of the structure of Zygeupolia shows that it is 

 a Heteronemertean, on account of the presence of the outer longi- 

 tudinal muscle layer, the position of the lateral nerves, the struc- 

 ture of the alimentary system, of the blood system, of the nervous 



