730 PROCEEDINGS OF THE ACADEMY OF [DeC. , 



The Lineidse are subdivided into the Araicrurse, forms without 

 a caudicle, and the Micrune, forms with a caudicle. 



Zygeupolia agrees with the above description of the Eupoliidse in 

 one point only, i.e., in the absence of lateral slits iu the head. 

 The number of the muscle layers of the proboscis is the same iu 

 both, but the position of the layers is reversed in Zygeupolia, the 

 longitudinal muscle being the outer; the circular, when present, 

 the inner. It will be recalled that the circular layer is absent 

 from the " anterior region " of the proboscis of Zygeupolia. 



The structure of the proboscis musculature of the " middle 

 region" in Zygeujiolia agrees with what Biirger evidently regards 

 as the less common condition in the Lineidte, namely, in the 

 absence of the innermost of the three muscle layers. 



Muscular crosses are found in the proboscis of Zygeupolia ; but 

 this is a very variable character, for different individuals may 

 have a dorsal and ventral cross of equal size, a strong dorsal with 

 a faint ventral cross, or a dorsal cross only. 



The absence of a definite head gland and the presence of a cau- 

 dicle are common to both Zygeupolia and the Micruran Lineidse. 



The neurochord cells of Zygeupolia are an evidence of speciali- 

 zation, for hitherto these cells have been found only in highly 

 organized genera. 



It is evident that Zygeupolia agrees best with the exceptional 

 members of the Lineidre — i.e., with those Lineidse whose cerebral 

 canals open directly to the exterior, and from whose probosces 

 the inner longitudinal muscle layer is absent. 



When we therefore consider the position of Zygeupolia in 

 respect to the Eupoliidse and the Lineidse, it seems to belong 

 entirely to neither, but to have affinities with both. It possesses 

 the leading characteristic of the Eupoliidse, but all the other attri- 

 butes of that family are greatly modified. In general structure 

 Zygeupolia comes nearest to the Micruran Lineidse, although it is 

 evident that it must be regarded as an aberrant member of that 

 family. 



But is Zygeupolia a retrograde member of the Lineidse, or 

 merely a more simple, primitive form in process of becoming more 

 complex ? 



It is generally accepted that the Eupoliidre are more primitive 

 than the Lineidre. Now Zygeupolia, in the absence of the lateral 



