1901.] NATURAL SCIENCES OF PHILADELPHIA. 731 



slits, possesses a character that is general in the Eupoliidse and 

 exceptional in the Liueid^e; the number and arrangement of the 

 muscular layers of the proboscis make a second character only 

 occasional in the Lineidu>, and, finally, variations occur in the num- 

 ber of the proboscis crosses in ZijgeupoUa, a character that is con- 

 stant in the Lineidie. Therefore Zygeupolia, with one primitive 

 character, a second corresponding to a reduced number of parts 

 in the Lineidte, and a third that varies in different individuals, 

 is undoubtedly a form in transition from a more simple and primi- 

 tive condition to a complex state. It may be regarded as the 

 most primitive member of the Lineidse yet described." 



In general external characters, Zygeupolia comes nearer to the 

 genus Mierura than to any of the other genera of the Lineidse. 

 The generally small size, the body more or less rounded poste-" 

 riorly and the presence of a caudicle are characters common to 

 both. To Mierura caca, Zygeupolia bears a most striking resem- 

 blance in size, shape and color, and the two can scarcely be distin- 

 guished except with a hand lens, which reveals the presence of 

 lateral slits in the former and their absence in the latter. 



The relation of the Heteronemerteans to the Protonemerteans 

 is an interesting question. The position of the lateral nerve 

 chords is relatively the same in both, the outer longitudinal muscle 

 layer of the Heteronemerteans being merely a later formation from 

 the ectoderm (Burger, 1894), and the cutis glands being formed 

 by the sinking beneath the surface of certain epithelial gland cells. 

 Lateral slits are absent among the more simple Eupoliidse, and I 

 hope to have shown in this paper that an inner circular muscle 

 layer, or its derivatives, is quite common among the Heterone- 

 merteans. 



It seems to me that the Heteronemerteans are very closely related 

 lo the Protonemerteans, the Lineidai being connected by forms like 

 Zygeupolia, the Eupoliidse and HuhrechUa, and that the Metane- 

 merteans and the Mesonemertean Cephalothrix are widely divergent 

 forms. 



'* Since the above was written the new Heteronemertean, jll/tcre??a ?•«/« 

 Punnett (1901 6), has been described as the most primitive member of the 

 Lineida-. But Zygeupolia, in entering the family of the Lineida% must 

 assume the lowest position until an even more primitive form is discovered. 

 Within the last few years so many new Nemertciin genera have been found 

 that we may look confidently lor further additions to the group. 



