1903] On the Norner teans of Norway. 15 



containing composite glands in its deeper portion. In comparing 

 the anatomy of the two groups it seems natural to regard the 

 circular muscle layer as homologous in each case; and on this 

 assumption it follows that the position of the nerve stems with 

 respect to the muscle layers has remained unchanged in the 

 two orders, i. e. that there has been no sinking in of the nerve 

 stems such as has undoubtedly occurred among the Metane- 

 merteans. This view of the homologies of the muscle layers 

 was advocated many years ago by Hubrecht 1 ) and has recently 

 reseived strong support from Miss Thompson^ discovery of an 

 internal circular muscle layer in Zygeupolia 2 ). Consequently, 

 starting with the condition found in Carinella (Pl. II, fig. 16 a) 

 we must imagine that something like the following processes 

 have occurred before ariving at the more complex Heterone- 

 mertean state. The composite glands of Carinella sink in 

 deeper and between them and the epithelium a layer of longi- 

 tudinal muscle fibres appears. This condition is found in 

 Carinoma (Pl. II, fig. 16 b). The separation of the cutis glands 

 continues and the longitudinal muscle fibres become more strongly 

 developed. At the same time a new basement membrane makes 

 its appearance just beneath the epithelium (Pl. II, fig. 16 c) whilst 

 the original basement membrane (b.m') becomes muen reduced. 

 This is the condition found in the nephridial region of Linens 

 niveus, and also in Zygeupolia and Micrella 3 ) with the excep- 

 tion that the original basement membrane (bm') has entirely 

 disappeared. Lastly the longitudinal muscle fibres develop 

 into a thick layer and take up a position entirely beneath 

 the cutis from which they often become cut off by a layer of 

 gelatinous connective tissue (Pl. II, fig. 16 d). At the same time 

 circular, and sometimes longitudinal, muscle fibrils are developed 

 just beneath the secondary basement membrane (bm.). Various 

 minor modifications are met with in this condition which however 

 obtains generally for the bulk of the Heteronemerteans. 



On the view advocated above the structure of the body 

 wall in L. niveus is more primitive than that of most Hete- 

 ronemerteans, and the primitive nature of the species is further 



2 ) Challenger Reports, The Nemertea, 1887, P. 70. 



2 ) Proc. Acad. Nat. Sc. Philadelphia 1901, p. H67. 



3 ) Quart, journ. mic. Sc. 1901, Pl. 39, hg. 7. A few circular muscle fibrils 

 occur beneath the basement membrane. 



