1901.] 



SOME ARCTIC NBMBRTEAKS. 



103 



missure, within the nervous ring. This last arrangement, which 

 I take to be the true one, is also shown by Biirger in section for 

 other Metanemei'teans, viz., for Eunemertes marioni (pi. xv. 

 fig. 10), for Drepanoplwrus cdholineatus (pi. xvii. fig. 2), for 

 Tetrastemma cruciatum (pi. xviii. fig. 6), and for Ototyj>hlone- 

 mertes duplex (pi. xviii. fig. 17). On pi. xviii. fig. 3, however, 

 Biirger figures a section through the brain and its commissures 

 in Mcdacohdella grossa, where the lateral vessels are shown 

 lying entirely outside the nervous ring. In a series of sections 

 made through a specimen of this species, I have been unable 

 to confirm this arrangement. In my specimen the cephalic 

 loop divided just before the brain, and whilst one branch ran 

 along the lateral edge of the body, as in Biirger's figure, the other 

 Isbj just over the ventral brain-commissure, and consequently 

 within the nervous ring. The vessels lying outside the nervous 

 ring are probably to be regarded as secondary. In many Metane- 

 merteans (esp. AmpJiiporus and Drepanopliorus') the cephalic and 



Text-figs. 4-6. 

 5. 



rCs 



S^ 



dorsal 

 ■vessel 



lateral 

 ..-' vessel "--- 



f 



Amphj^iorus, &c. Malacohdella. Malacobdella according 



to Biirger. 



lateral vessels are strongly bent just before entering and just 

 after leaving the nervous ring (text-fig. 4). It is probably by the 

 confluence of these arches that the outer lateral vessels of Mala- 

 cobdella are formed (text-fig. 5). If now we suppose the portion 

 of the lateral vessels lying within the nervous ring, anterior to the 

 point of origin of the median dorsal vessel, to disappear, we arrive 

 at a condition similar to that described ,by Biirger (text-fig. 6). 

 Such a condition must, however, be regarded as secondary, and 

 derived from the normal Metanemertean type. The conclusion 

 then, I think, is justified, that in the Metanemerteans, as in all 

 the rest of the phylum, the lateral blood-vessels pass throiigJi the 

 nervous ring formed by the brain and its commissures, and that 

 the limbs of the cephalic loop unite at the tip of the snout above 

 the rhynchodaeum. 



I have laid upon this point partly because of its morphological 

 importance, and partly because eiToneous statements on this head 

 tend to become perpetuated in text-books. Thus in the ' Cambridge 

 Natural History,' vol. ii. p. 106, Miss Sheldon gives a diagram in 



