1901.] 



SOME ARCTIC NEMBBTEAirS. 



tapering sacs, the widest portion of which, is near the ducts (Plate 

 VI. fig. 5). A most interesting feature is that they show ova at 

 all the various stages of development. These are referred to in 

 more detail on p. 104. The brain is well developed. In shape 



Drepwnqphorua borealis. Section through the commissural region of the brain, 

 showing the opening of the cerebral organ. X 30. 



hm., basement-membrane ; ec, ciliated canal of cerebral organ ; cl., cephalic 

 vascular loop ; dr.. dorsal nervous commissure ; dff., dorsal ganglion ; 

 cp., epithelium ; get., gelatinous connective tissue; mo., circular muscle- 

 layer; ml., longiiudinal muscle-layer; oes., oesophagus; ps., proboscis- 

 sheath; rd., rhynchocephalic diverticulum; rde., expansions of rhyncho- 

 ccelomic diverticula ; vc, ventral brain-commissure ; vg., ventral ganglia. 



it is considerably flattened, so that the dorsal ganglion comes to lie 

 on the outer side of the ventral, and not above it as is usually the 

 case. The ventral commissure is short, straight, and strong ; the 

 dorsal thinner and more curved. A Bingle large neurochord-cell 

 occurs on either side. It is oval in Bhape, measuring about 83 u x 

 40 /i. Its nucleus measures 25 /i x L8 /u and contains a well-marked 

 circular nucleolus 8 /< in diameter. 



The lateral aerve-cords li<' ventrally, and each is situated at the 

 same distance from its fellow as ii is from the lateral margin of 

 the body. They are united by ventral commissures al intervals. 



Proc. Zool. Boo.— 1901, Vol. EL No. A r II. 7 



