108 THE VOYAGE OF H.M.S. CHALLENGER. 



Carinellidae and in the Scliizonemertea, in fact in all Nemcrtea, with the exception of 

 the Hoplonemertea. In the latter the mouth is always in front of the brain ; it has 

 thus shifted forwards, the extreme range of this shifting process being reached when 

 the mouth becomes confluent with the opening of the rhynchodseum just recorded. 



Another difi'erence between the Hoplonemertea and the two other orders of the class, 

 with respect to the digestive system, is found in the relative j^osition of the oesophagus and 

 hind-gut. While in the two last-named groups these two subdivisions of the intestine 

 pass into each other along a straight line and do not overlap, we see that such an overlap- 

 ping does occur to a more or less considerable extent in the Hoplonemertea. In a number 

 of transverse sections the hind-gut is cut when the oesophagus is also still present in the 

 section, showing that the latter overlaps the former. Still, I should be inclined to adopt 

 the view that the gradual process by which this came about was not so much a further 

 extension backwards of the oesophagus, as a tendency of the hind-gut to spread out 

 and to reach forwards below the oesophagus. This would seem to be indicated by the 

 fact that in these Hoplonemertea the intestinal cseca, that properly belong to the hind- 

 gut, but that have come to be situated below the oesophagus (PI. XV. fig. 20), may even 

 reach so far forwards as to become situated close to the brain-lobes, a phenomenon which 

 is never observed in the lower groups, where the whole length of the oesophagus separates 

 the brain-lobes from the hind-gut. Possibly the shifting forwards of the hind-gut and 

 its diverticula may be a phenomenon that runs parallel with (if not due to the same cause 

 as) the disappearance of the lacunar blood-spaces round the oesophagus, and the substitu- 

 tion for them of cylindrical blood-vessels communicating by transverse ducts with the 

 medio-dorsal vessel. The latter arrangement is also typical of the region of the hind- 

 gut in the Schizouemertea, where, however, the circumoesophageal portion of the blood- 

 system is eminently lacunar. 



These speculations need not, however, be further insisted upon, and we may now 

 pass to a description of the oesophagus in the Challenger Palseonemertea. Here, again, 

 Cariniiia offers features of interest. In the first 23lace, the exceedingly close application 

 of the oesophageal epithelium against the muscular body- wall below and the thin muscular 

 layer of the proboscidian sheath above is peculiar (PL IV. fig. 3). There is no gelatinous 

 connective tissue between the cells and the bundles of circular muscles, not even a base- 

 ment membrane, and strong powers are wanted to demonstrate any intervening tissue 

 between the bundles themselves, so strongly are they interwoven, and so dense are the 

 muscular layers in this region of the body-wall. Anteriorly there is a sharp bend down- 

 wards where the mouth is situated, and in front of this a short bulging out forms a 

 prostomial extension to the oesophagus, which is seen to be cut through in PI. II. fig. 3. 



The cells of the oesophagus, as seen from the section figured, are finely granular, and 

 below those which actually clothe the lumen there are sometimes seen others also with 

 large nuclei (PI. IV. fig. 7) but less granular and with less distinct boundaries. This 



