BRITISH NEMERTEANS, AND SOME NEW BRITISH ANNELIDS. 385 



inner surface. The turning in of the borders of the region is an interesting cir- 

 cumstance, and demonstrates the distinction between it and the succeeding 

 region, even from the earliest condition of the worm, without for the moment 

 regarding the other cardinal facts relating to the peculiar arrangement of the 

 circulating channels on the walls, the thicker texture of the latter, and the total 

 absence of the gregariniform parasites. Moreover, it is only in this region that 

 the ciliated character of the digestive cavity is apparent, probably because the 

 greater firmness of the walls keeps the chamber somewhat distended. In certain 

 lateral views of the animal, the distinction between the oesophageal and the suc- 

 ceeding region is very evident. 



Though in the various drawings of transverse sections of Borlasia this chamber 

 (oesophageal) is seen in its normal condition, it is well to remember that 

 it undergoes very marked alterations in size, according to the condition of the 

 proboscidian cavity in its vicinity, for the proboscis most readily distends the 

 latter in this region, and bulges it so much that the walls of the former are 

 pressed flatly together at the ventral surface. In the contracted condition of 

 the worm, as after immersion in spirit, the communication between the oesopha- 

 geal and the succeeding portion of the digestive system is almost obliterated by 

 firm closure. 



The second or great division of the alimentary tube extends from the point 

 of inflection previously mentioned to the posterior end of the worm, as a ciliated 

 chamber with glandular and sacculated walls ; but the cilia, with the exception 

 of a streak near the tip of the tail, are only favourably seen on making a trans- 

 verse section of the living animal, though they are actually longer and more 

 active than those on the cuticular surface. In pale species, such as Borlasia 

 lactea, Mont. MS., the digestive canal is very distinctly divided, for the posterior 

 region is not only more opaque than the oesophageal, on account of the greater 

 development of its glandular elements, but its borders are crenate from the sac- 

 culations. The posterior aperture or anus is situated slightly in front of the tip 

 of the tail, and is well guarded by the muscular structures surrounding it, as 

 may be observed before granular matter escapes, for it requires the impulse of 

 numerous waves of fluid before yielding under pressure. In some favourable 

 specimens masses of cells and debris may be seen revolving within the dilated 

 anus before extrusion. In various examples a distinct anal papilla (Plate XII. 

 fig. 7), furnished with a tuft of longer cilia, is seen projecting posteriorly. 



In transverse section (Plate XII. fig. 3), the encroachment made on the 

 cavity by the ovaries, during the period of their activity, is well shown, and also 

 the gregariniform parasites, which often occur so abundantly in these worms. 

 The parasites were first alluded to and figured by Dr G. Johnston,* afterwards 



* Magaz. Zool. and Botany, vol. i. p. 534, pi. xviii. fig. 1 **. 

 VOL. XX V. PART II. 5 F 



