CIRCULATORY APPARATUS OF THE NEMERTHA. 17 
operate, and not in an insignificant manner, considerably to 
diminish the lumen of the lacune. 
These two different types of communication of the lacune 
in the head I have described from specimens cut into trans- 
verse sections, but it would not surprise me if there are more 
types or variations, having found these two types in only four 
specimens. I must remark that these two types occur as well 
in the first form as in the second. (See the beginning of this 
description, p. 15.) 
We now approach the csophageal region. In those indi- 
viduals where there is a large communication between the 
lacune of the head beneath the probiscidian sheath the 
mouth becomes visible when this lacuna disappears. The 
lacune, which down to this point have been situated on the 
sides of the probiscidian sheath, now descend to a level between 
the probiscidian sheath and intestine (figs. 19 and 20), as in 
Cephalotrix (fig. 21). They have not lost their circular 
fibres, and also preserve their inner investment of the thin 
hyaline basal layer, and the beautiful epithelium of little 
round cells, with relatively large nuclei. Still further back- 
wards the lacunz descend still further, and now lie on each 
side of the intestine, but so that their upper wall is just as 
high as the upper limit of the intestine. 
In the esophageal region I have to mention apeculiarity which, 
as far as I know, has not yet been seen in other forms. The 
lacune show numerous dorsal dilatations, a portion of which 
is separated from the bulk of the lacuna by a sometimes 
thin, sometimes large band. This band consists for the greater 
part of hyaline basal tissue, but some of the circular fibres 
also enter the band. I have represented it in figs. 2 and 4 
diagrammatically. On the ventral border of the lacune 
arches, as indicated in the figure, only now and then occur, 
A transverse section through such a region is shown in the left 
half of fig. 23, and the more common case of arches directed 
upwards, in the right half of fig. 23 and in fig. 24. Rarely 
an arch directed upwards and one downwards were seen in the 
same section. In a younger specimen these separating bands 
9 
ow 
