BRITISH NEMERTEANS, AND SOME NEW BRITISH ANNELIDS. 351 



immediately behind the hearts (ganglia), under the character of a csecal end, 

 and as "a perfectly closed sac, containing a milky fluid." The walls of this 

 cavity, he says, act upon the exuded food, after its passage through the walls of 

 the " oesophagus." He is correct in denying the ovarian character of the organ, 

 and in showing that the so-called ova consisted only of oil-globules. He has also 

 some reason for considering the transverse segmentation of the organ as an indi- 

 cation of annuli* Dr Max ScHULTZEf described it as a straight canal in Tetra- 

 stemma obscurum, ciliated on its inner surface, and opening anteriorly and pos- 

 teriorly, and figures | the cells in its walls as altered by extrusion into the water. 

 M. Claparede, in the before-mentioned figure of Prosorhochmus, shades the 

 region, but makes no mention of it in his description. 



The digestive cavity is a somewhat moniliform or lamellated canal, in so far 

 as its surface is increased by the numerous diverticula. Its appearance under 

 pressure is well seen in" Tetrastemma (Plate VIII. fig. 3) as a lobulated glandular 

 organ, usually, of a pale flesh or slightly pinkish hue, extending from a short dis- 

 tance behind the ganglia to the tip of the tail, and forming (in the individual in 

 which the reproductive elements are not developed) a lining to the body-wall, 

 except where interrupted by the proboscidian sheath. In the ripe animal, how- 

 ever, the gradual enlargement of the ova or sperm-sacs pushes in the yielding 

 organ, so that it occupies a more median position, and has its ventral portion in- 

 creased in bulk. It is also well to bear in mind that the body of the adult worm 

 is only rounded in contraction, and partly so when the ova or spermatozoa are 

 mature, but at other times it is flattened, and very mobile ; thus, what is space 

 in the transverse section is often filled up in the living animal by the collapsing 

 and contraction of the yielding tissues in the neighbourhood. Anteriorly the 

 only opening leading into this chamber is that of the posterior end of the rugose 

 oesophagus ; posteriorly it terminates in an anal pore, less easily seen than the 

 similar structure in Borlasia, from the absence of the strongly ciliated internal 

 line. In intimate structure the walls of this cavity resemble the anterior or 

 oesophageal portion, only the gland-cells are larger and more numerous, and 

 the fatty elements in greater abundance, so that although the type of struc- 

 ture remains, there are considerable differences in microscopic appearances. 

 I was for a long time in doubt about the ciliation of this chamber in 

 Ommatoplea, since I have seldom been able to see cilia satisfactorily in the 

 uninjured 0. alba, though in the latter, 0. purpurea, Tetrastemma, and especially 

 in Polia involuta, Van Beneden, peculiar motions of the cells were apparent. 

 When a specimen is kept for some time under pressure, a few moving granules 

 are observed at some particular point; these continue to increase in number, 

 and sometimes a few cells accompany them, the group gradually enlarging and 



* Philos. Transact. 1858. f Op., tit. p. 64. | Op. tit. taf. i. fig. 35. 



