

BRITISH NEMERTEANS, AND SOME NEW BRITISH ANNELIDS. 349 



leathern bottle, and is due to the narrowing of the sac in front. The pale portion 

 immediately behind the ganglia shows cilia in active motion very distinctly, but 

 I have never seen anything like an aperture ; indeed, the great and peculiar 

 stretching of this pale portion, as it is dragged backwards from the region in front 

 of the ganglia during the motions of the animal under pressure, at once demon- 

 strates the fallacy of supposing it connected with any post-ganglionic aperture, 

 as in Borlasia. The wall of the sac evidently contains some contractile fibres, 

 which cause it to dimple inwards here and there during its motions; and in 

 anterior transverse sections the cut ends of longitudinal muscular fibres are 

 shown very distinctly, though they are finer than those of the proboscis. Poste- 

 riorly, the organ opens into the digestive cavity ; but the communication is not 

 actually seen in ordinary views, from the folding together of the walls, and I 

 have not been so fortunate as to observe the animals feeding. In Polia involuta, 

 Van Beneden, the oesophagus is short and nearly globular under moderate pres- 

 sure, being also conspicuously tied posteriorly by strong transverse bands. In 

 this species the posterior aperture is very apparent. 



The relations of the oesophagus to surrounding organs may be observed in the 

 sections (Plate IV. fig. 5, and Plate V. fig. 2, at/). The walls increase in thick- 

 ness after passing the narrowed portion in front, form considerable parietes, and 

 again slightly diminish posteriorly. In transverse sections of specimens hardened 

 in spirit, and mounted in the usual manner, the structure has a streaked and 

 fibrillated aspect, or marked by a series of vertical stria?, and minutely granular, 

 an appearance due to the position of the glandular follicles with respect to the 

 inner surface, and the change caused by the preparation. It will also be 

 observed that in these sections the organ is thrown into numerous characteristic 

 longitudinal folds. In life considerable differences in appearance are observed, 

 according to the degree of pressure — as, for instance, between the flattened fol- 

 licles of the organ in a small Tetrastemma, and the thicker structure in a good- 

 sized 0. alba (Plate VII. fig. 7). In the latter, the inner edge (a) of the glandular 

 tube has a somewhat translucent and well-defined border, garnished with mode- 

 rately long and most vigorous cilia, whose activity is in strong contrast with the 

 motion of the same organs on the epidermis, and which seem to play an im- 

 portant part in the economy of the tube. Under the microscope the fresh speci- 

 men is always thrown into numerous wrinkles, and is crossed by pale streaks — 

 the ciliated edges of the folds {b). The entire organ is studded internally with a 

 series of granular glands or follicles, and numerous brownish pigment-granules. 

 The glands taper towards the free ciliated edge of the ruga?. 



In 0. melanocephala the organ is curiously narrowed posteriorly; and in 

 0. pulchra the granular glands are distinct and large. In T. varicolor the 

 glandular appearance in a small specimen under pressure is somewhat finer and 

 more translucent, but the structure is essentially the same as in Ommatoplea. 



VOL. XXV. PART II. 4 U 



