DIQESTIVE SYSTEM. 107 



large space is occupied by the proboscidian sheath (a), and the great lacunae {s, s), and indications 

 of some other vascular meshes are seen at the sides. The margins of the mouth (w) curve inwards, 

 and gradually merge into the ciliated glandular texture of the cavity. A little further back the 

 glandular substance is confined to the inner surface of the body-wall (though not closely 

 applied thereto), leaving a large central space. In full perfection the chamber and glandular 

 texture are seen in Plate XX, fig. 3. The minute structure of the wall of this portion of the 

 digestive cavity is similar to that of the ciliated oesophageal region in the Enopla, being composed 

 of a thick layer of granular gland-cells and basement-substance, raised here and there into pro- 

 minent rugae, and possessing a rich coating of cilia on the inner surface. The incurvation of the 

 borders of the region is an interesting circumstance, and demonstrates the distinction between it 

 and the succeeding division, 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 cilia of the digestive cavity are apparent, probably 

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

 lateral views of the animal (Plate XXII, fig, 4), the distinction between the oesophageal and the 

 succeeding region is very evident, the point of junction being inflected in a characteristic manner. 

 Though in the various drawings of transverse sections of Lineus 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 oesophageal and the succeeding portion 

 of the digestive system is almost obliterated. 



c. Alimentary Cavity Proper. 



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

 previously mentioned to the posterior end of the worm, in the form of 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 well seen on making a transverse section of the living animal, though they are actually 

 longer and more active than those on the cuticular surface. In pale species, such as Lineus 

 lacteus, 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 sacculations. 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 (Plate 

 XIX, fig. 6) masses of cells and debris may be seen revolving within the dilated anus before 

 extrusion. In various examples a distinct anal papilla (Plate XVII, fig. 22), furnished with a 

 tuft of longer cilia, projects posteriorly. 



In transverse section (Plate XXI, fig. 1), the encroachment made on the cavity by the 

 ovaries, during the period of their activity, is well shown ; and also the gregariniform parasites, 



