PROBOSCIS. 61 



curvature of the fibres cause the most powerful compression of the cavity, but the posterior aperture 

 has a tendency to be closed and slightly carried forward, the anterior being less subject to 

 interference. The closing of the posterior aperture (channel of communication) is also greatly 

 assisted by the circular fibres lying outside the longitudinal. 



In extrusion of the proboscis (Plate XIII, fig. 14), the entire spike of the central stylet projects, 

 the floor of the anterior chamber forms all round a thick and powerful umbrella-shaped cushion 

 (whose independent structure escaped Professor Keferstein), the marginal stylet-sacs are under 

 cover, and the region of the reservoir is shortened and widened. The position of the muscular 

 chamber (t), which forms a second small umbrella round the apex of the basal apparatus of the 

 central stylet, is characteristic. 1 The separation between the longitudinal fibres of the stylet-region 

 proper and the looped fibres (r) of the reservoir is distinctly indicated. It will also be 

 observed that the whole stylet-region is dilated by the forcible wedging forward of the reservoir. 



c. Posterior Region of the Proboscis. 



Behind the translucent region just described, the opaque-white long posterior chamber 

 (c, Plate XV, fig. 3) occurs. It communicates with the reservoir in front, but its posterior end is 

 csecal. The contractile nature of the parts renders comparison uncertain, but it is generally not 

 much shorter than the anterior chamber in the perfect animal; sometimes, indeed, it exceeds 

 the latter part in length, the simple structure of its wall giving it greater extensibility. In 

 young specimens and in regenerating organs, again, it assumes a nearly globular form in contrac- 

 tion. Externally it is covered by a very delicate investing layer. Within is a series of circular 

 muscular fibres, which towards the tapering posterior end become indistinct, and finally disappear 

 altogether after the csecal tip is reached (Plate XI, fig. 16). The next coat is formed of an equally 

 strong series of longitudinal fibres, the anterior or primary bundles being continuous with the 

 longitudinal layer of the reservoir, as previously mentioned. These run throughout the entire 

 length of the chamber, becoming proportionally more developed as the central cavity diminishes 

 posteriorly, and finally merging into the terminal muscular ribands. The mucous layer with its 

 glands lies within the latter coat (Plate XIII, fig. 16), though in several views, both in the living 

 animal and in transverse sections, I fancied some sub-mucous circular fibres were present ; they are at 

 any rate insignificant, and the two chief layers explain all the motions which ensue in this division. 

 The mucous coat in contraction of the organ forms many rounded folds. The glandular papillas 

 which clothe the surface of the latter — from the commencement of the region behind the trans- 

 lucent reservoir — almost, but not quite, to its csecal tip, differ materially in structure from those 

 of the previous parts. Viewed as a transparent object under moderate pressure (Plate X, fig. 12) 

 the field is covered with globular glands containing clear rounded vesicles in their interior. In 

 contraction, and when the wall is less compressed, the glands have a larger and coarser appearance, 

 the external wall only of each being visible. When the pressure has been increased, these bodies, 

 especially towards the posterior end (where, from their diminished numbers, a clearer view can be 



1 The appearances shown by this chamber ( £ ) in the extruded organ must not be mistaken for 

 " poison-glands/' since they are caused by the pressure of the glass cover rendering the central portion 

 of the continuous umbrella for the moment indistinct. 



