58 ANATOMY OF THE ENOPLA. 



the reservoir) is capable of a certain amount of constriction, as indicated in one of M. Claparede's 

 figures. In the latter state the inner or convex side of the duct is glandular, while the outer or 

 concave is not. A layer of longitudinal fibres, continued forward from the reservoir, constitutes the 

 proper wall of the tube, and it is this coat which causes the distinct ring round the duct in trans- 

 verse section. Internally it has a mucous lining, which generally possesses a few small glandular 

 papillae towards its posterior end. The duct is not very dilatable, the cavity becoming elongated, 

 but not much increased in diameter, even under violent expansive force. It can be firmly closed 

 by the contraction of the region surrounding it (Plate XII, tig. 9, and Plate XIII, fig. 7 /i) so as to 

 be indicated by a mere central streak. The whole structure of the channel, and its relations to 

 surrounding parts, show that it is formed, not for transmitting fluids from before backwards, but 

 entirely in the opposite direction. The mobile muscular space ( £ ), into which this duct opens, 

 forms a kind of sac that is occasionally distended with the cells and granules, before they reach, 

 through the central pore, the pit of the anterior chamber. 



c. The Central Stylet and its Apparatus. 



The central stylet projects straight forward into the floor of the anterior chamber in the 

 usual state of the parts, and is generally about the same size as the largest stylet in the marginal 

 pouches, with which (stylet) it likewise agrees in structure and composition (Plate XII, figs. 1 

 and 2). Its base is fixed to the granular apparatus (A) ; the arrangement being not inaptly 

 likened by Dr. Johnston to an awl, the anterior or smaller end of the structure sending its 

 investing substance over the head of the organ, and grasping part of the spike. The basal apparatus 

 (or awl-handle) is narrowed anteriorly, gradually widens backwards, is then marked by a constriction, 

 and again terminated by a dilated portion, which may represent the butt of the awl. This structure 

 is shorter in proportion to the stylet, and has its constriction placed further backwards than in 

 Tetrastemma Candida. It is opaque-white, and coarsely granular from an early age, the granules 

 disappearing with effervescence under the action of weak acetic acid, and rendered pale, in some 

 cases dissolved by caustic potash. These granules would not seem to be simply enclosed in the 

 structure, as if in an ordinary sac, but they adhere together and form a consistent whole, as 

 proved, amongst other things, by their not falling out of the fragment in transverse section, or 

 when the anterior part is cast off with the stylet, as will hereafter be described. I have also seen 

 the stylet and its granular apparatus thrown off together in a discarded organ in the probos- 

 cidian chamber of T melanocephala and other species. This peculiar granular structure or sac (0) 

 is set in a firm wedge of translucent, and, under ordinary or external appearances, structureless 

 substance \ but the addition of caustic potash or acetic acid shows distinct striae, chiefly of a trans- 

 verse character when viewed under pressure, and therefore of a radiating nature in relation to the 

 apparatus, as clearly proved by a transverse section. An interesting condition was found in two 

 specimens of Tetrastemma jlavida, which directly bears on the physiology of this part. In each 

 a fragment of the granular apparatus, with the central stylet attached, lay towards the anterior 

 end of the first region of the proboscis ; and since injury would scarcely have caused a result so 

 systematic, it is evident the stylet had been thrown off by the animal. In both cases the appa- 

 ratus of the central stylet was complete, only in one its anterior part appeared pale, and there 

 was a slight irregularity in its outline, similar to that in Plate XI, fig. 13. In each, the marginal 



