328 DR W. CARMICHAEL M'lNTOSH ON THE STRUCTURE OF THE 



again is terminated by a wider 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 algce. The entire sac is opaque 

 white, and coarsely granular from an early age, the granules disappearing with 

 effervescence under the action of weak acetic acid, and rendered paler (in some 

 cases dissolved) by liquor potassse. These granules would not seem to be simply 

 inclosed 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 when the anterior part is cast off with the stylet, as will be here- 

 after described. I have also seen the stylet and its granular basal sac thrown 

 off together in a discarded proboscis in the proboscidian chamber of O. melano- 

 cephala and other species. This peculiar body or sac is set in a firm wedge of 

 translucent yet compact muscular substance (marked 6 in the various figures) 

 which often has its posterior border curved in a saddle-shaped manner, projecting 

 backwards in the middle, and with a curve on each side directed forwards. The 

 anterior part of this wedge proceeds about as far forwards as the shoulder of the 

 first swelling of the awl-handle, and there becomes lost on the coat of the latter. 

 Though this generally appears like a wedge of translucent and structureless car- 

 tilage, the addition of liquor potassae and acetic acid shows distinct strite, chiefly 

 of a transverse character when viewed under pressure, and therefore of a radiating 

 nature with regard to the central granular sac. In front of the wedge-shaped 

 division lies the muscular cavity (e, Plate IX. fig. 3), into which the ejaculatory 

 duct opens (at //). This cavity is formed by the knuckling outwards of the floor 

 of the anterior chamber all round, and. it is furnished with a distinct inner mus- 

 cular coat. The walls are thus very mobile, and I have seen them form an hour- 

 glass contraction in the middle, quite distinct from the narrowing between the 

 sac (whose greatest diameter is in front) and the firm wedge behind. Its anterior 

 border can be projected to the tip of the central stylet ; while in the extruded 

 state of the parts (e, Plate VI. fig. 2) it forms, when seen from above, a com- 

 pressed process at each side of the basal sac of the central stylet ; more correctly, 

 however, and if viewed from the front, it has the shape of a muscular umbrella, 

 which slopes all round the anterior portion of the basal sac. M. Claparede does 

 not mention this arrangement at all, and M. de Quatrefages seems to have 

 mistaken it for a pair of glands, which, he explains, probably secrete poison for 

 cankering the wounds inflicted by the stylet, a supposition unsupported by any 

 anatomical basis as regards this spot. Prof. Keferstein's anatomy of the region 

 also requires correction, since he does not distinguish the separation between this 

 cavity and the floor of the anterior chamber; thus in his representation* of the 

 extruded proboscis, the central stylet projects smoothly into the water, and the 

 ejaculatory duct opens directly into the latter at a short distance from the stylet. 



* Op. cit. tab. v. fig. 3. 



