322 DE, W. CARMICHAEL M'lNTOSH ON THE STRUCTURE OF THE 



parts, with transverse markings or rugae ; but such an appearance does not of 

 necessity mean that it is composed of circular fibres, for the contraction of the 

 longitudinal layer underneath would cause even a very feebly elastic coat to 

 assume similar markings. The thin subjacent layer of longitudinal fibres is 

 likewise continued to a similar extent on the reservoir-region, and assists in 

 connecting the divisions. These two layers lie exterior to the stylet-sacs. 



The structure of the pit or termination of the anterior chamber (>;, Plate IX. 

 fig. 3) merits special notice, since it has certain important functions to per- 

 form. The large glands of the inner wall gradually diminish in size until the 

 floor is covered only by small, densely arranged, and minutely granular pro- 

 cesses, so that the whole forms a somewhat sharply defined border, which in 

 the ordinary state of the parts knuckles backwards all round the central 

 stylet in the manner shown in the figure, becomes firmly bound together so 

 as to co nstitute a sphincter for the aperture, and gently bending outwards and 

 backwards, is lost in the obscurity of the parts, caused by the external circlet 

 of glands — somewhat behind the anterior termination of the wedge-shaped 

 investment of the sac at the base of the stylet. This floor of the chamber 

 is composed of a series of muscular fibres, whose direction, in the ordinary 

 state of the parts, is outwards and backwards, as shown in the drawing, 

 but which assume various aspects during the motions of the organ. Thus 

 the floor passes from the conical form with the apex directed backwards to that 

 of a transverse platform ; and in the everted condition forms a cone whose apex 

 is directed forwards (Plate VI. fig. 2). In the latter position the secure binding 

 of the fibres which knuckle round the central aperture just permits the stylet to 

 project, but no more. The whole arrangement constitutes a large muscular pit with 

 very powerful and mobile walls, capable of many and varied alterations of form. 

 In firm contraction of the region the floor or pit of the cavity is pouted forwards 

 (Plate XII. fig. 9), causing a radiated or slanting appearance of the fibres. A firm 

 constriction of the tube just in front of the stylet-region often takes place, separat- 

 ing the pit of the organ from the more glandular region in front, and causing a 

 double swelling of the parts. Just in front of the stylet-sacs lie some coarse 

 granular glands, which, however, are less conspicuous than in O. gracilis and 

 others. Professor Keferstein* speaks of this region as having only a longitudinal 

 muscular coat (though the crenated border of the anterior chamber is continued 

 thereon in his figure), and as possessing much pigmentary and granular matter. 

 The latter is not well marked in Ommatoplea alba or Tetrastemma, as the 

 entire apparatus is either translucent or white ; but in certain species, as will 

 hereafter be shown, an increase in the granular matter occurs. The longitudinal 

 fibres of the last-mentioned author end at the posterior border of the stylet-region. 



The Lateral Stylet-Sacs — poches styligenes, Quatref., Taschen, Kef., &c. 



* Op. at. 



