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



reservoir in Tetrastemma is well illustrated in this species, where they form 

 very prominent structures with granular contents, and more nearly allied to 

 those in the posterior chamber than in 0. alba. In transverse section the micro- 

 scopic structure of the organ in the foregoing species agrees with that in Omma- 

 toplea. 



In Polia involuta, Van Beneden, the proboscis and its apparatus are reduced 

 to a minimum. The anterior region (Plate VII. fig. 5, a) is very short, and has 

 in general a somewhat conical outline, the base of the cone being formed by the 

 floor of the chamber. Its walls are proportionally thick and muscular, and 

 internally have a minutely granular aspect, a condition probably due to indica- 

 tions of papilla?. Posteriorly it terminates in the usual floor, into which, how- 

 ever, only one aperture leads, viz., that of the central stylet. The next, or stylet- 

 region proper, while still retaining the Ommatoplean type, differs much from that 

 of any other British species. Instead of the usual well-defined arrangement of longi- 

 tudinal and radiating fibres, the entire muscular structure is obscured by numerous 

 granular or cellulo-granular bodies (7), which give a characteristic appearance to 

 the somewhat conical region. There is no trace of lateral stylet-sacs. The central 

 stylet is minute, and furnished with an elongated and faintly granular basal sac, 

 which is fixed in the usual transparent muscular setting, the mobile muscular 

 chamber into which the ejaculatory duct opens being situated immediately in front. 

 Though the whole apparatus is very minute, I have seen the stylet thrust forwards 

 by the contraction of the fibres of its basal setting, so that its point projected 

 into the floor of the anterior chamber of the proboscis. The ejaculatory duct is 

 large, and, owing to its central position in ordinary examinations, causes the 

 stylet-region proper to appear bifid posteriorly ; but this is due only to the greater 

 translucency of the duct, which, for the time being, renders the denser granular 

 masses at the sides more conspicuous. The region of the reservoir is fairly 

 developed, the walls being striated with transparent muscular fibres in the usual 

 manner, and the granular glands lining the inner surface. The walls might be 

 seen now and then contracting with force, and driving the contents forwards 

 into the ejaculatory duct and muscular chamber behind the floor of the first 

 region. The posterior channel of the reservoir led into a posterior chamber of 

 comparatively small dimensions, but having thicker walls than usually found in 

 this region, and terminating in a cul-de-sac and rounded end, a short distance 

 behind the oesophageal apparatus. This chamber had a cellulo-granular lining 

 internally, and in some specimens the posterior end was observed under pressure 

 to be distended with a transparent fluid containing a few compound cells of 

 similar aspect to those found in other species. This posterior region is kept in 

 position by fibres from the strong bands at the posterior part of the oesophageal 

 apparatus. 



All that M. Van Beneden says with regard to the structure of this organ is 



