61 



Below the gizzard are seen several sub-globose sacculate viscera, 

 which the opacity of the case forbids to trace down ; they are doubt- 

 less the commencement of a constricted stomach, with its accessory 

 basal vesicles (pancreatic or biliary ?) ; as I have traced the passage of 

 coloured food through them (as far as the margin of the tube per- 

 mitted vision), in the form of a constricted canal. On one occasion, 

 the accidental desertion of its case by an individual enabled me to 

 submit the latter to compression, and to discover that the anal orifice 

 is situated near the middle of the body, on that side on which the 

 little curved horns (fig. 2, b) are placed, and consequently opposite to 

 that which is furnished with the respiratory tubes (e) and the little 

 rotating organ (fig. 1, g). The pellets composing the case are very 

 regular in form and position ; in a fine specimen (about ^ inch in 

 length when fully expanded, of which the tube was ^ inch) I could 

 count about fifteen longitudinal rows of pellets at one view, which 

 might give about thirty-two or thirty-four rows in all. 



Such was the amount of my acquaintance with this beautiful form, 

 on the 17th of November last. On that day a fine specimen fell 

 under my observation, attached to a submerged moss, from a pond 

 at Hackney. I had the great pleasure of seeing this individual en- 

 gaged in the building of its case, and at the same time of discovering 

 the use of the curious little rotatory organ on the neck. When fully 

 expanded, the head is bent back at nearly a right angle to the body, 

 so that the disk is placed nearly perpendicularly, instead of horizon- 

 tally ; the larger petals, which are the frontal ones, being above the 

 smaller pair.* Now below the large petals (that is, on the ventral side), 

 as in Limnias (fig. 3), &c, there is a projecting angular chin (i), which is 

 ciliated, and immediately below this, in Melicerta, is the little organ in 

 question (fig. 1, g). It appears to form a small hemispherical cup, and 

 is capable of some degree of projection, as if on a short pedicle. On 

 my mixing carmine with the water, the course of the ciliary current 

 was readily traced, and formed a fine spectacle. The particles are 

 hurled round the margin of the disk, until they pass off in front 

 through the great, sinus (fig. l,j), between the larger petals. If the 

 pigment be abundant, the cloudy torrent for the most part rushes off, 

 and prevents our seeing what takes place ; but if the atoms be few, 

 we see them swiftly glide along the facial surface (fig. 3, k), following 

 the irregularities of outline with beautiful precision, dash round the 



* I made no drawing of Melicerta in this position, but the figure of Limnias is 

 sufficiently like to illustrate the peculiarity spoken of. 



