HALACOZOA. GASTEROPODA. PULMOBRANCHIATA. 115 



late-brown colour, rarely reddish-bro^\^l, tinged with grey 

 toward the margins. Extending this foot, it produces in it a 

 series of undulations, by means of which it slowly glides along 

 the solid surface of a stem, leaf, or other immersed body. 

 Every now and then, the shell is jerked forward by a sudden 

 movement, which is not performed by the foot, but by the 

 muscles which pass from it or along the neck into the body. 

 Sometimes the shell is laid flat, the more advanced edge or lip 

 of its oblique aperture being above the anterior part of the 

 outer whorl, to the left hand, as in a Zonites. The dextral 

 nature of the shell is thus apparent. But the animal often also 

 keeps the shell inclined at an angle of from 15° to 20°, or even 

 so much as 45° or 60°, the right margin of the disk touching 

 the plane on which it moves. It can even raise the shell on 

 edge, so as to be quite perpendicular. Moreover, as the neck 

 is so slender, it can easily be twisted without receiving injury, 

 so that sometimes the animal is seen advancing at the bottom 

 of the water, with the shell reversed, or having the prolonged 

 lip of its aperture below, and the right margin of the disk 

 above ; or it may be turned over, so as to have its lower sur- 

 face uppermost. An individual seemed to prefer this arrange- 

 ment, and I thought it might be natural to it, or that it might 

 be sinistrorse ; but, on taking it out of the water and replacing 

 its shell in the ordinary position, it kept it so. Sometimes the 

 animal moves along with a continuous progress, without jerk- 

 ing the shell forwards at intervals. Generally however, it 

 combines both methods. Like many other mollusca, it can 

 move on the surface of the water. In this case, the plane of 

 the shell is parallel to the svu-face, the extended foot of the 

 animal is applied to the surface of the atmosphere in contact 

 with that of the water, forming a concavity, and the undulatory 

 contractions of the foot cause a slow advance ; but the animal 

 does not usually travel this way. It readily changes its direc- 

 tion, the foot turning to either side with ease, on the slender 

 pedicel formed by the body. The mantle forms but a slight 

 margin or collar; the head projects with a rounded exti'emity; 

 over the mouth is a broad flap of two rounded lobes, and be- 

 low it the foot, anteriorly rounded and decurved, projects a 

 little. The tentacula, nearly as long as the foot, or a twelfth 

 and a third in length, are extremely slender or setaceous, 

 dilated at the base, then contracted, and thence gradually 

 tapering to the point. They are contractile, but not in the 

 manner of the tentacula of snails and slugs, and can be moved 

 in any direction, but seem to be not generally employed as 



