1921.] Manipur Molluscs. 581 



normal, but rather small. Tlie measurements of a large shell are 

 14 x11x6 mm. The colour, when the shell is clean, is a deep 

 uniform chestnut brown, but the surface is usually covered with a 

 black deposit. The aperture is often somewhat contracted and 

 shows a slight tendency to be irregularis' folded round the margin, 

 and the vertical sculpture, though by no means strong, is rather 

 irregular. The animal is apparently quite normal. 



The shells taken in the middle of April are considerably smaller, 

 the measurements not exceeding 11 X 9^5X5 mm. The older part 

 is like that of individuals collected in summer, but the younger 

 part of the body-whorl, often for a length of at least 6 mm. is 

 distinctly abnormal. In this region the shell is very thin, strongly 

 and irregularly sculptured and often somewhat eroded on the sur- 

 face and as a rule much distorted round the aperture, which is 

 sometimes thrown into strong folds. At this season the soft parts 

 are also abnormal. A greater part of the animal is as a rule pro- 

 truded from the shell than is usual, and the mantle seems to have 

 shrunk in such a way that the siphon and the branchial process, 

 which is very large, are often completely exposed. The tentacles 

 also are as a rule strongly clavate, having an oval swelling at the 

 tip which, as its tip is densely pigmented, has much the appear- 

 ance of an eye. Major R. B S. Sewell has kindly examined speci- 

 mens of these abnormal tentacles. He finds in the tissues of the 

 swollen tips numerous individuals of a parasitic Protozoon, prob- 

 ably a Gregarine. Specimens taken at the beginning of June had 

 again become normal both as regards their shell and in their soft 

 parts. The tentacles were no longer clubbed and were free from 

 parasites. 



At least five factors may enter into the question of the origin 

 of these changes in the shell and soft parts of I. exusttis, namely 

 (i) changes in the salinity of the water, (2) abnormally high tem- 

 perature, (3) restricted space, (4) restricted food supply, and (5) 

 parasitism. The production of clavate tentacles is clearly due to 

 the last, but probably the abnormal structure of the youngest 

 part of the shell in April is due mainly to the first two. 



/. exustus is common all over the Manipur Valley, in ponds, 

 swamps, sluggish streams and in the Loktak Lake. Specimens are 

 for the most part normal, all the shells are of a deep chestnut 

 brown colour when clean. Some very large specimens with the 

 vertical ridges strongly but not excessively developed and the 

 central region deeply depressed both above and below, were found 

 in small artificial bathing-pools devoid of visible vegetation at the 

 edge of the swamp to the east of the Loktak Lake. The mea- 

 surements of one of these are 20 X i6'5 X 8 2 mm. 



Anatomy. — The foot of /. exustus when fully expanded is leaf- 

 shaped, not more than twice as long as broad, very broadly round- 

 ed or subtruncate in front and bluntly pointed behind. The 

 head is broad and short, it has an expanded and flattened lower 

 margin and is convex above. The tentacles are not very long and 

 are filiform. The eyes are entirely sessile and lie close to the inner 



