118 Journal of the Asiatic Society of Bengal. (April, 1905. 
Bionomics oF Chetogaster bengalensis. 
The Chetogaster of the Calcutta tanks is usually found clinging 
by means of a posterior sucker to the external surface of the body 
or the edge of the shell of a water-snail. When disturbed it with- 
draws itself entirely within the latter. It does not confine itself 
to any one species of snail, but generally chooses a Limnzus, 
Limnophysa or some similar species, apparently because these 
genera are common in its habitat, do not possess an operculum 
and have a wide aperture to their shells. In one instance I saw, 
in an aquarium in which snails were somewhat scanty, a solitary 
worm attempting to establish itself on a Planorbis; but the connec- 
tion was only temporary, not lasting for more than a few minutes. 
The mouth of the shell in this genus, though there is no oper- 
culum, is evidently too constricted to be suitable for the worm, 
which is generally gregarious. As shown by the figure (plate ITI, 
fig. 1), a considerable number of individuals may establish them- 
selves on a single snail. Occasionally Ch. bengalensis quits its 
host altogether and either wanders away in search of another or 
drops to the bottom. This happens whenever the water becomes 
foul or reaches too high a temperature (in an aquarium when the 
sun, falling directly on the surface, heats the water), or when too 
many individuals are settled ona single host after rapid asexual 
multiplication. Before fixing themselves on a fresh snail they 
frequently crawl over the external surface of the shell. 
Progression is mainly effected by a series of contractions and 
elongations of the body, aided by the two suckers. The posterior 
of these having been fixed to any surface, the body is stretched 
forward to its greatest extent. The anterior sucker is then applied, 
the posterior one set free, and the body contracted. As a rule, the 
ventral surface is not lifted, but something analogous to the “loop- 
ine” of a leech or a Geometrid caterpillar, but not so marked, takes 
place very occasionally. When sinking through the water, as it 
does when its hold is released, the worm can change its direction 
slightly by moving the posterior part of the body from side to side ; 
it cannot swim or raise itself upwards without support. The sete 
appear to play a very small part in ordinary progression, except 
as aids in adhesion. Hach bundle is capable of an independent 
rotatory motion somewhat resembling a rapid turn of the wrist. 
This movement is very useful when the worm is insinuating itself 
into a crevice, as it thrusts the body forward rapidly. All the sete 
are frequently moved at once, although each bundle can be turned 
separately. The anterior bundles have a different function, as we 
shall see. 
Although this species! lives in close connection with water- 
snails, it is not, strictly speaking, parasitic upon them; for it captures 
1 The European species also live on water-snails, but some of them at any 
rate are said to be internal parasites. Dr. J. H. Ashworth has sent me a 
specimen of an English species in which the food probably consists of 
diatoms and the like. 
