1. Ecological observations and experiments have been carried out since 1934 on the Japanese fresh water snails, Melanoides (Semisulcospira) libertinus (GOULD), M. (S.) libertinus japonicus (REEVE) and M. (S.) niponicus (SMITH). 2. M. (S.) libertinus and libertinus japonicus are omniphagous and feed on decayed remains of water plants and animals. M. (S.) niponicus is, on the contrary, phytophagous and only feeds on algae which grow on the stones and other substrata. 3. They are unisexual and ovoviviparous. Young shells are produced in all seasons, but most numerously in late spring and early summer. 4. The activity of M. (S.) niponicus depends on the water temperature. An experiment was made in which three to seven animals were placed in a tin box (bottom area 30×20 cm., height 10 cm.), and their crawled tracks were traced for ten minutes, marking with a piece of chalk on a glass plate covering the tin box. The protocols obtained are shown in Table 13. The relation between the walking distance for ten minutes and the water temperature may be expressed by two straight lines, (the formulae of which are respectively y=1.4x-5 and y=1.05x+0.05), intersecting each other at a point corresponding to 15℃ (figure 11). 5. The habitat of M. (S.) niponicus near the Otu Hydrobiological Station is restricted to a small area, being controlled by the degree of disturbance caused by the wave action and the nature of substratum. From spring to autumn, when the snails are in active condition, they are found crowded on the stony beach where the water is kept in motion, thus securing better oxygen supply, washing away the debris of plants and preventing the excessive growth of slimy algae. On the other hand, the stagnant loci where the algae or phanerogamous water plants flourish and the substrata consist of mud or smooth pebbles are rarely occupied by them. In winter, they prefer relatively still places, but in this case also it is difficult to find them on the mud substratum. These conclusions were obtained by field observations as well as by laboratory experiments. 6. From spring to autumn, when M. (S.) libertinus and libertinus japonicus are in active conditions, their rheotropotactic reaction is positive ; but in winter when their activities are reduced, the same reaction becomes negative. As a receptor to feel water current, the tentacle is of course important, but as the results of experiments we must assume the presence of some other receptors. 7. In current water, the orientation of the shells of M. (S.) libertinus and M. (S.) libertinus japonicus is varied according to the speed of current. At the places where the speed is about 0.15m/sec. or less, the orientation of animal is quite at random ; but where it reaches about 0.75m/sec. or more, the snails are all forced to orient themselves upstream and the majority lie quite parallel to the current. 8. The speed of dispersion from one place to another is about 5∿7m per day in libertinus and libertinus japonicus, and about 3m per day in niponicus. 9. In a experiment with Melanoides group, we must take care of the material used as a rearing vessel (because Melanoides is very sensitive to substratum, for example, the tin plates of a rearing vessel must be waxed thinly, otherwise the animals are damaged), and the amount of dissolved oxygen must always be kept in a degree of saturation.

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