Note AU — GLOSSARY OF TECHNICAL TERMS USED IN PART II.

In the Calcutta tanks operculate molluscs such as Vivipara are certainly more free from visible attack than non-operculate species. This is the case for instance, as regards the common aquatic glowworm (Luciola sp.), which destroys large numbers of individuals of Limnophysa, Limnæus, &c. If it has been starved for several days in an aquarium it will attack an operculate form, but rarely with success. Similarly Chætogaster bengalensis attaches itself exclusively to non-operculate forms. In the one case the polyp could do very little against an adversary with so stout an integument as the insect, while, in the other, it is doubtful whether the worm does any harm to its host. The polyp would afford very little protection against the snail's vertebrate enemies or against what appears to be its chief foe, namely, drought. As the water sinks in the tank non-operculate species migrate to the deeper parts, but Vivipara and Ampullaria close their shells, remain where they are, and so often perish, being left high and dry, exposed to the heat of the sun.