In the aquatic crayfish there are, besides the long antennæ, smaller antennules, each of which has two filaments, an inner and an outer. On the under surface of most of the joints of the outer filament there are two bunches of minute, curiously flattened organs, which were regarded by Leydig, their discoverer, as olfactory. Observation, too, seems to confirm the view that the sense of smell (or telæsthetic taste) is located in the antennule. I tried on a crayfish the following experiment: When it was at rest at the bottom of its tank, I allowed a current of pure water (the water in which it lived) to flow from a pipette over its antennæ and antennules. The antennæ moved slowly, but the antennules remained motionless. I then took some water in which a cod's head had been boiled, and allowed some of this to stream over the antennæ and antennules. The former moved slightly as before, but the antennules were thrown into a rapid up-and-down jerky vibration, and shortly afterwards the crayfish began moving about the bottom of its tank. If only one antennule be thus stimulated, or stimulated to a higher degree than the other, the crayfish seems generally (but not always) to turn to that side in search of food. Mr. Bateson[EV] has shown to how large an extent shrimps and prawns seek their food by smell, and states that a prawn, though blind, will often find his way back to his proper place, and stay in it. In the snail the anterior pair of "horns," or tentacles, are said to be olfactory. Near the end of each is a large ganglion, or nerve-knot, from which fibres pass to the surface, in which there are said to be developed sensory knobs. Snails, however, from which these tentacles have been removed are apparently still possessed of a sense of smell. Certain lobed processes round the mouth have been regarded as the seat of olfactory sensation, but this is doubtful. In the foot of the snail, the part on which it glides, there is a hollow gland, and in this there are special cells, each of which gives off a delicate rod, enlarging at the free end into a ciliated knob. These are regarded as sensory and, it may be, olfactory. In shell-fish like the mussel, in which the water is sucked in by an inhalent tube or siphon, and ejected through an exhalent siphon above it (see Fig. 2, p. 4), there is at the entrance of the incoming current a thin layer of elongated cells which are described as olfactory, and are in association with a special ganglion. Olfactory depressions have been described in some worms. But in a great number of the lower invertebrates very little or nothing is known concerning a sense of smell. * * * * *