26o Animal Life and Intelligence. 



water (the water in which it lived) to flow from a pipette 

 over its antennaB and antennules. The antennae 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 antennae 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 * 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, en- 

 larging 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 ex- 

 halent 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 concern- 

 ing a sense of smell. 



* Journal of Marine Biological Association, New Series, vol. i. No. 2, p. 211. 



