SMELL AND TASTE QOJ 



being aware of the stimuli in the way that we should be. 

 Their reactions appear to be automatic, and to correspond 

 more with the movements of the cations to the cathode in 

 electrolysis rather than conscious reactions. In the more 

 complex invertebrates definite chemo-receptor organs are 

 recognizable and they are particularly conspicuous in some 

 of the gastropod molluscs such as the whelk, periwinkle and 

 many others. These creatures breathe by means of gills that 

 are contained in a cavity near the front part of the body. 

 Sea-water is drawn over the gills through an opening the 

 edges of which are usually extended as a tube known as the 

 siphon. When the water enters the cavity it impinges on a 

 complicated chemo-receptor before it passes over the gills. 

 This water tester is thrown into numerous frilly tufts so that 

 its surface area is large and can hold many receptor cells. It 

 not only tests the water for suspended matter so that the gills 

 do not get clogged with muddy particles, but also acts as a 

 chemo-receptor. Whelks, which are carnivorous, find their 

 food by "smell" and can easily be caught in numbers by 

 letting down a small net baited with meat or fish on to the 

 bottom where they live. A striking example of the use of the 

 water tester for finding food is shown by a sea snail, Bullia, 

 that lives on the shores of South Africa. In tliis creature 

 there is no doubt that the water tester is for smelling, or 

 tasting at a distance. The snails live buried in the sand near 

 low tide mark, but feed upon beach carrion stranded near 

 the high water mark. Substances from the food dissolve in 

 the water when the waves reach it and are carried down 

 to the snails as the waves retreat. As soon as the scent 

 reaches the snails they come up out of the sand and let the 

 waves carry them up the beach to strand them near the tide 

 mark where their sense of smell leads them straight to the 

 food. 



