SIGHT IN INVERTEBRATES I59 



Most species feed on minute floating plants and animals 

 which they capture by drawing a current of water between 

 the shells and filtering their food out of it by means of the 

 gills. When the shells are opened a short way for this pur- 

 pose in many species the edges of the body lining each shell 

 move towards the opening so that light falls upon them. In 

 other species, which live buried in sand, part of the body- 

 edge is drawn out into a tube that can be pushed up to the 

 surface for the current of water to be drawn in through it. 

 All these animals are provided with eye-spots, often no more 

 than cells containing a speck of light-absorbing pigment, 

 though sometimes more complex. 



In the species with water-tubes the eye-spots are concen- 

 trated at the outer end of the tube, but in the others they 

 are scattered along the edge of the body just inside the 

 shells. Although these eye-spots are only light-gatherers and 

 can give no sight, they are extremely sensitive to changes in 

 the intensity of the light falling on them, and the faintest 

 shadow crossing them causes the animal to withdraw into 

 the shells and close them tightly. 



The eyes of the oyster are of this type, but those of another 

 favourite edible mollusc, the scallop, are very different. Un- 

 like the sluggish oyster the scallop often shifts its position 

 and swims rapidly about by flapping its shells. It does not, 

 however, swim by simple jet-propulsion with the hinge of 

 the shells leading and the closing of the shells giving the 

 power stroke, as one would expect. It swims with the hinge 

 trailing, and appears to be taking bites out of the water in 

 front — the water is taken in at the shell edges and is expelled 

 on each side of the hinge. This is, of course, a form of jet 

 propulsion, for the water squirted out beside the hinge is 

 concentrated into jets, and the method is thus more efficient 

 than what, at first sight, appears to be the obvious one. An 

 animal that swims about actively presumably needs some 

 power of vision, and it is therefore not surprising to find 

 that the eye-spots of the scallop are very prominent. There 

 may be over a hundred of them arranged round the body 

 edge immediately inside the shells. They are comparatively 

 large dark spots which shine like jewels with the light re- 

 flected from them; each contains a lens and retina and is 



