BEHAVIOR OF LOWER INVERTEBRATES 371 



Molluscs. Parker (20) finds that certain snails move by a 

 continuous or arhythmic movement of the foot, instead of by- 

 means of rhythmic waves of pedal contraction, as is the case 

 with most species. The foot is an organ of attachment in snails 

 either by means of the mucus it secretes, or by suction, or in 

 both ways. A wave in the foot is a portion that is lifted from 

 the support and is moved forward; this local forward move- 

 ment takes place in successive sections of the foot until- the 

 whole has been moved. 



A careful study of the movements of heteropod and pteropod 

 molluscs has been made by Polimanti (26). He has also inves- 

 tigated the functions of the ganglia and statocysts in these 

 animals. Lesion of the cerebral ganglia produces heightened 

 reflex irritability. The pedal ganglion is the organ of locomo- 

 tion, and when it is destroyed, motor cooordination is lost. 

 Destruction of one statocyst produces rotation towards the unin- 

 jured side; destruction of both results in complete disorientation. 

 This is more marked if the eyes are also extirpated. Ordinary 

 sound stimuli have no effect on movement, but jarring the 

 water so as to produce tactile stimulation does have an effect. 



Yung's papers (34,35) report his conclusion that the snail 

 Helix pomatia, although it has well developed eyes, is totally 

 blind and not even dermatoptic. The individuals he tested 

 were perfectly indifferent to shade or bright sunlight on the 

 floor of a room, and to dark or light compartments of a box; 

 they did not respond to sudden variations of light intensity 

 such as the turning on of an electric lamp or the flashing of a 

 light by means of a mirror; and they gave no evidence of being 

 able to avoid obstacles by vision. A pin-head was repeatedly 

 approached to the eye and then struck against it : the tentacle 

 bearing the eye was withdrawn at the touch, but never, in spite 

 of repeated experience, before it. Snails with the eyes removed 

 behaved in all these respects precisely like normal individuals. 

 It is certainly interesting to find that an organ apparently in 

 perfectly good condition can be without function. The danger 

 of arguing from purely morphological evidence is well illustrated. 



Pieron (24, 25) makes use of the light reactions of a common 

 snail (limnee) to investigate the laws of memory. In the first 

 of these two papers he undertakes- to find out how rapidly the 

 effects of adaptation to a repeated visual stimulus die away 



