524 THE WONDER OF LIFE 



way by pressing their throats against the ground, and in 

 these cases the fragments of shell pass down into the 

 stomach and are dissolved away. One would like to know 

 more about an Indian snake, Elachistodon westermanni, 

 which has a structural peculiarity like that seen in Dasy- 

 petiis, but there does not seem to be any certainty as to 

 how it uses it. As the Indian snake belongs to a different 

 group the occurrence of a similar peculiarity of structure 

 is very interesting, and the interest would be enhanced 

 if there is also an egg-eating habit. 



Aristotle's Lantern. Commanding our admiration 

 as a piece of mechanism which can discharge several differ- 

 ent functions is ' Aristotle's lantern ' which surrounds the 

 beginning of the food-canal in the ' regular ' sea-urchins. 

 Aristotle saw it more than two thousand years ago a neat 

 contrivance with five continually growing teeth in five 

 sockets which are united by * braces ' and ' compasses ' 

 and swayed about by strong muscles attached to five 

 ' standards ' on the test. It is capable of rhythmic move- 

 ment which helps in mastication, in boring, in respiration, 

 and perhaps in keeping up a certain turgescence in various 

 internal cavities of the body of the sea-urchin. But perhaps 

 the most remarkable thing about the lantern is that dis- 

 covered by Dr. J. F. Gemmill : it is an organ of locomo- 

 tion in certain conditions, and acts as an auxiliary to the 

 suctorial tube-feet and the spines. The sea-urchin can 

 stumble along on the tips of its teeth which seems a most 

 extraordinary statement to make. In each step the urchin 

 is raised on the tips of the teeth and a forward impulse 

 is given, (a) by strong pushing or poling on the part of the 

 lantern, (6) by similar but usually less effective pushing 

 on the part of the spines, and (c) after a certain stage, 



