560 SEMIVERTEBRATES. 



water, it will burrow its way to the lowest depths of the sand-hillock in the 

 twinkling of an eye. Its usual modus vivendi is to bury the whole of its body in 

 the sand, leaving only the mouth with the expanded buccal cirri [tentacles] pro- 

 truding. When obtained in this position in a glass jar, a constant inflowing current 

 of water, in which food-particles are involved, can be observed in the neighbour- 

 hood of the upstanding mouths. The food consists almost entirely of microscopic 

 plants (diatoms, desmids, etc.) and vegetable debris . . . Occasionally it emerges 

 from its favourite position in the sand, and after swimming about for some time it 

 will sink to the bottom, and there recline for a longer or shorter period upon its 

 side on the surface of the sand. When resting on the sand, it is unable to maintain 

 its equilibrium in the same position as an ordinary fish would do, but invariably 

 topples over on its side, — indifferently, the right or left ; " this inability to maintain 

 its balance being due to the absence of certain structures of the internal ear, to 

 which this function is assigned in fishes. According to another observer, lancelets 

 occasionally attach themselves to another by their mouths in a chain-like manner, 

 as represented in our illustration. That lancelets indicate an extremely archaic 

 type, and also that they are more nearly allied to the Vertebrates than to the 

 Invertebrates, may be considered certain ; although there is still a difference of 

 opinion whether they should be looked upon as simple or degraded forms. 



