THE ARCHIPELAGO OF BREHAT. 117 



of small oyster, which attaches itself to various sub- 

 stances under water. When it has exhausted the 

 food around it, or when it wishes to change its posi- 

 tion, it extends its long, dark-coloured, riband-like 

 body, which is terminated by a head, bearing some 

 resemblance to that of a serpent, although it has 

 neither the large mouth nor the formidable teeth of 

 the latter animal. In observing it in motion, the eye 

 is unable to detect any contraction, or any apparent 

 cause by which it is enabled to move, and it is only 

 by the aid of the microscope that we learn that the 

 Nemertes glides through the water by means of ex- 

 cessively fine vibratile cilia, Avhich are protruded 

 from every part of the surface of the body. It 

 pauses, gently moves from side to side, as if endea- 

 vouring to investigate the ground, until it at length 

 succeeds in finding a stone to suit its purpose, lying 

 perhaps some fifteen or twenty feet from its former 

 retreat. It then begins to unwind its coils, in order 

 to arrange itself in its new domicile, and in proportion 

 as one knot is loosened another forms at the op- 

 posite extremity. We may remark that the con- 

 tractility of the tissues of this animal is so great, 

 that a Nemertes thirty feet long scarcely exhibits 

 one tenth of this lens^th after beins: immersed 

 in alcohol, when it will be found to measure no more 

 than two and a half or three feet. 



All the great apparatus of life is represented in the 

 organisation of the Nemertes, although it is here 

 reduced to its simplest expression. The nervous 

 system does not form that oesophageal ring, which 



I 3 



