354 OLIGOCHAETA 



connected by a circumoesophageal commissure with a ventral 

 ganglionated cord. From the cerebral ganglia arises a system 

 of nerve -fibres and nerve -cells, which represents the stomato- 

 gastric nerves of other Invertebrates. 



Senses and Sense-Organs. The only organs that can be 

 regarded with anything like probability as sense-organs are the 

 pigmented eyes of certain Naids and the tactile cells of many 

 worms. The latter are usually elongated cells provided at their 

 free extremity with a stiff process ; they occur associated in 

 groups, and often these bundles of cells have a segmental arrange- 

 ment. The head end of many of the lower Oligochaeta, for 

 instance the genus Aeolosoma, has delicate processes projecting 

 here and there ; these appear to be also of a tactile nature, and 

 are of course connected with cells of the epidermis. The eyes of 

 certain Naids are little more than lenticular bodies embedded 

 in a mass of pigment. In the genus Eudrilus and in many 

 Eudrilidae are peculiar integumental bodies, which were in- 

 dependently discovered by Dr. Horst l and myself, and com- 

 pared by us to the Pacinian bodies of Mammals. Whether 

 these structures are connected with nerves or not is doubtful. 

 In spite of the poor development and the simplicity of their 

 sense organs, the higher Oligochaeta at any rate can feel, and 

 can distinguish light from darkness. Darwin 2 came to the 

 conclusion that " light affects worms by its intensity and its 

 duration." And furthermore, it is only the anterior end of the 

 body which is thus affected. Of the sense of hearing these 

 animals appear to be utterly devoid. Some kept by Darwin 

 "took not the least notice of the shrill notes from a metal 

 whistle, which was repeatedly sounded near them ; nor did they 

 of the deepest and loudest tones of a bassoon." But it is always 

 necessary to discriminate between sound and vibrations passing 

 through any solid body, which would appeal rather to a sense of 

 touch. Here worms are most sensitive. It is quite easy, by 

 digging with some vigour, to arouse the worms in the neighbour- 

 hood, who will crawl to the surface and away from the scene of 

 action ; a proceeding on their part which is sometimes put down 

 to a desire to escape from their enemy the mole. 



Smell appears to be another sense which is somewhat deficient. 



1 Mim. Soc. Zool. France, iii. 1890, p. 223. 

 2 Vegetable Mould and Earthworms, London, 1881. 



