150 ANNELIDA [CH. 



It further serves to put each organ and each part of each organ in 

 communication with all the others, and thus this vast accumulation 

 of tissues and cells acts in an orderly way and towards a set end. 

 It is built up by the repetition of a special type of cell called the 

 neuron or nerve-cell. This cell consists of a body containing 

 the nucleus and processes. From the body is given off a straight 

 fine process called the ax on, which ends in a tuft of processes 

 called terminal dendrites. From the other end of the cell a 

 coarse process proceeds outwards, ending in a tuft of processes 

 called receptive dendrites. These receive impressions which 

 are transmitted to the axon and passed on either to other neurons 

 or to muscles. 



The swelling called a ganglion is due to an aggregation of a 

 number of the bodies of neurons, so that in this region the nerve- 

 cord is broader than at other places, though everywhere some bodies 

 can be seen in transverse section of the cord. 



The earthworm has no specialised sense organs, it has neither 

 eyes to see, nor nose to smell, nor ears to hear with. Still, 

 although it is apparently deaf, it is not devoid of the power of 

 appreciating those stimuli which in us excite the sensation of sight 

 or smell. A strong light suddenly turned on the anterior end of 

 the body will cause the worm instantaneously to withdraw into its 

 burrow, and worms readily recognise the presence of such favourite 

 food as onions and raw meat. Their sense of touch is well 

 developed and they are very sensitive to vibrations ; for instance, 

 a stamp of the foot on the ground will cause all those in a 

 certain radius to disappear into their burrows. It is further 

 possible that earthworms possess other senses with which we are 

 totally unacquainted. 



In each segment of the worm scattered here and there amongst 

 the ectoderm cells are a number of sense cells. Each of these has 

 a minute sense hair which projects upwards through a hole in the 

 cuticle, and by means of this hair stimuli oi various kinds are 

 received from the outer world. The body of the cell is small just 

 large enough to contain the nucleus and from the base proceeds 

 an axon which runs inwards and terminates inside the central 

 nerve- cord in a brush of terminal dendrites in close contact with 

 the receptive dendrites of a neuron. In this way the neurons 

 receive impressions from the outside world. A bundle of the 

 axons of sense cells proceeding inwards is known as a sensory 

 peripheral nerve. The receptive dendrites of the neurons 

 receive the impressions from the sensory nerves and pass them on 



