vi ANNELIDA 77 



from a scarcity of oxygen in their environment, or which 

 have small respiratory surfaces. 



There are no special sense organs in worms. 

 The only sense which is highly developed is that 

 of touch, which is possessed by the whole surface of the body, 

 so that the lightest touch, or any very small vibration of the 

 soil, seems to be perceived by them. This is no doubt of ad- 

 vantage in warning them of the approach of such a ferocious 

 enemy as a mole, although on the other hand it does not 

 seem sufficient to cause them to withdraw deep into their 

 burrows when a thrush alights on the ground near them. 



A worm has no eyes, but all its front segments are specially 

 sensitive to a change in illumination, this sensitiveness being 

 shared to some extent by the whole body. That they have 

 some sense of taste seems indicated by the preference they 

 show for certain foods, e.g. onions and celery, and the ease 

 with which they find such food suggests some sense of smell, 

 although Darwin's experiments, in which he tested them 

 with various strong-smelling substances such as paraffin and 

 tobacco, prove that the sense is decidedly weak, at any rate 

 for any odours that are strange to them. In the sense of 

 hearing they seem entirely deficient. 



Nervous The nervous system is much more highly 

 System, organised here than in any Coelenterate. There 

 is a special paired mass (ganglion) of nerve cells lying above 

 the alimentary canal in the third segment of the body. This 

 is known as the brain, and from it two short cords of nerve 

 fibres pass down, one on either side of the oesophagus, the two 

 uniting -below to form a double ventral nerve cord which rims 

 the whole length of the body below the alimentary canal, with 

 a swelling in the middle of each segment. From this cord 

 three pairs of lateral nerves arise in each segment. 

 Reproduc- Worms are hermaphrodite (see p. 30), but in 



tion. them, as in hermaphrodite flowers, cross-fertilisa- 

 tion and not self-fertilisation is the rule. 



After mating has taken place, each worm secretes from 

 the glands in the " clitellum " a quantity of mucus con- 

 taining a horny substance which hardens on exposure to 

 the air. Out of the ring so formed, the worm slips back- 

 wards, depositing in it, as it does so, three or four eggs 

 from the ovaries which communicate with the exterior by 



