120 THE EVOLUTION OF CONTINUITY 



more closely ; and it is consistent with our theory 

 that in the earthworm the nerve-collar surrounds the 

 pharynx. 



In the segments posterior to the third (Fig. 41) no nerve- 

 collars are present, but in their place, ventrally, we have 

 what we may call ganglia of absent nerve-rings, united in 

 series to form the ventral ganglionic chain. This system 

 of circum-pharyngeal collar and ventral chain is peculiar 

 to the Invertebrata, and will be referred to again in 

 connection with the Vertebrate's nervous system. 



Fio. 43. — The evolution of the nervous system of the earthworm. 

 a, diagram of medusoid nervous system, r, the ganglionic 

 cord running round the rim of the bell, b, imaginary figure 

 showing a primitive serially-medusoid organism in whom the 

 successive nerve-cords are linked up to form several longitudinal 

 chains, c, diagram of anterior segments of the earthworm 

 in which, as it were, only the first nerve-ring of B is represented, 

 and the ganglia of this occupy a dorsal position ; while the 

 succeeding rings of b are only hinted at by the presence of a 

 ventral line of ganglia, one for each segment, and united to 

 form a chain. (See also Fig. 62.) 



Fig. 43 illustrates the suggestions made above. 



With respect to the reproductive organs of the worm it 

 may be noted that these are formed at special points in 

 the walls of the ccelom. This is in harmony with our view 

 that the ccelom walls are derived from ancestral medusoid 

 bell-walls, as in these, closely applied to the radial canals, 

 is the common position for the sexual organs of present- 

 day medusoids. 



The segments of the worm are very small and simple 

 in comparison with those of a vertebrate like the horse. 



