TYPE ANNELWA. 233 



and correspoud to tlie metamere formed by the prostomial 

 lobe. Behind the transverse band are four additional gangli- 

 onic masses, apparently forming with the other six the supra- 

 cesophageal ganglion, but in reality forming together with two 

 additional masses on the ventral side of the nerve-cord below 

 the oesophagus the ganglion of the second somite. Immedi- 

 ately posterior to the two ventral masses is a chain of eight 

 ganglia lying one behind the other on the mid- ventral line of 

 the cord, and corresponding to these there occur on each side 

 along the dorsal surface of the cord other eight masses, 

 between each successive pair of which a nerve passes out. 

 There are therefore four metameric ganglia represented in 

 this complex structure, each consisting of six ganglionic 

 masses and each giving rise to a pair of nerves. The sub- 

 cesophageal ganglion accordingly consists of the ganglia of 

 four metameres, to which must be added the two ventral 

 masses of a fifth metamere, the supra- and suboesophageal 

 ganglia representing together six metameric ganglia. Behind 

 the suboesophageal ganglionic aggregate there lie twenty-one 

 ganglia separated at some distance from each other, especially 

 anteriorly, each one representing a metamere ; and finally at 

 the posterior end of the body is another ganglionic aggregate, 

 representing, to judge from the number of nerves arising from 

 it, seven metameric ganglia. Thus there are in all thirty- 

 three, or, counting the ganglion which innervates the prosto- 

 mium, thirty-four, metameric ganglia — numbers exactly corre- 

 sponding with those obtained by counting the rings. 



The sense-organs of the Hirudinea have especial interest 

 as showing an adaptation of what may be considered tactile 

 sense-organs to a different purpose. On each metamere of 

 the body in all Hirudinea, with a few possible exceptions, 

 small sensory papillae (Fig. 106, sp) are to be seen, arranged 

 in definite lines. They occur in the majority of forms on the 

 first ring of each segment, though in some species of Nephelis 

 they occur on all the rings. On the dorsal surface of each 

 sensory ring there are three papillse on each side of the 

 middle line, and the same arrangement occurs on the ventral 

 surface, and in addition a single papilla is found at the 

 margin of the ring on each side. There are thus fourteen 



