No. I.] THE METAMERISM OF NEPHELIS. 2 "J 



Somites. 



The number of rings in the typical somite is five, but this 

 number is reduced at each extremity. Unlike Clepsine, Mac- 

 robdella, Hirudo, and some other leeches, Nephelis does not 

 have the first ring of each somite, except in the anal region, 

 marked by especially large sensillae, and the study of a large 

 number of individuals showed the arrangement of sensillae 

 to be constant in this region. The last nephridiopores lie 

 between rings 96 and 97, and 97 is well marked by sensillae 

 (see PI. VI, Fig. 7). This, then, is the first ring of a somite. 

 The next four rings following have no prominent sensillae, but 

 ring 102 is again strongly marked with them ; 103 is a broad 

 double ring ; 104 is another double ring, the latter half of 

 which bears sensillae. The anus sometimes divides this part 

 of the ring and so comes to be bounded anteriorly by 103, but 

 generally a thin portion of 104 forms the anterior lip of the 

 anus. Rings 102, 103, and the anterior half of 104 make up a 

 pre-anal abbreviated somite, while rings 97 to loi form a com- 

 plete post-nephridial somite. Now going forward as far as the 

 first pair of nephridiopores (Fig. 3) the somites may be readily 

 traced by the nephridial openings, and they consist of five rings 

 each. At this point another criterion enables us to determine 

 one complete pre-nephridial somite. The ganglion in each 

 typical somite lies almost wholly in the first ring. If we count 

 five rings forward from the first nephridiopores, we find the 

 first ganglion of the nerve cord lying in this ring, the 12th. 

 The innervation of these five rings also proves that they make 

 up a complete somite. To recapitulate : The somite anterior 

 to the anus is reduced to two and a half, morphologically four, 

 rings ; thence forward to the 12th ring inclusive we find eight- 

 een complete somites, innervated by the eighteen separate 

 ganglia of the nerve cord. 



The reduced somites of the head region are innervated from 

 the "brain" and sub-oesophageal ganglia, while the reduced 

 somites of the anal region are innervated from the anal 

 ganglia. 



