A STUDY OP METAMERISM. 409 



the line between the 11th and 12th. In these last three cases 

 all failures were above, and in all the vasa deferentia were 

 normal. 



In fig. 11 the line between the 1st and 2nd failed to meet 

 below ; in fig. 12 between 13th and 14th below. In fig. 13 

 the line between the 5th and 6tli metameres failed to meet at 

 the sides. Both in figs. 11 and 13 the vasa deferentia were 

 normal, but in fig. 12 there were indications of an additional 

 opening on the left side of the 16th metamere. 



Conclusion. — In nine out of the ten cases the position of 

 the openings of the vasa deferentia were not affected by the 

 failure to meet of metameres anterior to the openings, nor was 

 a shifting to be expected on a priori grounds, since the modi- 

 fication is clearly only a lack of perfect union between the 

 right and left half-metameres (fig. 13 perhaps excepted). 



Fig. 12 is, then, a case of doubling of the openings of one 

 side, like those described in the last section, and can have 

 nothing to do with the lack of perfect union of the more 

 anterior segments. 



It is noticeable that in seven out of these ten cases the 

 imperfect union is on the dorsal surface. In only two cases is 

 it found below, and in one at the side. We will return to a 

 consideration of the point at another time. 



C. Figs. 14—19, 20—26. 



Two cases are to be considered under this head : 1st, those 

 in which the spiral introduces no new half-metaraeres on 

 either side ; 2ndly, those cases where a new half-metamere is 

 introduced in front of the 15th metamere, so that one half of 

 15 becomes united to 16 (15 — 16), and the other half that 

 remains is united to half of 14 (15 — 14). 



1st case. — In fig. 14 a spiral is found involving the 7th, 

 8th, and 9th metameres. It belongs to category viii (PI. 40, 

 fig. viii). No new half-metameres are introduced, and the 

 vasa deferentia open on their normal (15th) metamere. 



In fig. 15 a similar spiral involves the 12th, 13th, and 14th 

 metameres. Vasa deferentia as before, normal. 



