400 T. H. MORGAN. 



Fig. XI^ A, B, shows a further extension of viii, with three 

 turns. 



There are several combinations of the preceding categories 

 that are possible, and some of these 1 have found. It is not 

 particularly important to discuss these possibilities, as they all 

 reduce back to forms already described. As an example, 

 however, one such is given in fig. XII, a, b, c ; the reconstruc- 

 tion, c, sufficiently explains the conditions. It will be noted 

 that five half- segments of one side correspond to three of the 

 opposite. 



Whenever a double half-compound metamere is introduced 

 a more complicated form of spiral results (see fig. XIV, and 

 PL 42, fig. 47, at x^). This causes a double spiral, i.e. two 

 spirals, to take a parallel course, as shown in fig. XIV. One or 

 both of the spirals may end in a half-compound metamere — 

 both so end in fig. XIII. This reconstruction will, I think, 

 sufficiently explain the conditions, so that a further description 

 would be superfluous. 



The double spiral may be formed in other ways. Two suc- 

 cessive compound metameres may be introduced in such a way 

 that a new spiral is started along with the one already present. 

 Such cases are shown in fig. XIII, and in fig. 47, PL 42, at 

 x^. Several variations of these combinations suggest them- 

 selves, and several have been found amongst the worms, but 

 the discussion of these variations would not add materially to 

 the former cases, and may be omitted. Even a triple spiral 

 was found in one case, as shown in fig. 47 at x^. It was of 

 short duration, owing to the introduction of compound meta- 

 meres in such a way that the spirals were quickly absorbed. 



Lastly, the series of compound metameres, double and 

 triple spirals shown in fig. 47, PL 42, were all drawn from 

 the same worm. We find 134 half-metameres on the right 

 side and 118 half-metameres on the left. In all there were 

 fifty-two perfect rings, and the numbers in the plate between 

 consecutive groups of compound metameres, spirals, &c., in- 

 dicate the number of perfect rings in that locality. An exa- 

 mination of this reconstruction will show how far it is possible 



