158 



MERISTIC VARIATION. 



[part I. 



like the spiral valve of an Elasmobranch's intestine.] Cori, 

 Z.f. w. Z. t Liv. 1892, p. 573, figs. 5, 6 and 7. 



* 



I B 



Fig. 22. Spiral segmentation in Lumbricus terrestris. 



I, A, the case No. 92 ; I, B, diagrammatic representation. 



II, A, the case No. 93 ; II, B, diagrammatic representation. 



III, the case No. 94. (After Cori.) 



Two other cases described by Cori may be mentioned here, though 

 there is a presumption that they are not really examples of Variation 

 in the segmentation along the axis of a Primary Symmetry, but rather 

 belong to the class of Secondary Symmetries. They are alluded to 

 here as it is convenient to illustrate this distinction by taking them 

 in connexion with the examples just given. 

 95. Hermodice carunculata. (Fig. 23, III.) Between two normal 

 segments is what seems at first to be a segment double on the left 

 side with two complete sets of parapodia, but imperfectly divided 

 on the right (left of figure), the septal groove stopping short before 

 it reaches the parapodial region. The lower half on this side is re- 

 presented with a normal ventral ramus of the parapodium, but the 

 ventral ramus in the upper was itself partially doubled, having in 

 particular two cirri Cv. I. and Cv. II. and two branches of setae. The 

 condition of the dorsal ramus is not described. Of course without 

 seeing this specimen it is impossible to say more than this, but the 

 figure strongly suggests that the division between the two halves of 

 this parapodium was a division into images and not into successive 

 segments. The figure represents the lower cirrus Cv II. as standing 

 in the normal position for the cirrus, on the posterior limb of the 

 parapodium, but the anterior cirrus is distinctly shewn as placed on 

 the anterior limb of the elevation and anterior to the bristles. If 

 this were actually the case, this double parapodium must be looked 

 on as a kind of bud, with a distinct Secondary Symmetry of its 

 own. Described afresh from Cori, C. J., Z.f. w. Z., liv. 1892, p. 574, 

 fig. 3. 



