CHAP. VI.] 



ANNELIDS. 



157 



enclose a small spindle-shaped island of tissue. Three segments 

 lower a wedge-shaped half-segment is similarly formed. At 



Fig. 21. Examples of imperfect segmentation in Annelids (after Cori). 



I. Lumbriciis terrestris (No. 88). II. L. terrestris (No. 89), as seen when laid 

 open on the dorsal side. III. Lumbriconereis (No. 90). IV. Halla parthenopeia 

 (No. 91). 



N, nephridium ; Np, nephridial pores; D, alimentary canal; dG, dorsal vessel; 

 vG, circular vessel. 



The letters a, b, c, &c. indicate the parts belonging to the respective segments. 



another point in the same animal (not shewn in Fig. 21) one 

 of the segments was partly divided into two in the right dorso- 

 lateral region. Cori, p. 572, figs. 8 and 9. 



Sjriral Segmentation \ 



92. Lumbricus terrestris. Fig. 22, I. A shews a part of an 

 Earthworm seen from the dorsal side, the ventral side being 

 normal in appearance. By following the groove indicating the 

 plane of the septum between b and c on the right side to the 

 ventral surface, it could be traced to the left side between b and c, 

 so across the dorsal surface, between c and d on the right side, 

 across the ventral surface and between c and d on the left, reach- 

 ing nearly to the middle dorsal line again. This is shewn dia- 

 grammatically in Fig. 22, I. B. 



93. A simpler case affecting one segment only is shewn in Fig. 

 22, II. 



94. Another specimen exhibited a similar arrangement near the 

 tail-end (Fig. 22, III.). The lettering of the figure sufficiently 

 explains the course of the spiral septal plane. [Cori does not 

 state that the septa internally formed a spiral division, but it can 

 scarcely be doubted that they did so, following the external groove, 



1 Further observations on this subject have been lately published by Morgan, 

 T. H., Joum. of Morph., 1892, p. 245, and by Buchanan, F., Q. J. M. S., 1893. 



