CHAPTER VI. 



linear series — continued. 



Ch^etopoda, Hirudinea and Cestoda. 



Imperfect Segmentation \ 



Though from the circumstance mentioned at the beginning of 

 Chapter n, that the total number of segments in the Annelids is 

 generally indefinite, true Meristic Variation cannot be easily re- 

 cognized in this group, there is nevertheless a remarkable group of 

 cases of imperfect segmentation, in which by reason of the incom- 

 pleteness of the process of Division, the occurrence of Variation is 

 at once perceived. The following cases were all originally described 

 by Cori, who speaks of them as instances of "intercalation" of 

 segments. For reasons sufficiently explained in the Chapter on 

 Vertebras, there are objections to the use of this term, if only as a 

 mode of expression, and the evidence concerning these cases has 

 therefore been re-cast. 



*88. Lumbricus terrestris : the 46th segment having the form 

 shewn in Fig. 21, I. being normal on the right side, but double on 

 the left. Internally a septum divided the two parts a and a from 

 each other. Each of them contained a nephridium, setae, &c. 

 Cori, C. J., Z.f. w. Z. t liv. 1892, p. 571, fig. 1. 



*89. Specimen having, in the region close behind the clitellum, 

 three consecutive segments, each resembling that just described. 

 Of these the first was double on the right side, the second on the 

 left, and the third on the right again. Fig. 21, II. shews the 

 internal structure, the nephridia and other jmrts having doubled 

 in each of the doubled half-segments. Cori, /. c, p. 572, fig. 2. 



90. Lumbriconereis : case similar to the first case in Lumbricus, 

 Fig. 21, III. Cori, C. J., I c, p. 572, fig. 4. 



91. Halla parthenopeia. A specimen 50 cm. long presented 

 numerous abnormalities of which two are represented in Fig. 21, IV. 

 At the point marked a the lines of division between the segments 



1 Numerous facts illustrating this subject are given in a recent paper by 

 Buchanan, F., Q. J. M. S., 1893. 



