CHAPTER VI. 



LINEAR SERIES continued. 



< 'II T.T'tPODA, HlRUDINEA AND CESTODA. 



1 in perfect Segmentation '. 



Though fr.'in the circumstance mentioned at the beginning of 

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

 generally indefinite true Men-tic Variation cannot be easily re- 

 cognized in this gn>ii]>, there is nevertheless a remarkable group of 

 cases "C I'/iijH'rfect segmentation, in which by reason of the incom- 

 pleteness <>t' the process of Division, the occurrence of Variation is 

 at once perceived. The following cases were all originally described 

 by CORI, who s] teaks of them as instances of "intercalation" of 



meiits. For reasons sufficiently explained in the Chapter on 

 V.-rt'-l.r.-e, there are objections to the use of this term, if only as a 

 mod.- of expression, and the evidence concerning these cases has 



lllei ef ( iiv lieell re-cast. 



*ss. Lumbricus terrestris : the 4>0th 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. Ka.-h of them contained a nephridium, setce, &c. 

 CORI, C. J., /./. w. Z., LIV. i,s!>2, p. :,71, fig. 1. 



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

 three consecutive segments, each resembling that just described. 

 Of the>e the first was double <m the right side, the second on the 

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

 internal struct nre. the nephridia and other parts having doubled 

 in each of the doubled half-segments. CORI, I.e., p. 572, fig. 2. 

 !)0. Lumbriconereis : case similar to the first case in Lumbricus, 



Fig. 21. III. CORI, C. J., I.e., p. 572. tig. 4. 



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

 numerous al.iiorinalit ies of which two are represented in Fig. 21, IV. 

 At the point marked ,/' the lines of division between the segments 



facts illustrating this subject are given in a recent paper by 

 X, F., V. ,7. M. .S., is-..:;. 



