1890.] . Annelid Descent. 1143 
ANNELID DESCENT: THE ORIGIN OF METAMERISM 
AND THE SIGNIFICANCE OF THE MESODERM: 
BY EDUARD MEYER. 
HE recent publication by Claus of a paper “ On the Morpho- 
logical and Phylogenetic Significance of the Body of the 
Tapeworm "? has induced me to defer no longer the long-enter- 
tained and elaborated conception of the origin of metamerism 
amongst Annelids, and thus amongst all.segmented Bilateralia, 
but to communicate to my fellow-workers, provisionally at least, 
a general sketch of it. 
Claus, in the paper quoted above, refuting the conception of 
the tapeworm as a colony due to strobilation, brings forward 
evidence that the segmented Cestode body is to be derived from 
a non-segmented form in which, at first, the internal organs 
(sexual organs) appeared in metameric sequence, and later a 
corresponding external segmentation was introduced. This has 
finally attained a maximum in the superlative individualization of 
pieces of the body that separate completely from it in the forma- 
tion of proglottids. 
In my opinion, the Annelids owe the metamerism of their bodies 
to a process quite analogous, which here has produced a certain, 
but never, indeed, a complete, individualization of. the segment, 
and which in some cases has finally degenerated into a non-sexual 
reproduction by fission. 
This last phenomenon, in which I can see, when occurring in 
the higher Metazoa, a considerable degree of degradation, is often 
used as an argument in favor of their hypothesis by the supporters 
of the doctrine that animal segmentation had its origin in budding, 
We should not forget, however, that reproduction by fission 
among the Annelids has either been observed in only such forms 
as have a structure implying undoubtedly not primary but degen- 
! Translated by Dr. E. A. Andrews from the Biologisches Centralblatt, X., July, 1890. 
? Arbeit. a. d. zool. Inst. Wien, VIII., 1889. 
Amer. ber.—3. 
