1890.] Annelid Descent. 1145 
lids are regarded as descended from Echinoderms, as has been 
‚done (R. Wagner), we can scarcely come nearer to the solution 
of the above question, since we are driven to resort either to 
budding, strobilation, or to circumlocutions, such as “intermittently 
progressive processes of growth and differentiation.” We will, I 
believe, arrive much sooner at the goal if we derive the Annelids 
from Planarian-like ancestors, for which the development, espe- 
cially the differentation of the mesoderm, gives ample ground. 
We find in Annelids, typically, as a chief constituent of the 
mesoderm, the two mesoderm-bands growing forward from two 
pole-cells. These bands, at first solid strands, subsequently break 
up, in the trunk, into the paired mesodermic somites, which be- 
come hollowed out and give rise to the definitive body-cavity ; 
this constitutes the so-called secondary mesoderm. Besides this, 
larvae as well as embryos of Annelids have a “primary” meso- 
derm, which not only functions in the larva before the formation 
of the mesoderm-bands, but also furnishes a considerable part of 
the permanent organs. To the former category belong the various 
simple muscles and the excretory organs of the larva; to the 
latter also a number of muscles, namely, the circular muscles, 
the transverse muscles, the muscles of the setae sacs, septa, and 
mesenteries, the special muscles of the various parts of the diges- 
tive tract, also the retro-peritoneal connective tissue whenever it 
is formed, and in certain cases the excretory part of the definitive 
nephridia, which I have called the nephridial tube. Such a state 
of things I have established as existing in various Polychzte 
larvae, and the same may be inferred with tolerable certainty from 
many statements in the literature, though the interpretations there 
are manifoldly different. The distinction between the Primary 
mesoderm, or embryonic mesenchym, as it might be better called, 
and the secondary, or celomic mesoderm, appears most plainly 
when a spacious primary body-cavity occurs between the ectoderm 
and the entoderm, separating the parietal mesenchym next the 
skin from the mesoderm-band next the digestive tract. As an 
example of this I would cite the larva of Psygmobranchus.? 
3See Pls. 23 and 24 in Mitth. Zool. Station Neapel, VIII., 1888. I have there called 
i mesoderm “parenchyma,” from consideration of the possible 
reference to the corresponding tissue of the Turbellarians. As, however, this designati 
carries with it the idea of a more compact tissue, it is thus not quite a fitting one, and so 
I return to the old name, “ mesenchym " 
