722 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
Vermes. 
a. Annelida. 
Descent of Annelids.* — Herr E. Meyer makes this essay a text for 
some remarks on the origin of metamerism and the significance of the 
mesoderm. Claus has recently suggested that the jointed Cestoda may 
be derived from the unsegmented forms owing, primarily, to the 
metameric repetition of the gonads. The author believes that an 
analogous process is the cause of the metamerism of the body of 
Annelids; though it never produces complete individualization of the 
segments, it does in some cases lead to asexual reproduction by 
division. 
The ancestors of the Annelids appear to have been strong, predatory 
Turbellaria which lived a pelagic life. Their body was elongated, and 
they may have had some resemblance to the Nemertines, which, how- 
ever, were not their ancestors, for they form a distinct side-branch. 
The gonads were placed in the body-parenchyma, which was partly 
surrounded and partly traversed by muscles, and were, in the young, a 
single pair of compact cell-cords, but in the adult long hollow tubes 
opening at the hinder end of the body by a pair of simple dermal pores. 
It is conceivable that these tubes, when filled with sperm or ova, would 
affect the flexibility of the whole body ; this may have led to the two 
tubes being broken up into two rows of metameres of equal size. They 
would naturally become centres of metamerism, around which the other 
organs, diffused and scattered through the body, would become meta- 
merically grouped. This grouping would affect the spaces in the 
parenchyma, and so would give rise to the paired and segmentally 
chambered secondary body-cavity. 
The extension of the gonads would cause the filling up of a large 
part of the primary coelom, which in the ancestors of Annelids was 
probably an irregular system of lymph spaces and ducts ; only a small 
portion would, therefore, remain over to form the definite blood- vascular 
system. The author offers the following hypothesis with regard to the 
origin of the neural and haemal longitudinal muscular areas. Some of 
the non-productive elements of the walls of the gonads may be looked 
upon as epithelio-muscular cells, the distal fibrillar portions of which 
became drawn out into two ends ; these fibrillar parts, by their con- 
tractions, exerted a pressure on the contents of the follicular cavities, 
and were, therefore, primitively of use in ejecting the generative 
products. When the walls of the follicles became attached to the 
integument and enteron, the fibrils lost their function and disappeared, 
except in the longitudinal areas of the outer body-wall, where they at 
first strengthened the primary longitudinal musculature, and subse- 
quently completely replaced it. The author further developes the 
consequences of this change. 
Herr Meyer holds to the opinion that the nephridial tubes should be 
regarded as parts of a pair of longitudinal canals, such as are possessed 
by the Turbellaria ; the presence of intersegmental constrictions of the 
body hindered the flow of fluid and caused the formation of metameric 
orifices. The peritoneal funnels are neomorphs. 
* Biol. Centralbl., x. (1890) pp. 296-3C8. 
