The adult organisation of Paragordius varius. 399 
polygonal (Fig. 37, Pl. 39). But in those muscle cells of the medio- 
ventral region of the trunk (Muse Fig. 25, Pl. 38), and to less extent 
those of the medio-dorsal (Muse Fig. 42, Pl. 39), the cells are more 
voluminous in size and less flattened, and in most of them the fibrils 
appear much larger on cross section, and are more irregular in size 
and distribution: this relation is to be seen from the carefully drawn 
Fig. 25, Pl. 38, and Fig. 42, Pl. 39. Here there is frequently a 
grouping of the fibrils into pairs, sometimes several are so closely 
apposed as to form one large one, and the fibrils often appear ribbon- 
shaped rather than rounded. Thus the structure of the muscle elements 
in the median line is different from that of those elsewhere on the 
body wall, the principal difference being the greater volume of the 
cell and the smaller number but larger size and more irregular distri- 
bution of the contractile fibrils. Now in death the worm generally 
contracts so as to bend ventrad, due certainly to the ventral muscu- 
lature having a greater power of contraction; perhaps this is due 
again to the larger size of the fibrils of these cells, or perhaps the 
larger size of these fibrils is due to their greater degree of con- 
traction. 
No transverse striation of the muscle cells could be found on 
longitudinal sections. 
The muscle cells are not branched, and by proper use of the 
iron-haematoxylin stain the muscle cells can be sharply differentiated 
from the intermuscular substance. The latter is found in the greater 
region of the trunk, it separates the muscle cells from each other and 
surrounds each of them on all sides. It is a dense substance (1. M.S 
Fig. 25, Pl. 38; Figs. 37, 42, Pl. 39) of a different chemical nature 
from the contractile zone of the muscle cells, and contains sparsely 
vacuoles with deep-staining rounded bodies in them, perhaps de- 
generated nuclei. The septa made by this substance between the 
muscle cells are very regular in diameter (Fig. 37, Pl. 39), while the 
diameter of the muscle cells on a given plane of a cross section is 
very variable. Except with suitable staining it is very difficult to 
distinguish between the clear cores of the muscle cells and this inter- 
muscular substance. The latter shows the characteristic of the 
parenchym in having vacuolar cell bodies with a large amount of 
dense intercellular substance; but whether it is a degenerate parenchym 
layer, or is a product of the muscle cells, cannot well be determined 
in the adult stage. In the anterior trunk region (Fig. 14, Pl. 37), as 
in the posterior ends of the tail lobes (Fig. 71, Pl. 41), and in the 
