52 ILLINOIS BIOLOGICAL MONOGRAPHS [172 



cavity lies around the dorsal part of the intestine and may be divided by 

 the gonads into a median canal over the intestine and two lateral canals, 

 or the median canal may be absent (Figs. 145, 158). In the males 

 frequently the parenchyma fills the entire space, leaving no cavities around 

 the intestine. In the females there is usually also a cavity on the dorsal 

 side between the ovaries. This is seldom present in the males. In later 

 stages, when part or all of the reproductive products have been discharged, 

 other cavities appear in both males and females. 



Muscles. As in the previous species, these consist of a cylinder of 

 longitudinal fibers just beneath the hypoderm and of the cloacal muscula- 

 tures. The cylinder of longitudinal muscles is interrupted only on the 

 ventral side by the connection between the nerve cord and the hypoderm, 

 and is lost in the parenchyma before the extreme ends of the body are 

 reached. 



Longitudinal muscles. The longitudinal muscles begin their differ- 

 entiation slightly later than they do in Gordius rohustus. In the youngest 

 specimens sectioned they are still similar to mesenchyma cells, and appear 

 rounded or cuboidal in cross section, but have the shape of short spindles 

 when viewed from the side (Figs. 147, 155, 156, 165). Later the ends elongate, 

 and the main bodies of the cells become crowded inward by the intercala- 

 tion of the elongating ends near the hypoderm. Soon, however, the bodies 

 of the cells also elongate and the diameters become nearly equal at the 

 inner and the outer edges. The ultimate shape of the muscle cell is essen- 

 tially the same as in the previous species, but the cell is even more elong- 

 ated and the nucleus is very much elongated so that it extends even into the 

 narrower parts of the cell (Figs. 6, 158, 174). The adult muscle cell shows 

 a layer of longitudinal fibrils, similar to that of the previous species, lying 

 just inside the cell membrane and completely surrounding the remainder 

 of the cell. Some of the cells that are crowded inward by the nerve cord 

 and come to lie at the side of the connecting lamella develop into muscle 

 cells, so that some of the muscles appear to be inserted on the lamella. 



Cloacal musculature. The radiating cloacal musculature of the male, 

 so prominent in the previous species, is lacking in Paragordius varius 

 except for a few longitudinal fibres at the posterior side of the cloaca 

 which are clearly continuous with the longitudinal body muscles. 



The circular muscles around the sperm ducts are located a short dis- 

 tance anterior to the cloaca and are more highly developed than in the 

 previous species (Fig. 1 64) . They develop from mesenchyme cells (Fig. 142) . 



As in the female of Gordius rohustus, so in the female of Paragordius 

 varius, circular fibers are found chiefly around the duct connecting the cloacal 

 gland with the seminal receptacle, but also surrounding the gland itself 

 and in a thin sheet even surrounding the cloaca behind the gland. Very 

 thin layers of these fibers also surround the oviducts. The fibers develop 



