38 ILLINOIS BIOLOGICAL MONOGRAPHS [158 



nuclei and with the cell boundaries only very faintly indicated (Figs. 50, 72, 

 73,75,79,84,86,106). Around the outside of the intestine is a heavy 

 membrane (Fig. 77), easily demonstrated when stained with Mallory's 

 connective tissue stain. The inner edge of the wall is frequently dififerenti- 

 ated into a loose, porous or spongy structure, with no definite membrane 

 on the inner surface (Fig. 106). During the early part of the formation 

 of the fibrous cuticula the cells of the intestine begin to decrease in size, 

 in the adults they are excessively shrunken, and by the time the reproduc- 

 tive products have been discharged there is little left but the skeletal struc- 

 ture of the cells (Fig. 46). 



At the posterior end the intestine in the very young forms opens slightly 

 ventral (Figs. 11, 15), but by the twelve day stage has become nearly ter- 

 minal (Figs. 63, 64). At that stage diverticula are formed at the points 

 where the seminal receptacle and oviducts in the female and the sperm 

 ducts in the male later enter the cloaca (Figs. 64, 65). The part of the intes- 

 tine behind these diverticula is in the young stages lined by a heavy mem- 

 brane and in the adult condition lined by the homogeneous cuticula 

 (Fig. 78) and must be regarded as consisting of invaginated hypoderm. 

 In the female the opening of the intestine remains terminal, but in 

 the male it is again shifted to the ventral side. The lobes of the fork 

 first grow out beyond the cloacal aperture (Fig. 59) and then by an over- 

 growth of the dorsal wall the aperture is turned to the ventral side (Figs. 

 60, 61). The larval cuticula does not enter the space between the prongs, 

 but leaves this space to be filled out by a substance similar to the homo- 

 geneous intermediate layer, the intestine opening terminally thru a passage 

 in this substance (Figs. 36, 60). 



Excretory system At no stage in the development is there present any 

 trace of an excretory system corresponding in any way to the excretory 

 systems found in other animals. 



Circulatory system. There exists no definite circulatory system, but 

 there are present at all stages in development spaces in diflFerent parts 

 of the body that undoubtedly aid in the distribution of the body fluids. 

 Very early the intestine becomes surrounded by an open space, remaining 

 attached to the other tissue only on the ventral side. This space is later 

 invaded to a great extent by the parenchyma but is seldom entirely 

 eliminated. It is the only space that is usually present. In the females a 

 second space later appears on the dorsal side between the points of attach- 

 ment of the ovaries. These spaces usually do not approach the ends of the 

 body which are filled with parenchyma. 



Muscles. In the very young forms the muscles appear as a layer 

 of longitudinally arranged, spindle-shaped cells, lining the hypoderm 

 (Fig. 67) . The cells are at first rounded in cross section but soon become 

 flattened by crowding against each other so that they appear as columnar 



