78 ILLINOIS BIOLOGICAL MONOGRAPHS [78 



The nervous system of this species has been made the basis of the dis- 

 cussion on page 49, and needs no further consideration here. 



The musculature is ahnost all parietal, except for the walls of the intestinal 

 tract. Parietal and splanchnic muscles are similar in structure. The former 

 consist of external transverse and internal longitudinal bundles, while the 

 latter consist of external longitudinal and internal transverse fibers (Figs. 

 118, 119). Each fiber can be traced to a myoblast, the central figure of which 

 is the large oval nucleus, with karyosome and radiating processes, so that 

 the whole figure appears stellate. There are several fibers originating from 

 each myoblast; they always nm along a single axis. The longitudinal mus- 

 cles of the tail are prominent (Fig. 133). 



The body is filled with a parenchyma complex, consisting of undifferen- 

 tiated cells, connective tissue fibers, and cystogenous gland cells. In the tail 

 of an immature cercaria (Fig. 133), there is a partition of parenchyma cells, 

 separating the caudal excretory canals into right and left tubules. This 

 condition disappears as the animal matures, altho vestiges of these cells may 

 be found in the mature cercaria. 



The cystogenous cells are differentiated parenchyma cells, filled with a 

 mucoid in the form of oval granules. In the ordinary cystogenous cell (Fig. 

 113), the granules are about l/i by 0.6/i in section. There is a central nucleus 

 to each of these cells, with poorly defined membrane separating it from the 

 cytoplasm. It is conspicuous because of its large number of chromatic gran- 

 ules, composed of elongate flecks. These flecks are also present in considerable 

 numbers thruout the cytoplasm; they are especially massed against the cell 

 walls. The glands are best developed in the middle of the body (Fig, 109, 

 sections hb and dd). Since the cystogenous glands are well developed, the 

 cyst wall is heavy (Fig. 115, A-C). In crawling over the surface of any object 

 the mature cercaria squeezes off the tail by a constriction of the posterior 

 transverse muscles. A final jerk of the tail frees it from the body. Immedi- 

 ately the cystogenous glands pour out a mucus aroimd the contracting worm, 

 so that at first an oval cyst is formed. Later, as it hardens, it assumes a more 

 spherical outline (Fig. 116). Thru this cyst membrane the excretory and 

 digestive systems of the body and the collar spines are readily distinguished. 

 The cysts are so well walled and so numerous that they suggest a considerable 

 period of wintering over. 



Cercaria biflexa Faust 1917 



Cercaria biflexa belongs to a type of echinostome cercariae distinguished 

 by a smooth body outline, a long powerful taU, blunt oral hood spines and a 

 reflexed excretory tube. The excretory system worked out by Looss (1894: 

 Fig. 191c) for Distomum echinatum approaches the system in this species to 

 some extent, but differs from it in many details. 



The body of Cercaria biflexa is elongate ovoid, with a slight constriction 

 just behind the oral hood (Figs. 134, 135). Both the body and the tail are 

 extraordinarily muscular. The body measures 0.45 mm. to 0.5 mm. in length 



