52 ILLINOIS BIOLOGICAL MONOGRAPHS [52 



are connected by commissures. Those around the acetabulum are of especial 

 importance. The preacetabular commissure arises from the lateral trunk 

 and proceeds in a posterior oblique course, then around the anterior reach of 

 the acetabulum to meet the branch from the other side. The postacetabular 

 branch arises from the lateralis also, but continues caudad along the course 

 of the ventralis. Behind the acetabulum it meets the branch from the oppo- 

 site side to form the span. From these trunks circumscribing the acetabulum 

 two ring commissures arise, a superficialis and a profundus, of which the latter 

 with the nerve endings is found in the section (Fig. 71). 



The nervous system of the monostome, as worked out by Jagerskiold (1891, 

 Taf. I) for Ogniogaster plicatus (Crepl.), has been substantiated for the most 

 part in the study of Cercaria pellucida (see figure 23). The three paired 

 posterior trunks are evident. The posterior ventralis is the most important 

 of these and can be traced to the caudal extremity of the animal in all cercariae 

 of this species, as well as in C. konadensis and C. urhanensis. The dorsalis is 

 less conspicuous, yet it is usually traceable far caudad. The posterior lateralis 

 is delicate. It arises near the origin of the ventralis and swings out laterad in 

 a great bow. It, too, can usually be traced to the posterior extremity of the 

 body. 



The anterior trunks can also be definitely traced. The ventralis arises 

 from the extreme lateral horn of the ganglion center in conjunction with the 

 posterior ventralis. More median, the anterior lateralis is found. The two 

 anterior dorsales arise as a single structure along the median line. Their 

 primary function in the cercaria is the innervation of the median pigment eye, 

 altho branches may be traced farther anteriad. Likewise, an important 

 branch of each posterior dorsalis constitutes the nerve tract to the lateral eye. 



The ganglion cells in the monostomes are superficial to the nerve strands. 

 A considerable number of them are not even in intimate contact with the 

 fibers, but have fibrillar communications with them. The ganglion masses 

 from which the nerves arise are distinctly cornuate, wdth a wide dorsal com- 

 missure. 



Pigmentation and eye-spots in the monostomes. The monostome cer- 

 cariae that have come under the direct observation of the writer, together 

 with those described in the literature, may be placed in two groups according 

 to their eye-spots, namely, those with a single pair of eye-spots, disposed 

 laterally to the brain center, and those with an additional median eye, anterior 

 to the central nervous system. The amount of pigmentation is considerably 

 larger in the trioculate species than in the binoculate type. Figures 1 to 3 

 show in dorsal view a series of stages in the pigmentation of Cercaria pellucida. 

 The pigment originates anteriorly over the brain center and proceeds caudad 

 along six lines of growth. A very careful study of the pigmentation in these 

 species indicates that a very simple but reasonable relation exists between 

 the pigmentation and the underlying nervous system. The pigmentation is 

 found to be a delicate superficial index of the underlying nerve fibers. This 



