54 ILLINOIS BIOLOGICAL MONOGRAPHS (54 



monostome cercariae seems to indicate that it is the melanoidin fraction of 

 the oxidative processes in the nervous system. The possibility of utility as 

 a receptor of light or heat is a secondary item and must not be confused with 

 the primary meaning of the pigmentation. 



In the free-Uving Platyhelminthes the fully developed eye is present in the 

 mature individual. In the ectoparasitic trematodes the eye-spot is well 

 developed in the young animal (Hesse, 1897:560, 561), but degeneration takes 

 place as the animal matures. Goto (1894:81), speaking of Tristomum, ob- 

 serves: "Morphologically speaking they are certainly degenerate eyes; and 

 have probably been derived from such eyes as are found in Turbellaria; but 

 I do not think they are functional. In the first place the pigment granules 

 are situated on the dorsal side and thus prevent the light from reaching the 

 lens, since the dorsal side is the only direction from which light can come. In 

 the second place there is not always a distinct retina. If these 'eyes' are 

 really still useful to the animal, they may possibly be a temperature sense 

 organ; and for that purpose their structure seems to answer well." Goto 

 goes on to show that the more degenerate condition of the eyes in Tristomum 

 ovale is due to the greater degree of internal parasitism of this species than 

 that of Tristomum molae. In the monostome, the eyes are well developed in 

 the cercaria but become fully degenerate, with a loss of all the pigment in the 

 adult, so that the adult of one species has been described by Creplin as "albi- 

 dus" (Jagerskiold, 1891 :4). In some species of Allocreadiidae {Crepidostomum 

 farionis O. F. M. and C. cornutum (Osborn)), pigment eye-spots are found in 

 the adult. A still further stage of degeneracy is found in Cercaria racemosa 

 (Fig. 100) and C. gracillima (Fig. 144), where the optic nerve is still present 

 but the pigmentation is absent. 



In Cercaria gracillima, the representative of the furcocercariae, the ner- 

 vous system is narrow, in correspondence with the attenuate condition of the 

 animal. The posterior laterales are not found in the mature cercariae, altho 

 the bud is present in the germ ball (Fig. 151). One is struck by the significant 

 resemblance of the main nerve complex of this cercaria and that of Schistosoma 

 haematobium, described in detail by Looss (1895). The three pairs of anterior 

 trunks are readily made out, altho, in addition, a prominent dorsolateralis is 

 found (Fig. 150). The posterior dorsalis arises from the dorsal side of the 

 ganglion cell mass and proceeds caudad to the region of the acetabulum, where 

 it fuses with the ventral trunk. A prominent subesophageal commissure and 

 a small pharyngealis are present. The fundamental resemblance between 

 the system described for this cercaria and that for the schistosome adult 

 seems to the writer to be sufficient morphological evidence for the correlation 

 of these apharyngeal furcocercariae with the Schistosomatidae. 



A study of the nervous system of the Holostomata has been made of Cer- 

 caria ptychocheilus, based on both toto mounts and sections (Fig. 53). No 

 adequate idea of the nervous system of this group can be secured from the 

 meager data of Brandes (1891) and Thoss (1897). The dorsal commissure is 



