55] LIFE HISTORY OF TREMATODES— FAUST 55 



indistinct and thoroly fused with the ganglion masses. The latter are wide, 

 with a construction in each in the region of the origin of the lateral trunks. 

 The trunks figured by Thoss are probably the ventrales, since they supply the 

 main innervation of the animal. The anterior ventralis arises along with 

 the posterior ventraUs just anterior to the latter. The anterior trunk soon 

 divides. The major portion runs around the oral sucker, while the external 

 branch is traceable anterolaterad. The anterior lateralis is represented by a 

 blunt stock just outside the pharynx. It runs cephalad but soon ends in two 

 delicate branches. There is no posterior dorsal of posterior lateral. The 

 posterior ventral is stout and thick. It gives off one prominent branch ex- 

 ternally soon after it reaches its most external position. At regular intervals 

 it gives off branches internally which have the indication of rudiments of 

 commissures. These transverse trunks just anterior and posterior to the 

 acetabulum are still well developed; they are similar to those described for 

 the distome. 



Contrary to the opinion of Looss (1894:245, 246), the writer has found 

 without exception that the general trematode nerve anatomy can be traced 

 from the early germ balls up thru various stages of growth, and that the 

 cercaria shows not only the potentialities of the adult system, but actually 

 the details of this system. Moreover, the study of various groups of cer- 

 cariae has demonstrated that the fundamental deviations and modifications 

 from type are recognizable in the mature cercaria. Thus this study has 

 shown that the nervous system of the cercaria is constant for the group to 

 which it belongs, and is a definite basis for the natural classification of the 

 groups. 



In contrast to the highly developed nervous system found in the cercaria 

 is that of the parthenita. In the sporocyst no definite nerve complex is found, 

 altho Looss (1892) has observed nerve elements in miracidia of Amphistomum 

 subclavatum. In the redia, however, with the continued functioning of the 

 highly muscular pharynx, there is a nerve complex practically embracing the 

 entire anterior portion of the gut (Fig. 125). Viewed from the dorsal, ventral, 

 or lateral aspect, the system in surface view appears as an H. It is resolvable 

 into four anterior trunks, four posterior trunks, and a ring commissure. On 

 the dorsal side are two swellings, the rudiments of the cerebral ganglion masses 

 of the cercaria. The nerve cells of the system are very prominent. They 

 are usually bipolar or multipolar (Fig. 126), but, as might be expected, the 

 more superficial ones are more often the multipolar cells. 



This redia nervous system constitutes a very primitive type, in which the 

 nerve cells are much more frequently diffuse and more discrete than in the 

 systems in the cercariae. It is probable that the pharynx is responsible for 

 keeping the system from total degeneration. 



The size, number, and location of the ganglion cells vary according to indi- 

 vidual species of cercariae. They may be situated within the gangUon centers, 

 as in Cercaria micropharynx (Fig. 97) and C. glandulosa (Figs. 69, 70); they 



