50 ILLINOIS BIOLOGICAL MONOGRAPHS [50 



laterales and proceed forward with them for some distance. After separation, 

 the dorsalis runs more median, then flexes outward and dorsalward over the 

 oral hood, proceeding toward the apical sensory center. The lateralis at the 

 point where it leaves the dorsalis proceeds outward and forward, so that it 

 reaches a level shghtly below the plane of the central nervous system. Slightly 

 after diverging from the dorsalis the lateralis gives back a commissure to the 

 dorsalis. This dorsolateral connective fuses with the dorsaUs just behind 

 the posteriormost branching of this trunk. Arising from the most ventral 

 reaches of the gangUon center is the ventralis, a broad flat trunk, which courses 

 outward and downward to the oral Up of the oral sucker. Internally it gives 

 off the palatine, which Ues just lateral to the pharynx. 



Caudal to the central nervous system arise the four pairs of posterior 

 trunks, the dorsales, the laterales, the ventrales, and the pharyngeales. The 

 dorsalis branch arises dorsal and slightly lateral to the junction of the ganglion 

 and the commissure. It runs straight backward, slightly dorsal to the plane 

 of the central nervous system. The lateralis arises slightly lateral to the 

 dorsalis; its path lies outward and backward. The ventralis arises from 

 the very heart of the ganglion mass. It spreads outward and then runs back- 

 ward parallel to its mate. The ventral commissure, arises just median to the 

 origin of the posterior ventralis flexing below the prepharyngeal opening. It 

 is stout and bowed considerably downward. Between the commissure and 

 the ventralis arises the pharyngealis. 



In a germ ball of Cercaria trisolenata some 45/i by 60ju in size, the central 

 nervous system is well developed. This may be called the butterfly stage 

 (Fig. 121). At this period there are two pairs of main trunks anterior and 

 four pairs posterior to the central ganghon mass. The anterior trunks are 

 the ventrales and the laterales. By reference to the next stage in the devel- 

 opment (Fig, 122), the dorsales are found to arise from the sinus between the 

 laterales buds of figure 121. They arise at first as a single bud and bifurcate 

 later. Caudally the most conspicuous trunks are the dorsales, which arise 

 in a median plane and conspicuously dorsal to the other posterior trunks. The 

 dorsal commissure at this period is practically neghgible. The outer- 

 most ventral trunk-buds are the laterales, short and stubby at this 

 period of growth. The ventral trunks arise from the ventral portion of the 

 cerebral masses. Between them and the laterales arise trunks which are 

 present in the embryonic stage only. They may be designated as the posterior 

 intermedins nerves. 



In stage II (Fig. 122) a very decided change has occurred in the outUne of 

 the central nervous system, altho the fundaments of the first stage described 

 are present. Anteriorly the intermediate space between the laterales has 

 disappeared and from that region has arisen a wedge which is the fundament 

 of the paired dorsales. The ventral trunks have been set off to themselves by 

 a lateral growth and elongation of the intermediate fibers. On the caudal side 

 of the ganghon the posterior dorsalis has been separated from its mate by the 

 growth of the dorsal saddle commissure. Most noticeable, however, is the 



