426 University of California Publications in Zoology. [ V L - 6 



stems and rings, associated with the development of the powerful 

 musculature of the organ of attachment, the scolex. 



That this is in harmony with the course of development 

 elsewhere in the phylum is shown by consideration of the 

 probable construction of the nervous system of the primitive 

 turbellarian-like ancestor and the changes it has undergone. 

 Throughout the phylum there is remarkable uniformity in the 

 ground-plan of the nervous system. The primitive structure in 

 the free-living Turbellaria is a sub-dermal plexus of fibres and 

 ganglion cells with a marked concentration of these at the 

 anterior end, and an increase in their number and size on the 

 ventral or creeping surface. This differentiates in two direc- 

 tions ; first in the segregation from the plexus of from six to eight 

 longitudinal strands, with irregular transverse connecting fibres ; 

 and second, in the increase in size of the main bridge-commissure 

 or brain, and in the development of secondary commissures in 

 the region of the brain and in the neighborhood of specially 

 developed musculature, notably in the pharynx and in organs of 

 attachment. This is to be seen in the remarkable development 

 of the posterior commissure in the heterocotylean in connection 

 with the development of the posterior sucker as the principal 

 organ of attachment. Further, two of the longitudinal nerve- 

 strands, the ventral, become more highly developed than the 

 rest. In the trematodes there are two longitudinal strands, con- 

 nected near the anterior extremity b}^ a bridge-commissure rich 

 in ganglion cells from the region of which arises a pair of sensory 

 nerves. There are in addition numerous peripheral longitudinal 

 strands and an indefinite number of cross-nerves, anastomosing 

 among themselves to form irregular ring-commissures about the 

 body. In the posterior region, in close relation to the large 

 sucker, is developed a complicated system of commissures and 

 rings. From this type the nervous system of Gyrocotyle has 

 been derived. The longitudinal strands are the same in both; 

 there are however two bridge-commissures in Gyrocotyle. But 

 the sensory function of the acetabulum and the forward direc- 

 tion of that extremity in locomotion, together with the well- 

 developed nature of that commissure throughout as compared 



