84 WILLIAM A. KEPNER AND ARNOLD RICH 



NERVOUS SYSTEM 



The central nervous system of this triclad consists of a pair 

 of dorsal ganglia, connected by a transverse commissure and of 

 two ventral nerve-trunks which give off at more or less regular 

 intervals lateral and mesial branches. We have been able to 

 trace a pair of the mesial branches of the ventral nerve-trunks 

 towards the base of the proboscis, so that there is good reason 

 for believing that the proboscis has a definite connection with 

 the central nervous system, such as Gamble ('01) describes for 

 triclads in general. As early as 1897 R. Monti ('97) wrote 

 of the ventral nervous system of Dendrocoeles, '^J'interprete 

 done les cordons longitudinaux comme une chaine ganglion- 

 naire non encore differenciee." Our own histological studies of 

 Planaria albissima show that the ventral nerves are not mere 

 bundles of nerve-fibers, but present a series of ganglionic masses 

 along their entire extent. Steiner ('98) found that these ganglia 

 exercised local control, for isolated posterior portions of Pla- 

 naria neapolitana moved under control of ganglia in these parts 

 of the body. The mesial branch of each of the ventral nerve- 

 trunks Ifeaves the ganglion that lies near the base of the proboscis 

 to enter the proboscis. 



ANATOMY OF THE PROBOSCIS 



The proboscis, when freed, is a slightly tapering, almost cylin- 

 drical tube, its posterior end being the wider. Near the ante- 

 rior end — the fixed end under normal conditions — there is a 

 muscular ring, which acts as a sphincter. Both the inner and 

 the outer surfaces of this tubular organ are densely covered 

 with cilia. The wall is highly muscular. A definite nerve- 

 plexus has been describe and figured (Gamble, '01) for the 

 proboscis of planarians. From what we have seen of the his- 

 tology of this organ, the proboscis does not greatly differ from 

 the description of the anatomy of the proboscis of other Pla- 

 naria. We have left the detailed histological study of this 

 organ for a later piece of work. It is important, however, to 

 mention here the presence of numerous glandular ducts which 



