ORGANS OF SPECIAL SENSE 165 



as to how a species of Prorhynchus without a penis would defend 

 itself. On placing some Prorhynchus applanatus with some other 

 species of Prorhynchus, which had chitinous penes, the former 

 animals defended themselves with their pharynges. This organ 

 can be thrown out of the mouth to a considerable distance and 

 with enough force to tear other flat worms to pieces. We think 

 that it is a point well to be noted that here we have a species 

 of Prorhynchus in which the pharynx has taken the place of the 

 penis as an organ of defense; or if not this, then the other alter- 

 native that the penis has not as yet developed to supplant this 

 function of the pharynx. 



Von GrafT's second statement that nothing is known of the 

 ciliated pits leads us first to consider the central nervous system 

 with which the pits are intimately associated. In the living 

 specimen at each side of the anterior end of the pharynx, lying 

 close to the epithelium of the proboscis sheath, are two color- 

 less, refractive, plano-convex bodies, the dorsal ganglia (text 

 figs. A and B). In life these ganglia appear to have their axes 

 parallel to that of the pharynx sheath. However, histological 

 preparations show that these ganglia are slightly concavo-convex 

 bodies, with their axes dipping ventrally at their posterior ends. 

 Thus the ganglia obliquely girdle the sides of the pharynx sheath. 

 At their more dorsal, anterior ends the ganglia are connected 

 by a dorsal commissure which arches over the mid-dorsal wall 

 of the pharynx sheath (text figs. B and C). The posterior, ven- 

 tral extremity of each ganglion is continued as a long, tapering, 

 ventral nerve which passes beneath the enteron (text figs. B and 

 C). This sheath is supplied with two special nerves, though the 

 ganglia appear at places to lie in contact with the pharynx 

 sheath. Each of these nerves leaves the mesial, ventral side 

 of the anterior end of the ganglion and passes to the ventral 

 wall of the pharynx sheath (text figs. B and C, NPh). Just a 

 little anterior and lateral to the origin of each nerve of the 

 pharynx sheath a second nerve arises which, when near the 

 ciliated pit, sends a branch to the anterior end of the body and 

 a second branch to the posterior dorsal wall of the ciliated pit 

 (text fig. C, NCP). 



