MUSEUM OF COMPARATIVE ZOOLOGY. 29 



except that Rhynchodesmus has no marginal nerve. Both Phagocata 

 and Planaria abscissa are intermediate between these extremes, PI. 

 abscissa possessing only the dorsal portion of the superficial system (in 

 which a special dorsal longitudinal nerve has arisen), and Phagocata 

 having only the ventral portion of that system. Both possess, however, 

 the marginal nerve found in Gunda, and I believe that it probably sus- 

 tains in PI. abscissa the same relations to the deep portion of the ner- 

 vous system that I have found to exist in Phagocata. 



It is evident, I think, from what I have shown in Phagocata, that the 

 marginal nerve is to be regarded as one of the means of communication 

 between the central and superficial parts of the nervous system ; or per- 

 haps rather as a differentiation of that portion of the superficial system 

 which is put in connection with the deep system by means of the lateral 

 branches from the main trunks. 



It may perhaps be reasonable to suppose that the more concentrated 

 condition in Gunda has been brought about by a process of centraliza- 

 tion from the more diffuse and more primitive (1) condition shown in 

 Rhynchodesmus. 



The brain is formed on the same plan as that of Gunda (Lang, '81, 

 p. 67 ; '81 a , p. 213). I find two commissures, a larger anterior commis- 

 sure which Lang calls in Gunda the sensory, and a smaller posterior one 

 which he calls motor (Plate III. Figs. 23, 33, and Plate IV. Figs. 39, 46). 

 The posterior commissure lies somewhat behind and below the anterior 

 one. It directly connects the longitudinal nerve trunks, since it lies in 

 the same ventral plane with them, while the anterior commissure, occu- 

 pying a higher plane, is only indirectly united to these ; viz. by means 

 of the lateral masses of the brain from which vertical commissural fibres 

 (the motor-sensory commissures of Lang) extend to the nerve trunks. 

 Lang describes four pairs of nerves as arising from the lateral sensory 

 masses of the brain. I cannot discover that there is any fixed number 

 in Phagocata. The only well defined one is the optic nerve (Plate IV. 

 Fig. 40, n. opt.). A great sheet of fine nerves is given off from the 

 lateral surface of the brain, and, spreading out fan-like, runs forward to 

 the anterior margin of the body (Plate III. Figs. 25 and 34, ».). It is 

 from these nerves that the " Tastorgane " of this highly sensitive portion 

 of the body receive their nerve supply. 



A comparison of the figures will make clear the relation of the differ- 

 ent parts of the brain. Figures 26 to 31 are from cross sections through 

 the region of the brain taken at intervals of 60 /x. Figures 32 to 36 are 

 consecutive sections in the horizontal plane, Figure 32 being the most 

 dorsal of the series. 



