1907.] NATURAL SCIENCES OF PlIILADELIMIIA. lOO" 



thicken, I'onnini:: i^ wcll-niai'kcd zone. Ccntrad of tliis zone the rods are 

 just visible, but can with care be seen to terminate against the base- 

 ment membrane. On the inner side of the basement membrane, at 

 this point, a group of oesophageal muscle fibres are inserted {oes.m.). 

 In the belief that this curious structure was sensory in function, a 

 careful search was made to determine whether a demonstrable con- 

 nection with the nervous system was present, but none could be found. 



From the description given above it is apparent that D. conklini 

 possess a very much simplified nervous system of the ladder type, and 

 one which is embryonic in many particulars, as, for example, in the 

 close association of the whole with the hypodermis (ectoderm), and 

 especially the slight differentiation of the portion belonging to the 

 trunk, where the lateral trunks with their ganglia are not merely in 

 contact with the body wall, but included in it. 



The first description of the nervous system of Dinophilus is that 

 given for D. apatris by Korschelt (1882). The studies of this investi- 

 gator were confined almost wholly to entire individuals, living or 

 mounted in balsam. From one of the latter, stained in alum carmine, 

 he described and figured a brain, from which arose two pairs of nerves : 

 one pair running forward to the eyes, while the other, which passed 

 backward, he regarded as the roots of longitudinal nerves. 



The next paper on Dinophilus, that of Weldon (1886), gave a fairly 

 complete account of the nervous system of this species (D. gigas). 

 According to Weldon, the brain, which fills the preoral lobe, consists 

 of a central mass of nerve fibres, surrounded by ganglion cells, which 

 are, as indicated by his figures, in close contact with the hypodermis. 

 From the brain arise the lateral nerve cords, which are also in close 

 contact with the hypodermis, and consist of nerve fibres and ganglion 

 cells. The latter, however, almost entirely disappear in the posterior 

 region of the body, and apparently show no traces of separation into 

 ganglia. Transverse commissures also appeared to be wanting. In 

 regard to the last two points, there seems room for reasonable doubt 

 if more perfect series of sections than it was at that time possible to 

 make wovild not reveal the presence of both ganglia and transverse 

 coramissm"es in this species. 



In respect to the points last mentioned, the account of Harmer 

 (1889a) is much more satisfactory. This author found in D. keniatus 

 a brain definitely separated from the hypodermis, and a nervous sys- 

 tem of the ladder type, which, although remaining in contact with the 

 body wall, lies outside of its basement membrane. The ganglion cells,. 

 although investing the nerve cords along their whole length, are united 



