68 Jdurnal dI^ Ent()m()lot;\ ami Zoology 



level of the anal opening and the nephridial tubules. This thickest 

 portion of the nervous system directly continues with the epithelium 

 •of the surface of the body and is dorsal to the anal papilla in the 

 depression caused by the anal prominence; from here the thicken- 

 ing passes toward the tentacles sending fibers to the jophophore and 

 the tentacles. The lophophore depression on each side marks off 

 the chief thickening of the nervous system. This central part, 

 although continuous with the epithelium is made up of a distinct 

 mass of fibers and cells. At this point three chief centers of cells 

 are found among the fibers while out laterally strands run to the 

 lophophore dcfressions and out to the tentacles. There is quite a 

 mass of fibers and cells in the region of the lojjhophoral depression. 

 Running out vertrally on the left side just medial to the lophophoral 

 depre.ssion and between it and the left nephridium is the clear cord 

 of unknown function noticed first by Caldwell. This cord sur- 

 rounded partly by cells comes to run farther ventrally until it 

 passes through the basement membrane of the liody-wall and comes 

 to lie .iust under the ejiithelium. This end does not seem to be of 

 nervous tissue, although it is connected with the central part of the 

 nervous system. 



If I understand Torrey's descriiition aright his material mu.^^t 

 have been too poorly fixed to show the relationship of the nervous 

 system for in well preserved sjiecimcns the cerebral nervous sys- 

 tem is continuous laterally with the lophophoral organs as well as 

 with any lateral or longitudinal extensions of the nervous system. 

 My observations both on Pharonis and Phoroiiapt^is agree closely 

 with tho.se of Pixell. In Phoronapsis the centra] nervous system 

 seems more elongated, as Pixell found. 



With the exception of the central part of the nervous .system 

 the nerve cells are not clearly different from the eiiithelial cells, but 

 careful study shows at the liases of the cells as well as farther down, 

 nerve cells with their fibers directed into the basal mass of fibers. 

 Ill the epithelium are bipolar cells, some of which may be sen.sory, 

 although many of the ]n-ominent strands are tho.se of supportive 

 cells. 



ACTINOTROCHA. 



It seems best to consider the larval stage of Plii)r())iis iiriefly 

 at this place. Schn(>i(ler, 18(i2, in his discussion of the develop- 

 ment of Arti)Hitri>rh(i does not consider the nervous system. Cald- 

 well has the first work of imiiortance but his account, according to 

 MacBride, implies that the apical jilate and adjacent ganglion of 

 the larva are lost, and the cerebral ganglion of the adult mu.st be a 

 new structure. But in every trochophore so far studied the apical 

 plate with its ganglion forms the material which persi.sts to the 

 adult condition. 



