70 jiiunial iif Entiimolojiy ami Zoolopy 



4. A ring about the anal end of the trunk into which dorsal 

 and ventral tracts lead. 



5. A ring about edge of prae-oral lobe, joined at each side to 

 the ganglion and in median front region by three main tracts run- 

 ning in mid-dorsal line. 



6. A diffuse plexus of fibers at the base of all the epiblastic 

 layer, including fibers of ventral collar region, which pass forward 

 and dorsally to meet the ganglion. 



Ineda, 1901, found no collar, nerve ring or dorsal or ventral 

 commissure in the larva. He also failed to make out presence of 

 (he peri-anal ring. If present it is represented by a small number 

 of parallel fillers. The main nerves were three in number close to 

 each other and parallel along the mid-dorsal line of the trunk but 

 confined to only a few sections posterior to the first pair of tentacles. 

 There was found however a very complex and beautiful system of 

 nerve fibers seen on the prae-oral lobe. Fibers are very numerous 

 and fine and radiate from the ganglion on all sides towards the free 

 margin of the prae-oral lobe. In the median line and anterior to 

 the ganglion fibers are three long parallel strands on which the 

 apical sensory spot is situated, not far from the ganglion. After 

 passing through the sensory spot .strands fray out into fine fibers 

 which continue to the free margin of the prae-oral lobe. Fibers 

 from (he ganglion do not show a regular radial arrangement, but 

 ari.se from the lateral edge of the ganglion and soon take an anterior 

 direction. Sometimes near the ganglion there is an anastomosis of 

 fibers, but probably more apparent than real. There are nerve end- 

 ings in the prae-oral ciliated belt. There is probably an incomplete 

 development of nerve elements in the collar and trunk region. He 

 finds no neuropore and believes that Ma.sterman's structure is due 

 to contraction. 



De Selys-Longchamps, 1902, gives a rather complete descrip- 

 tion of the nervous system. The central ganglion is a dorsal ex- 

 pansion of the epidermis with fibrillar substance below the surface. 

 The depression which Mastcrnian calls neuropore is not such a 

 structure. There are three cords of the nervous system, the median 

 is most developed. The apical organs are organs of sense. 



BIBLIOGRAPHY 

 .Andrews, E. 



1900. On a New American species of the remarkal)lc animal I'horonis. 

 Ann. Map. Nat. Hist. vol. 5, pp. 44.'>-l49. 



Benham, W. B. 



188il. The Anatomy of Phoronis australis. Q. Jour. inic. so. vol. :!(>. pp. 

 125-158, pi. 10-1.-?; N. syst. pp. I.'JS-ISS. 



Caldwell. W. H. 



18S:i. Preliminary note on the structure, development and affinities of 

 Phoronis. Proc. Roy. soc. vol. 34, pp. 371-.383, 1 fig. 



