138 BULLETIN OF THE 



fairly with their neighbors, whereas, if they had by division given rise 

 to nerve cells, they should all be smaller, (t;.) In Figure 67, which is a 

 section immediately in front of the advancing tip of the nerve, the po- 

 sition corresponding to that opposite the nerve in the preceding sections 

 is indicated by an asterisk (*). The nuclei are here crowded together, 

 indicating pressure. Fourthly, there is a consideral)le diti'erence in size 

 between the nuclei of the cells of the layer i. of the lophophore and the 

 nerve cells. This is not what one would expect upon the assumption of 

 the formation of the nerve directly from the overlying cells. Fifthly, a 

 longitudinal section through the young lophophoric nerve (Plate \IL. 

 Fig. 71) shows a more active cell division in it than in the walls of the 

 arm (compare Fig. G-i, h. loph.), and a crowding together of nuclei of 

 the outer layer of tlie arm, i, at its distal end, rather than a passage of 

 nuclei into the nerve. 



The conclusion to which I have arrived from considering these facts 

 is that the peripheral nervous si/stem in riiyladolcemata arises from the 

 brain as an outgrowtli of its walls. 



5. Development of the Epistoine. — The epistome was regarded by 

 Lankester at one time ('74, p. 80) as homologous with the foot of 

 Mollusca, and on another occasion (^^o, p. 43-i) as representing the 

 preoral lobe of Annelids, — a view for which Caldwell ('83) first pro- 

 duced evidence from comparative embryology. In view of such diver- 

 gent opinions, and of the occurrence of an organ which is possibly its 

 homologue, in quite aberrant genera, such as Phoronis, llhabdopleura, 

 etc., a careful consideration of its origin and development is desirable. 



After the ganglion is fully formed, its oral face remains in contact 

 in front with the posterior M'all of the oesophagus (Plate V. Fig. 52, 

 Plate IX. Fig. 77), and on each side with the outer wall of the lopho- 

 phoric pockets b}' means of the lopl:y3phoric nerves (Plate VII. Fig. 63, 

 n. loph.). The outer layer of the bud penetrates between the gan- 

 glion and rectum, but not between the ganglion and the oesophagus 

 (Fig. 51,*). This layer also comes to lie between the floor of the 

 atrium above, the ganglion below, and the lophophoric nerves on either 

 side, having made its way in from behind as a double cell-layer enclosing 

 a flat cavity (Plate V. Fig. 52, Plate VI. Fig. 5(S, Plate VIII. Fig. 74, 

 can. € stm.). My description of the process by which tlic inner layer 

 comes to envelop tlie ganglion above and behind differs considerably 

 from Nitsche's, already quoted (page 114). As the ganglion becomes 

 farther removed from the floor of the atrium, the cavity above it (m;/. e 

 stm.) enlarges, and the two lateral walls of this canal, each composed of 



