ANATOMY AND CLASSIFICATION OF THE ARENICOLID^. 475 



The second series of changes in the brain involve the for- 

 mation of ganglionic outgrowths on the dorsal surface of the 

 anterior lobes, the segregation of large cells ('04 mm. in 

 greatest length) into centres, from which definite fibrous 

 tracts arise and pass into the oesophageal connectives, and 

 also into other parts of the brain. Separation of the smaller 

 kind of ganglion cells into discrete masses also takes place. 

 The neuroglia develops rapidly, and the fibrous part of the 

 brain assumes a much more complicated structure. 



These are no doubt only the most gross of a long and im- 

 portant series of steps in the gradual differentiation of the 

 brain. In the post-larval stages, the ganglionic covering and 

 the delicate, almost homogeneous fibrous I'egion show no 

 separation into centres and tracts, nor even a clear division 

 into differently constituted elements. In lugworms 60 mm. 

 long, at least two kinds of elements are distinct, but the cover- 

 ing of ganglion cells is uniform and scarcely subdivided. The 

 neuroglia is but slight in amount, the fibrous material still 

 neuropilarin aspect. Specimens of 150 mm. in length (mature 

 examples of the 'Mittoral" variety) exhibit all the changes 

 we have just indicated above, but they are even better marked 

 in still larger individuals (250 mm.). 



Both series of changes occur in the other species of 

 Arenicola. In A. ecaudata and A. Grrubii the subdivision 

 of the primitively uniform ganglionic covering occurs some- 

 what earlier than in A. marina, and there are indications of 

 segregations of large pyriform and of smaller cells to form the 

 centres of tracts to different parts of the brain, and from the 

 brain to the connectives. In a specimen (26 mm. long) of A. 

 Clap are dii (which, however, is quite a young one and not 

 by any means mature) the brain resembles in form and 

 structure that of an individual of A. marina 50 to 60 mm. 

 long. In spite of the more strongly developed prostomium in 

 A. Claparedii the brain does not appear so complex as in 

 the common lugworm, and this is probably due to the fact 

 that the brain is in intimate contact with the prostomial epi- 

 thelium through its dorsal ganglion cells, and not by special 



