ANATOMY AND CLASSIFICATION OP THE ARENICOLID^. 473 



the rough method of sections shows that many obvious changes 

 are at work to which we are unable (in the case of Arenicola 

 marina at least) to fix an age limit. The brain of the oldest 

 specimens exhibits additional changes from those of slightly 

 younger and smaller examples^ although in this respect the 

 species of Arenicola differ ; for the limit of growth, aud with 

 it the definitive arrangement of the brain-elements, appear to 

 be reached sooner in A. ec an data, A. Grubii, and A. 

 Claparedii than in A. cristata or A. marina. Indeed, in 

 the two latter, growth in the length and bulk of the body 

 seems to be coterminous with the life of the individual. 



The changes occurring in the brain of A. tnariua (in 

 which we have examined a larger series of stages than in 

 the case of the remaining forms) are of two kinds : first, an 

 increase in the number of elements, involving a growth of the 

 brain in the later as well as in the earlier and post-larval 

 stages ; and second, a differentiation of the form and arrange- 

 ment of the ganglionic centres and the fibrous tracts. 



In a post-larva 4*5 mm. long the brain is '11 mm. long, 

 •065 to "7 wide, '02 to "03 deep; while in a specimen 

 7"5 mm. long the brain has almost exactly twice these 

 dimensions.^ 



Again, a lugworm 17*5 mm. in length, i. e. rather more 

 than twice as long as the latter and four times the length of 

 the first, has a brain of correspondingly greater dimensions. 

 Beyond this point the rate of increase of the brain is much 

 slower, but the limit is scarcely reached ("9 mm, long) when 

 the animal attains a length of ten inches, for still larger 

 specimens, fourteen and fifteen inches long, have a slightly 

 larger brain, 1 mm. (see the table giving dimensions of the 

 brain and nerve-cord in specimens of different body-lengths). 



• The measurements taken from preserved material are probably somewhat 

 less tlian the actual ones, on account of the contraction of the coelomic spaces, 

 wiiich to a certain extent distend the brain in the livin? animal. 



