ANATOMY AND CLASSIFICATION OP THE ARENICOLID^. 469 



(1898) and Hamaker (1898) have obtained good results on 

 Polychaetes by its use. The cord after fixation and washing 

 was treated with pyrob'gneous acid or with pyrogallol until 

 quite blackened. It was then hardened and cut, but the 

 results were inferior to those obtained by fixing with the 

 corrosive-acetic mixture and subsequent staining by one of 

 the haematoxylin solutions. 



There are the usual components of the nervous system in 

 Arenicola, the "brain/' the circumoesophageal connectives, 

 the stomogastric system, the ventral cord, and the peripheral 

 nerves. Professor Ehlers (1893) has described the general 

 form of the brain in A. marina, A. Claparedii, and A. 

 Grubii; and Retzius (1891) has figured one or two examples 

 of the nerve elements stained with methylene blue, but thei'e 

 has been very little work published on the minute histology 

 of the nervous system. We propose in this section to go 

 rather more into the detail of this subject than we have done 

 in the other organs. 



I. The "Brain" (PI. 27). 



As with so many organs, the structure and form of the brain 

 follow one modification in the ''mar in a" section of the genus, 

 another in A. ecaudata and A. Grubii. Both, however, 

 belong to the same plan, and thanks to the comparative work 

 byRacovitza, Retzius, and others, it is now possible to regard 

 this plan of cerebral structure as common to all Polychfetes, 

 although there is still much difference of opinion as to the 

 relative importance of many structural details. According to 

 Racovitza (1896) this common plan may be considei'(>d to be 

 composed (1) of a pair of anterior lobes connected with the 

 epidermis at the point of origin of the median or paired 

 prostomial palps ; (2) of a median or secondarily subdivided 

 middle region connected with the upper surface, of the pros- 

 tomium, and the sense-organs (eyes) borne on that surface; 

 (3) a pair of postero-dorsal lobes continuous with the epi- 

 dermal invaginations which form the ciliated grooves or 

 nuchal organs. According to this view the brain is regarded 



