550 F. W. GAMBLE AND J. H. ASHWORTH. 



and apparently (at least opposite the cliEetigerousannuli) from 

 the median fibres, enter the spinal nerves. 



As far as histology can go, the cells and fibres certainly 

 seem to be truly nervous, while their arrangement strongly 

 suggests that they are motor elements. Apathy's contention 

 that (on histological grounds alone) giant-fibres are sensory 

 appears in the present state of knowledge to be only partially 

 true. 



IX. The brain exhibits two types of structure, not, however, 

 widely different. The first is seen in A. marina, A. Clapa- 

 redii, and somewhat less distinctly in A. cristata. In 

 these forms there are three regions : an anterior pair of cere- 

 bral lobes, specially connected with the anterior sensory 

 prostomial epithelium, and througli nerves with the buccal 

 papillee and buccal mass ; a posterior pair of histologically 

 different lobes continuous with the epithelium of the nuchal 

 organ ; lastly, a region intermediate both topographically 

 and histologically, supplying the upper surface of the pro- 

 stomium. In addition to these centres, the anterior lobes 

 contain near the median line a pair of groups of large pyri- 

 form cells, from which fibi-es pass to the circumoesophageal 

 connectives. The ganglion-cells for the most part form a 

 dorsal covering to the brain. The more ventral fibrous region 

 is composed of cell -processes and their branches, which form 

 a delicate neuropile. This arrangement becomes more com- 

 plicated as the animal gets older, and there is an actual 

 increase in the size as Avell as in the complexity of the brain 

 apparently throughout life. 



In the second section, which includes A. Grrubii and A. 

 ecaudata, the brain is a transverse band of nervous tissue. 

 Its upper surface is early in life produced upwards into short 

 irregular lobes, and from its outer angles the connectives are 

 given off. It appears to correspond to the united anterior 

 cerebral lobes of the other section of the genus. Occasion- 

 ally a short median process extends backwards so as to 

 underlie the nuchal organ, but this sensory structure is also 

 supplied by several nerves from the commissural brain. 



