SOME POINTS IN THE ANATOMY OF POLYCH.ETA. 269 



the same way as that illustrated so well by F. Nansen 1 in his 

 memoir on the Myzostomidae. But I have been unable to 

 trace any connection between the neural canal, alias giant- 

 fibre, and a ganglion cell. Indeed, in Sigalion the canal or 

 tube becomes very small long before the brain is reached, and 

 I cannot even distinguish it in the oesophageal commissures, 

 or in the cord immediately behind them. 



In the central part of each cord in the middle region of the 

 body are one or two tubes, which are similar in structure to 

 the large neural canals, but much smaller. 



A few words as to the character of the nerve-cords in Siga- 

 lion boa. The cords are nowhere separated from the epi- 

 dermis. Ganglion cells are abundant beneath the ventral 

 cords, both in the ganglia and between the successive ganglia. 

 Above the cords is a striking development of a very peculiar 

 tissue whose function is problematic. In the middle of the 

 body this tissue consists of waved fibres or laminse, which 

 are often arranged in parallel curves. These form a network 

 whose meshes occasionally contain cells with nuclei, but 

 usually are filled with a stained granular substance. Close 

 behind the head this mass of tissue is of very great size, 

 and is much more cellular. In the oesophageal cords it is 

 reduced to a very small quantity, but it forms a thick envelope 

 round the brain. The tissue stains with difficulty. It is in all 

 probability a kind of connective tissue not directly concerned 

 in nervous functions. 



With regard to Aphrodite, I entirely agree with Rohde that 

 colossal fibres or neural canals are altogether absent; and in 

 this genus the nerve-cords are quite separated from the epi- 

 dermis. 



Polynoe I have not examined, but in Harmothoeim- 

 bricata I find a pair of neural canals corresponding in posi- 

 tion to those of Sigalion boa, but I have not seen any in a 

 ventral position, such as those mentioned by Rohde in Polynoe. 

 In Harmothoe the ventral nervous mass is distinctly defined, 

 but not separated, from the epidermis. 



1 'Bidrag til Mjszostomernes Anatomi uud Histologi,' Bergen, 1885. 



VOL. XXV111, PART 2. NEW SER. T 



