182 DE. J. GILCHEIST OlST THE MINUTE STBUCTURE OE 



forms having been observed to differ only in the degree of concen- 

 tration of the ganglia, a fact well worthy of further investigation. 

 Thus, in the very interesting oesophageal ring of Fleurohranchus 

 the ganglia are so concentrated as to form a sort of brain ; and 

 they present several other striking peculiarities. The object 

 which I had in view with this animal was the determination of the 

 minute structure of these ganglia, vrhich could be to a certain 

 extent arrived at by ordinary methods. Aniline-black (not the 

 Continental, which is different from the English) was found a 

 very suitable stain for the purpose ; but all were quite inadequate 

 to bring out the finer details, such as are shown in the accom- 

 panying figures. 



The buccal ganglia of Aplysia can be dissected out with ease, 

 and as they took the stain readily they were suitable objects 

 for preliminary study. Fig. 2 delineates the general arrange- 

 ment of the cells and fibres in these ganglia. It appears from 

 the preparations that the cells consist of (1) typical motor- 

 cells {a, a) each with a long axis-process leading away into the 

 nerve and other processes which break up into fine branches 

 in the ganglia ; (2) of cells (5, V) the smaller processes of which 

 break up in one ganglion while the main process passes over 

 into another, and probably there breaks up into small branches, 

 though this latter point was not observed. 



A great many of the fibres pass from the nerves over the 

 ganglion, and some of these (c) were observed to give ofi" fine 

 branches. The two fibres e and e , the detailed relationships of 

 which I have not determined, appeared unexpectedly in one pre- 

 paration ; in another, at d and d' , exceptionally thick fibres 

 appeared, and it was found that they came directly from the 

 cerebral ganglia and joined the nerve which runs to the mouth- 

 region. Though these were not actually traced to the sensory 

 organ which has been shown to exist there, they probably inner- 

 vate it. As all the other sense-organs (including the osphradium, 

 as demonstrated by Pelseneer for one group) have been shown 

 to be in direct connexion with the cerebral ganglia, this is of 

 interest. Not only is the exceptional thickness of the fibres, as 

 compared with those in the other nerves, noteworthy, but also 

 their small number — only two having been detected in each 

 nerve examined. 



The curious fact that corresponding cells and fibres in the 

 opposite ganglia become coloured at the same time, which has 

 been noted by other observers in other groups, was well illustrated 



