228 DISCOVERY REPORTS 



Branchiopoda (Fig. 13 and Plate XL, figs. 7, 8). Actually only the second and third 

 cross connexions are commissural. The first cross connexion results from the two trunks 

 swelling up and touching in the middle line. The fourth is a peculiar cross connexion 

 on which is situated a median ganglion. From this ganglion a minute nerve is given off 

 which runs forward and has been termed by Liiders the sympathetic system. I can 

 give no explanation of the existence of this ganglion. It is obvious in the photograph 

 (Plate XLI, fig. 2) and also occurred in the specimen from which the nerve ring was 

 reconstructed. In the second specimen from which the ventral chain system was drawn 

 I am equally certain however that this ganglion was absent. There is really, therefore, 

 only one commissural connexion which is divided into two by a vacuity. In addition, 

 the nerves to the limbs which are given off from the ventral chain system bear no definite 

 relation to the cross connexions. It would appear therefore more probable that the 

 relatively long ventral chain is merely a secondary effect, another result of the 

 enlargement of the body. 



CARDIAC NERVOUS SYSTEM 



I have been able to follow the nervous connexion between the heart and pericardium 

 and the central nervous system, first from sections and subsequently, to confirm this, 

 by actual dissection. 



The heart itself possesses one large neurone situated on the inner surface of the roof 

 at the convergence of the two ostia (Figs. 10, 15). This sends out two axons laterally 

 which extend close against the wall to a point just dorsal to the hepatic valve where 

 a branch runs downward to the floor of the heart, while another branch penetrates the 

 wall and makes its exit into the pericardium. There are other branches from this single 

 neurone which are shown in Fig. 10. I have not, however, been able to investigate the 

 complete cardiac system with certainty. The axons do not stain and are only visible 

 by their peculiar meandering arrangement. It is quite possible that the latter effect 

 would vary with the varying degree of contraction or relaxation of the cardiac muscles 

 on fixation. It is not surprising therefore that these nerves appear to vary considerably. 

 What I have described above and figured, however, appear to be constant in all my 

 preparations. 



The cardiac nerve, on leaving the heart, is thus a single axon and is naturally very 

 difficult to follow in sections, not only because of its size but because it does not readily 

 stain. With the method of projection reconstruction, however, I found it possible to 

 follow this minute nerve through all the sections to its central connexion. I then 

 attempted to find it in dissection and found that this also was possible by using a very 

 dark background and a powerful beam of light focussed almost horizontally across the 

 dissection. The light reflected off the axon was sufficient to enable me to trace it and 

 finally to dissect out the isolated nerve. As the nerve approaches the central nerve ring 

 it joins up with other branches and becomes more easily manipulated. 



On emerging from the heart (Fig. 15) it curves upwards and gives off a side branch 

 to the first dorsal longitudinal muscle (Fig. 9). It then loops over the group of the 



