INTERNAL ANATOMY OF BDELLA. 499 



Viewed from the side, or in sagittal section, the supi'a-a?sophageal ganglion is almost 

 triangular; tliis form is caused hy its being thin at its anterior edge, and thick, dorso- 

 ventrally, at its posterior, its uj)per surface sloping rapidly upward fi'om before back- 

 ward, and then falling, almost perpendicularly, to the sub-oesophageal ; the upper surface 

 is somewhat concave, and faces upward and forward ; the posterior edge forms a wall 

 standing almost, but not quite, uj) right upon the sub-oesophageal ; round this wall great 

 bundles of tracheae are wrapped, as hereafter described in the section on the respiratory 

 organs {wfra, p. 518). 



The neurolemma is very fine and delicate, and difficult to see, extremely different 

 from the substantial neurolemma of the Hydrachnidse. The layer of cortical dark-staining 

 cells, which is so conspicuous in many of the Acarina, is but feebly developed in Bdella, 

 and is entirely absent from those parts of the sub-oesophageal from which the six pairs 

 of large nerves, which arise from that ganglion, start. 



With regard to the Nerves I have, in the above-named j^ajoer upon Thyas fetrophilus 

 (14), stated fully what previous writers have said respecting the nerves of such species 

 of the Trombidiidoe and allied faimlies as have been studied from the point of view 

 of their internal anatomy ; I do not therefore intend, in this paper, to refer more to 

 the bibliography of the subject than is necessary for the proper understanding of any 

 doubtful or disputed points, the settlement of which may be facilitated by what I have 

 been able to ascertain in the present investigation : in other cases I shall confine myself 

 to stating what I find in Bclella. 



The ample supply of specimens of Bdella Basferi which I have received has enabled 

 me to devote a large number to tracing the nerves only, sacrificing all other organs : 

 this has been done partly by dissection and partly by sections ; but, on the whole, I have 

 found the former method rather the more efficient on account of the extreme difficulty 

 of obtainin"' anv stain which will differentiate the nerves in the Acarina from the 

 surrounding tissues — with me gold chloride, in whatever way employed, has entirely 

 failed to do so. In no case has a single dissection or section been relied on for a nerve 

 or branch of one ; I have not put anything into fig. 13 until I have seen it in several 

 cases : I think, therefore, that all the detail which I am able to give may be relied 

 on ; I have been able to trace a fair number of the finer branches, but I have little 

 doubt that plenty of others exist which I have not seen. The creatures are so small, 

 and tlie nerves so extremely fine and difficult to trace, that it can only be by slow 

 degrees, and a certain amount of good luck, that jinything like completeness can be 

 obtained in their study. 



As might be anticipated, I find that the nerves agree fairly well with what I found in 

 Thyas petrophihts, without more difference than would be looked for between two families 

 which are very distinct, although allied, and differ so much in the anatomy of other 

 organs. 



I have been able to trace in Bclella Basferi one median nerve and ten pairs of bi- 

 laterally symmetrical nerves ; of these the median nerve and three pairs of the paired 

 nerves arise from the supra-cesophageal gangKon, and seven pairs of paired nerves from 

 the sub-a?sopliageal ; those arising from the supra-oesophageal are all small, those from 

 the sub-ffisophageal mostly large nerves. 



