498 ME. A. D. MICHAEL ON THE 



any parts; there is not any sign of commissures, and the fusion between the supi'a- and sub- 

 CESOjihageal ganglia is so intimate that it is impossible to say where one terminates and 

 the other commences. This will be well seen in Schaub's figures and descriptions of the 

 brain of Hydrodroma *, and in my own figures and descriptions of Thyas petrophilus f . 



It will be seen by Henkin's figure of a section of the brain of TromMdium fuliginosumX 

 that the form tbere is not so globular, notwithstanding the close connection between 

 Tromhidium and the Hydrachnidse ; it will also be seen, by my paper on the brain in 

 Oribatidaj and other forms §, that the spherical form is not usually approached so closely 

 in other families of Acarina as it is in the Hydrachnidce ; still the whole usually forms 

 one mass, which may be irregular in shaj)e, but in which all sharp demarcation of parts is 

 lost. Bdella can hardly be said to form any excej)tion to this rule ; it would scarcely be 

 possible to say where the commissures are ; the supra- and sub-oesoj^hageal ganglia 

 practically join one another ; still the origin from the two separate ganglia is far more 

 clearly shown, and the two are more distinct, than in any other Acarid with which I 

 am acquainted. Taking Bdella Basteri (which is an exceedingly good example) as a 

 type, the sub- oesophageal ganglion (fig. 13, 34, gsu) consists of an almost oblong 

 layer of ganglionic matter having an average length (in the male) of about '4 or '42 mm.,, 

 by an average breadth of about "17 mm. It is much flattened dorso-ventrally, the 

 thickness not being above 08 mm. where it meets the sujora-oesophageal ganglion, 

 diminishing almost to an edge at its posterior end ; it lies almost touching the ventral 

 surface, and extends from a little in front of the first pair of legs anteriorly to about 

 midway between the second and third pairs of legs posteriorly. 



The supra-oesophageal ganglion {gso) is about % of the length of the sub-oesophageal ; 

 it is situated upon (above), and slightly in advance of, the sub-oesophageal, so that the 

 anterior edge of the former overlaps the anterior edge of the latter. The oesophagus 

 runs between the two ; both, but chiefly the supra-oesoj)hageal ganglion, are excavated 

 to afi"ord it a jiassage ; the sub-oesophageal is thickest, dorso-ventrally, just where it 

 meets the posterior edge of the supra-oesojjhageal, and then thins away toward its 

 own anterior edge, and the under surface of the supra-oesophageal follows and joins it. 

 The oesophagus does not run in a horizontal line through the brain, but in a diagonal, 

 sloping upward and backward ; it enters the anterior edge of the brain at its ventral 

 surface and sloping strongly upward emerges at the posterior edge of the supra- 

 oesophageal and lies upon (above) the part of the sub-oesophageal which lies posterior to 

 the supra-oesophageal, i. e. about two-thirds of the length of the former. 



Looking at the whole brain in a general view, the supra-oesophageal ganglion appears 

 quite sharply divided from the sub-oesophageal and perched on top of it ; it is only when 

 one attempts to separate the two, or cuts sections, that the fusion of theii- peripheral parts 

 is appreciated. What may be called the ground-plan of the supra-oesophageal ganglion, as 

 seen from above, is almost a square, with the corners somewhat rounded, and a projection 

 near the middle of each lateral edge, from which projection the palpal nerve arises. 



* i8 ; his Taf. V. figs. 2, 3. t 14. figs. 20, 23, and p. 1!)9. J 6 ; his Taf. xxxiv. fig. 7. 



§ " On the Form and Proportions of the Brain in the Oribatida; and in some other Aoarina,'' Journ. R. Micr. Soc. 

 1895, pp. 274-28.;. 



