500 ME. A. D. MICHAEL ON THE 



The median nerve (fig. 13, wjjA) is the pharvugeal nerve; this, as in all other 

 Acarina in which it has been traced, is a thin nerve which arises in the centre of the 

 anterior edge of the supra-oesophageal ganglion immediately above the oesophagus, and 

 follows straight along, or rather parallel to and just above, that organ to its distal end : 

 there it breaks up into a number of fine twigs, some of considerable length; one of these 

 twigs is sent to each muscle of the sucking-pharynx. In Bclella, however, a compli- 

 cation of this pharyngeal nerve exists which has not been recorded, and probably does 

 not exist, in any Acarid the anatomy of w'hich has been studied ; I believe it to be 

 entirely novel : it is that the pharyngeal nerve, a sliort distance from the brain, splits 

 into two main branches, an uj^per and an under — the under goes on, as before described, 

 to supply the pharyngeal muscles, and in fact represents the whole nerve in other families : 

 the upper branch {nc) runs between the oesophagus and the receptaculum cibi, and 

 terminates just where the two join; it innervates the ring of sphincter muscle which 

 surrounds the neck of the receptaculum cibi (sucking-stomach), and possibly sends twigs 

 to other portions of the sucking-stomach ; but I was not able to make certain of this. In 

 fig. 13 this upper branch of the pharyngeal nerve is shown turned a little to the right; 

 if it were left in its natural position, it would entirely hide the lower branch and there 

 would not be anything to show that two existed. 



Of the paired nerves from the supra-oesophageal ganglion those situated a trifle the 

 nearest to the median line are a pair of very delicate nerves (««;) which innervate the 

 dorso-ventral muscles near the base of the rostrum ; in reality this pair of nerves, the 

 mandibular nerves, and the palpal nerves lie almost in a perpendicular line one below 

 the other, the palpal being lowest, the pair now treated of next, and the mandibular 

 nerves highest ; in the drawing the mandibular has been bent a trifle to the left, and the 

 present ])air a trifle to the right in order to show them ; the present pair is, however, a 

 very little nearer to the median line than the mandibular. JNotwitlistauding this I think 

 the mandibular must really be regarded as the tirst, and the present as the second pau", 

 which is tiie same as in Thyas. At a distance from the brain rather less than half the 

 length of the brain itself this nerve {nv) branches dichotomously. I was not able to 

 trace its further branching. 



The next pair of nerves from the supra-oesophageal ganglion are the mandibular (;iw); 

 these are situated considerably further towards the lateral edge of the ganglion than 

 they are in Thyas ; and the reason for this is not difficult to understand. In Thyas the 

 mandibles are much compressed laterally and lie together close to the median line ; in 

 Bdella the proximal ends of the mandibles are swollen into large bulbs, tlse two of 

 which together are as wide as the rostriun. About as far from the brain as two-thirds 

 of the length of the brain itself there arises from the mandibular nerve a branch-like 

 structure, which will be best understood from the drawings (figs. 13, 14). The nerve 

 gives off what may be called a branch, or may be considered a gangHon ; I am inclined 

 to look upon it in the former light : it is a thin sheet of nervous tissue, rather thicker at 

 tlie edges, and resembles in form a flattened hour-glass, Avith the corners drawn out into 

 points and the waist not so thin as in that instrument. Trom each of the two distal corners 

 of this expansion proceeds a very thin nerve-branch ; these tw^o are, I think, distributed to 



