INTEENAL INATOMl OF BDELLA. 501 



some of the mandibular muscles, but I was not able to ascertain precisely which : the 

 main trunk of the nerve proceeds into the mandible in the usual manner. 



The third and last pair of nerves which I could trace from the supra-oesophageal 

 ganglion are the optic nerves (no) ; these arise from small eminences about the middle 

 of the respective lateral edges of the ganglion ; they are the stoutest i^air of nerves 

 which arise from the ganglion, Ijut are not proportionately as thick as they are in Thy as, 

 Tromhidium, Uydrodroma, and other forms. I was able to trace one fine branch («o 1) 

 abovit as far from the brain as two-thirds of the length of the brain itself, which branch 

 ends in a very small ganglion. The principal trunk of the nerve proceeds to the hinder 

 and outer of the two eyes (on each side), and there on its outer side expands into a sort 

 of retinal disc {rd) (figs. 13, 27), which is almost sessile upon the nerve ; the principal 

 trunk then inclines more forward and less outward, and terminates in an almost similar 

 disc for the other eye on that side of the body. These retinal discs, although rather more 

 flattened, are practically the same as those described by Henkin (6) in Tromhidium 

 fuliyiuosum (his fig. 11, vn). 



I was not able to trace any homologues of the thin \)vav of nerves from the posterior 

 part of the supra-oesophageal ganglion which I found in Tlujas, still it may be possible 

 that they exist ; they were exceedingly fine in Tlu/as. 



The nerves arising from the sub-oesopliageal ganglion are six pairs of thick and odh 

 pair of thin nerves. The courses of the six thick pairs may be most distinctly ti'aced in 

 the ganglion itself, running almost to its centre ; they are even cons^iicuous. 



The first pair of thick nerves are the palpal nerves. Croneberg, speaking of Eylals 

 extendens (4), considered that the palj)i were innervated from the sub-oesophageal ganglion. 

 Nalepa (i6), speaking of Trichodactyliis anonymus (Tyroglyphidse), considered that the 

 maxillae were served by the sub-, and the maxillary palpi by the supra-oesophageal 

 ganglion. Schaub (i8) (speaking of Sydrodroma) stated that the palpi were innervated 

 from the sapra-oesophageal ganglion. Henkin (6) did not trace the i:)alpal nerve. In 

 Thyas petrophilus the two ganglia are so fused into a ball, and the palpal nerve arises so 

 exactly on the level of the oesophagus that I was unable to say which ganglion it arose 

 from. In Bdella Basteri, however, the matter is clear; the palpal nerve {up) sj)rings 

 from the upper part of the anterior outer corner of the sub-oesophageal ganglion ; it is 

 a thick nerve, much thicker in proportion than the same nerve in Thyas ; this doubtless 

 is ex2)lained Ijy the great size and importance of the palpi. It seems to me most 

 natural to sujijjose that the palpi would be innervated from the sub-oesophageal gauffliou, 

 because that is the centre whence the maxillary nerves of insects arise ; but of course in 

 insects the mandibnlar nerves also arise from the sub-oesophageal ganglion, whereas in 

 the adult Arachnoidea the nerves which serve the mandibles (chelicerse) spring from the 

 supra-oesophageal ganglion. This, however, is subject to three i-emarks, viz., first, that 

 Winkler considered that in G<(masus he had found tiie root of the mandibular nei've in 

 the svib-oesophageal ganglion, and that it passed right througli the substance of the 

 supra-oesophageal ganglion before emerging — an observation which I have not been 

 able quite to confirm, although I have a great respect for Winkler's general accuracv ; 

 secondly, that many Avritei'S, such as Lang and others, consider that the very fact of 



SECOND SERIES. — ZOOLOGY, VOL. VI. 68 



