326 Mr. A. Tulk on the Anatomy of Phalangium Opilio. 



fasciculi which radiate from the sides of the thoracic ganghon, 

 where they are attached by short tendons. Their arrangement is 

 such, that, according as either one or the other set of fibres act, 

 they will draw the nervous mass either forwards or backwards, 

 horizontally, or in the vertical direction. I am not aware that 

 this voluntary power of moving the nerv^ous centres exists in any 

 of the other Articulata. 



Organs of Sense. — With the exception of that of vision, the 

 precise location of the other organs of sense in Phalangium rests, 

 for the most part, as in insects, upon conjecture. A very delicate 

 touch no doubt resides in the extremities of the palpi, and the 

 slender terminations of the anterior and two hindermost pairs of 

 legs, while the second pair, longer than the rest, and which the 

 harvest-spider keeps raised, chiefly in resting, from the surface, 

 may serve to convey the vibratory impulses of the atmosphere, 

 and thus apprise it of any coming danger. The form of the che- 

 licerse, the usually regarded analogues of the antennae, would 

 seem to ill adapt them for the auditory function. The eyes are 

 four in number, as stated in the commencement of this paper, 

 two being situated in the centre of the cephalo-thoracic shield, 

 and the other pair near to its anterior angles. In the first of 

 these, the large size, prominence, and elevated position of which, 

 appear to supply their deficiency in number as contrasted with 

 the median ocelli of the Araneida, I have succeeded in tracing 

 out similar structures to those described by Miiller and others as 

 composing the eye of the scorpion ; namely, first of all a layer of 

 black pigment, retained doubtless in place by a choroid coat, which 

 is thicker laterally than above and below, and which must sup- 

 port upon its inner surface the nervous expansion of the retina ; 

 secondly, a vitreous humour, seen as a convex transparent spot 

 imbedded in the centre of the pigmentary layer ; and lastly, a mi- 

 nute, round and compressed crystalline lens formed of concentric 

 laminae, and apparent in the living animal through the cornea, 

 which is simply a smooth transparent portion of the general in- 

 tegument of the body. At fig. 32. om are represented a pair of 

 muscles, passing upon each side beneath the middle pair of eyes, 

 which are united laterally to each other. Their use must surely 

 be to effect some change of position in the internal humours of 

 the eye beneath their immoveable cornea, and so accommodate 

 them to the movements of the thoracic ganglion, and through it 

 the cephalic and optic nerves, when acted upon by its powerful 

 muscles. One fact is deserving of notice, as tending to throw 

 some light upon this opinion, — that in every specimen of P. Opilio 

 from which I have removed the cephalo-thoracic plate, the layer 

 of pigment with the vitreous humour has invariably remained at- 

 tached to the extremity of the optic neiTC, leaving the crystalline 



