312 



MEMOIIIS OF THE NATIONAL A(3ADEMY OF SCIENCES. 



Another organ which is characteristic of the scorjiioii's braiu we have failed to detect iu that 

 of Limuliis, uor does Viallaiies mention it. Saint Eemy thus describes that of the scorpion: 



The structure -n-hicli, for wnut of a better name, we shall designate under the name of walleUsltaped organ 

 (orijane en bissac) to recall its general form, occupies the internal and middle part of the cerebral lobe. It comprises 

 two segments united by a much smalUr middle jiortion iu the form of a neck (p. 224). 



Still another organ also present in the scorpion's brain which we have not found in the brain 

 of Limulus, and which is not mentioned by Viallanes, is the olivary body. It is situated iu the 

 posterior region of the cerebral lobe below the wallet-like organ near the median line; it is an 

 olive-shaped mass of dense punctured substance and situated obliquely. 



Considering, then, the fact that the deutocerebrum or its homologue is in Limulus separate 

 from, where in Arac^hnida it is fused with the brain ; also the differences iu the shape of the cerebral 

 lobes, the imnieuse development of the ruffle-Uke plates or masses of small chromatic ganglion 

 cells forming the pedunculated or mushroom bodies; the small number of large ganglion cells; 

 the absence of the wallet-shaped and of the olivary organs found in the braiu of. scorpions, and 



men 



aa 



Fig. XV. — Brain of Limvlus polyphemvs (h.■l^Ml by "^ i.illaiies. from .1 iiioilel in relief of the brain of Limulus, madeof -wax, seen from 

 the dorsal aspect, the neurilemma having been remo\e(l ( e I , lateral-eye lobe; m. e. !., median-oye lobe., m.e.n., median-eye nerve; «.n., 

 nerve passing to the neuropore of Patten \fosBette sensitive Viallanes) ; vi.b.,7n.b.,m. b.,in.l.,\oha\Gii of the mushroom or pedunculated body ; 

 c. L, cerebral tubercle in part, giving rise to the stallc of the mushroom body ; c. L, cerebral lobes ; r. t. n. and r «. (., rec^rent tegumental 

 nerve; ji.a.p., nerve of first pair of appendages (cheliceral nerve of Viallanes) ; c. com., cerebral commissure; d.c. com., deutocerebral com- 

 missure; d. c.i., deutocerebral lobes; v. n., root of visceral or sympathetic nervous system — After Viallanes. 



It is to be observed tb:it the median-eye ganglia are not correctly represented by the model, the horseshoe shaped mass simply repre- 

 senting tlie fibres. See my PI. Vll, Fig. 1. This empty median oval space is, in nature, tilled with large and small ganglia cells, and the 

 roots of the fibres originating from them. 



the absence of the peculiar medullary plates i^resent in the optic lobes of si^iders, we see that 

 while the brain of Limulus is iu most respects simpler, it also presents noteworthy dift'erences from 

 that of the Araclmida.* 



* Since this paper was put in type, and a few day.s before obtaining the proof, I received the Annales des 

 Sciences Naturelles, Zoologie, Nos. 4, 5, 6 for March 10, 1893, containing M. A^iallanes' final paper on the brain 

 of Limulus. 



The account of the internal structure of the brain is brief, occupying but five pages, and ends with an " Apcrfu 

 geuCral de V organisation da synteme nerveux des ArticuUs." The article is lllnstrated with two folding plates. Only 

 two actual sections of the Ijrain of Limulus are represented. One (PI. 10, Fig. 17) represents a horizontal section 

 passing through the lateral-eye ganglion (" ganglion optiiiue") of one side, and the other (Fig. 18) of a horizontal 

 section lower down passing through the st:ilk of the pedunculated body. 



This last figure clearly shows what my sections has failed to do, except p:irti;illy, the well-defined stalk of the 

 mushroom, or iieduncuhited body, and shows that what in the first place I had described as the " micleogenons 

 bodies," are lobules or branches of a pair of organs probably homologous with those of insects. This leads me to 

 accept M. Viallanes' view of the homology of tlu^se extraordinarily developed organs with the two mushroom bodies 

 of in.sects. In accordance with this view I have corrected the text of the jireseut article. Viallanes has also pointed 

 out and drawn the chiasma of the median eye, structures which I had failed to see; and he shows that the the basal 



