.mi:m()ii;s ok tiih national acadkmv of sciknckh. 297 



iuiti. Finally, what roiidcrs tliom spwially intorestiiif,' is that wc only iiumI with tlicin in tlic 

 siMisorial (•fnt«'is of the hiain, wlicrc tliey form very dense masses. These elements are almndant 

 in the .Myrioptids, where they are espeeially small and brif,'htly eolored; lliey likewise (leenr in the 

 Arachnids and Onyehophores. They are always in direct relation with the dilVerentiations of the 

 punctuated substance, and, in a gt-neral way, we have stated that they are far more numerous 

 and i)resent characters far more trenchant where the brain is higher in or^janizatiou. Thus in 

 the Myriopods they are especially abundant in the comi)li(ated brains of .lulus and of Seuti^era, 

 and do not exist in the rmlimentary brain of (ieoi>liilus. In the Arachnids we notice (heir iiresencc 

 in the hijjhly developed oiilie lobes of Lycodids or of IMialauKids, while they are replaced by small, 

 less deeply stained nucleated cells and with visible protoplasm in the very simi)le lobe of Apilcnids, 

 or even by ordiuary cells in the rudiint-ntary lobe of the Pholcids." lie calls these cells ''cilltilcs 

 chroinatiqum,'^ in reference to one of their salient characters, and we shall often use this term tVir 

 those of Limulus, thouf-h rather preferring the name aprotoplasmic cc^lls or small f;anglion cells. 



These small, aprotoplasmic, unipolar fiauglion cells as contrasted with the large jian-^'lion cells 

 are represented in IM. xii, fig. (ia, smyc, where their size as compared with the ordiuary large 

 ganglion cells is well brought out. 



They are seen to be much smaller than the nuclei of the large ganglion cells, and their inidei 

 have usually two <n- three nucleoli, though the number may vary from 1 tofi. In the large gang- 

 lion cells the number of nucleoli varies from 1 to 4, there being usually two more distinct than the 

 others. 



In this ganglion, that of one of the median eyes, the fibrillafromeachof the unipolar chromatic 

 or aprotoplasmic ganglion cells is seen entering the tangled central white mass, and among the 

 convoluted masses of fibrilhe are seen irregular masses of myeloid substance. 



This myeloid or puuctuated substance, common to the central white fibrillar substance of all 

 Arthropod brain ganglia, and usually enveloped by the ganglion-cells, we have referred to as 

 follows in our essay on the structure of the brain of the sessile-eyed Crustacea: 



The ptiuktuubsltin::, markaubst(tii: or myeloid substance, as we may designate it, differs in its topographical relations 

 from that of the brain of Dccapoda. This myeloid substance, which seems to be peculiar to the worms, mollusks, 

 and especially the Crustacea aud insecta, has been most thoroughly studied by Leydig. This is the central fine'y- 

 granular part of the braiu, in which granules have short irregular fibers passing through them. In his f'om llaii 

 dm thierischcn KiirperK, p. 8!», Leydig thus refers to it: 



" In the brain and ventral ganglia of the leech, of insects, and in the brainof the Gastropods (Schnecken) I observe 

 that the stalks (stiele) of the ganglion-cells in nowise immediately arise as nervc-tibers, but are planted in a molec- 

 ular mass 01 punktuubHlunz situated in the center of the ganglion, and merged with this substance. It follows, from 

 what I have seen, that there is no doubt that the origin of the neire-Jihemfuxl taken place from this central piinktsiibstanz. 



"This relation isthe rule. Kut there also occur in the nerve-centers of the invertebrates single definitely situated 

 gangliim cells, whose continuations become nerve-tibers without the intervention of a superaddeil punktsubstan/,." 



Leydig subseiiuently (p. 91) further describes this myeloid substance, stating that the granules composing it form 

 a reticulated mass of fibrilhe, or, in other words, a tangled web of very fine fibers: 



"We at present consider that by the passage of the continuation of the ganglion cells into the pMnKjuftsMns this 

 continuation becomes lost in the fine threads, and on theothersideof the iJMni<»M6«(aHC the similar fibrillar substance 

 forms the origin of the axis-cylinders arranged parallel to one another; so it is as good as certain that the eingh axix- 

 cylindcr derives itn fibrillar subiitance an a mixture from the moat diverse ganglion-cvlh." 



The brain of Limulus is, as we have seen, composed of three pairs of ganglia, viz, the lateral 

 optic lobes, the median optic lobes, and the cerebral lobes with their oflshoots, the i)air ol 

 mushroom bodies. We will begin our account of the topography of the brain with the first of 

 these. 



V. TuK Lateeax-Eye Lobes or Ganglia. 



(I'ls. I, II, VI, VII, IX, XI.) 



These are recognized without much difficulty, and are situated at the uppermost part of the 

 brain, the convex contour of their anterior portion being visible from a surface view without a lens. 

 The lobes are situated a little behind the middle of the brain, and are placed rather far apart, 

 situated one on each side of the biain, there being a well marked valley or depression between 

 them. The brain .seen from above is, then, slightly bihtbed, but not so uuukeiUy so as in 

 Arachuida. The two lateral-eye nerves arise from the lobes at a jioint considerably behind the 



