MEMOIES OF THE NATIONAL ACADEMY OF SCIENCES. 297 



acid. Finally, what renders them specially interesting is that we only meet with them in the 

 .sensorial centers of the brain, wliere they form very dense masses. Tliese elements are abundant 

 in the Myriopods, where they are especially small and brightly colored ; they likewise occur in the 

 Arachnids and Onychophores. They are always in direct relation with the differentiations of the 

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

 and present characters far more trenchant where the brain is higher in organization. Thus in 

 the Myriopods they are especially abundant in the complicated brains of Julus and of Scutigera, 

 and do not exist in the rudimentary brain of Geophilus. In the Arachnids we notice their presence 

 in the highly developed optic lobes of Lycodids or of Phalangids, while they are replaced by small, 

 less deeply stained nucleated cells and with visible protoplasm in the very simple lobe of Agalenids, 

 or even by ordinary cells in the rudimentary lobe of the Pholcids." He calls these cells ^^ cellules 

 chromatiques," in reference to one of their salient characters, and we shall often use thia term for 

 those of Limxrlus, though rather preferring the name aprotoplasmic cells or small ganglion cells. 



These small, aprotoplasmic, unipolar ganglion cells as contrasted with the large ganglion cells 

 are rei^resented in PI. xii, fig. Ga, smgc, where their size as compared with the ordinary large 

 ganglion cells is well brought out. 



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

 have usually two or three nucleoli, though the number may vary from 1 to C. 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 fibrilla from each of the unipolar chromatic 

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

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



This myeloid or punctuated 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 piinkisubstans, marksubstanz or myeloid substance, as we may designate it, differs iu its topograpliieal lelatious 

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

 and especially the Crustacea and insects, has been most thoroughly studied by Leydig. This is the central fineiy- 

 granular part of the brain, in which granules have short irregular fibers passing through them. In his fom Ban 

 des thierischen Korpers, p. 89, Leydig thus refers to it: 



" In the brain and ventral ganglia of the leech, of insects, and iu the br.iin of the Gastropods (Schnecken) I observe 

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

 ular mass oi punktsuhstans 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 nerve-fihers first takes place from this central imnktsuhstanz. 



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

 ganglion cells, whose continuations liecome nerve-fibers without the intervention of a superadded punktsubstanz." 



Leydig subsequently (p. 91) further describes this myeloid substance, stating that the granules composingit form 

 a reticulated mass of fibrilla% 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 iuto the punktiuhsianz this 

 continuation becomes lost in the fine threads, and on the other side of the j;Mn7c<8it6s/o«2 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 single axis- 

 cylinder derives its fibrillar substance as a mixture from the moat diverse ganglion-eclls." 



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 witU their ofl'shoots, the pair of 



mushroom bodies. We will begin our account of the topogi'aphy of the brain with the first of 



these. 



V. The Lateeal-Eye Lobes or Ganglia. 



(Pis. I, II, VI, VII, IX, XL) 



These are recognized without much difBculty, 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 brain, there being a well-marked valley or depression between 

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

 Arachnida. The two lateral-eye nerves arise from the lobes at a point considerably behind the 



