100 MEMOIES 01<' THE NATIONAL ACADEMY OF SCIENCES. 



The other sketches of Isopod brains by Brandt and Ratzeburg, Eathke, Lereboullet, and Milue- 

 Edwards, as well as those in our "Zoology,''* are drawn on a small scale, are iu some cases rather 

 indifl'erently drawn, and only represent a dorsal view, the antenna! and those ganglia posterior to 

 it being concealed from view in dissecting from above downward. t 



The observations I have made are based on vertical, longitudinal sections kindly made for me 

 by Mrs. C. O. Whitman, under the direction of Dr. C. O. Whitman. The sections were thin, clear, 

 well-mounted in Canada balsam, iu consecutive order, and made from alcoholic specimens, which 

 bad, however, been kept for several years, though the uerv^ous system had beeu well preserved, 



THE HISTOLO(HCAL ELEMENTS OF THE GANGLIA. 



Unlike the central nervous system of Vertebrates, in which there are but two kinds of nerve 

 tissue, viz, ganglion cells and fibers, there are in the Asellidae, as in insects and Decapods, three 

 kinds of elements in tlie brain and other ganglia, viz: (1) ganglion cells; (2) nerve fibers; and 

 (3) Leydig's puntzsuhstam {ma rksuhstx nz of Leydig and Kabl-Riickhard, nnd especially Dietl), which 

 might be called the myeloid tissue or substance. 



(1) GaiHjHoit cells. — These have not, as iu the brain of the lobster, a simple nucleus and 

 nucleolus, but they usually have numerous, from 10 to 20, nuclei, the nucleolus of each nucleus 

 readily receiving a stain and forming a distinct dark mass. They resemble those of the locust.:]: 

 They are, as a rule, much smaller, however, tlian in the locust. As seen in most of the sections 

 they appear to be spherical, being cut through transversely by the microtome, but as shown by 

 Pig. 3a they are of the usual pyriform shape. In size they are very much smaller than those of 

 the lobster and much more uniform in size, very few of the cells being twice as large as those of 

 the average size; as already remarked, the nucleus in the gauglion cells of the American lobster 

 are almost uniformly simple and homogeneous, with a single nucleolus. The largest ganglion 

 cell of the lobster's brain which we have found is six times as large as the largest ganglion cell ot 

 Asellus. 



The ganglion cells appear to be entirely unipolar; no bipolar or multipolar cells were observed, 

 though special search was made for them. Nothing noticeable was observed in respect to the 

 nerve-tibers. The j)uii]d.sul>st(tii:, marksuhstdiiz or myeloid substance, as we may designate it, dif- 

 fers iu its topographical relations from that of the brain of Decapoda. This myeloid substance, which 

 seems to be peculiar to the worms, mollusks, and esi)ecia!ly the Crustacea and insects, has beeu 

 most thoroughly studied by Leydig. This is the central liuely-granular part of the brain, in whicli 

 granules have short irregular fibers passing through theui. In iiis Vom Ban dex thierischen Korpers, 

 p. SO, Leydig thus refers to it: 



111 tbe brain aud ventral ganglia of the leech, of insects, iinaiii tlio brain of tin- Gasrropcxls (Schnecken; loViserve that 

 the stallis (stiele) of the {janfjiion-celts iu nowise imiuediately arise as neive-libfi-s, but areplaiitetliu a molecular ni.ass 

 or jxiKktsuhxIan: sitnateil in the center of the ganglion, aud merged with this substance. It follows, from what I 

 have seen, that there is no donlit Ihiit ilie origin of (he ne.n/t-Jilitrs firxt taken place from this central jiiiiiklniihstam. 



This relation is the rule. But there also occur in the uerve-ceuters of the invertebiates single detinitely situated 

 ganglion cells, whose coutiunatious become nerve-fibers without the intervention of a superadded iiuuktsubstanz. 



L(\vdig subsequently (p. 91) further describes this myeloid substance, stating that the gran- 

 ules composing it form a reticulated mass of fibrilhe, or, in other words, a tangled web of very line 

 fibers. 



We at present consider that by the passage of the continuatiou of the ganglion cells into the 2>^inktxnbst(im this 

 colli ill nation becomes lost in the tine threads, aud on tbe other side of the 2>'inktsnl»ilnn: the similar tibrillar substance 

 forms the origin of the axis-cylinders arranged parallel to one another; so it is as good as certain Ihot the xinf/lc axin- 

 ei/lindcr derices its fibrillar snbslance as a mixinre from the moat dirersc yaniilion cells. 



The myeloid substance iu the brain of Asellus is not however differentiated into distinct spher- 

 ical masses, the punktsubstanzballen of Krieger (Balken of Dietl) or whitish ball like masses 



' Fig. 255, Idolaa inorata, and Fig. 2.3(1, Scrolls, drawn by ,T. S. Kiugsley. 



t Since this essay has been prepared I have obtained Dr. Btdlouci's excellent memoir on the nervous system of 

 SjihaToiiia, iu which he figures and describes the brain and nervous system in general of that Isopod. 



t Second Report United Slates Entomological Commission, ch. xi. The Urain of the Locust, 1880 (PI. xi. Fig 

 :!b--3e). 



