4 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



The other sketches of Isopod brains by Brandt and Eatzeburg, Eathke, LerebouUet, and Milne- 

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

 indifferently drawn, and only represent a dorsal view, the antennal and those ganglia posterior to 

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



Tlie 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, in consecutive order, and made from alcoholic specimens, wliich 

 had, however, been kept for several years, though the nervous system had been well preserved. 



THE niSTOLOGICAL ELEMENTS OF THE GANGLIA. 



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

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

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

 (3) hey Aig^s puiitzaubstaiiz [marksuhstaitz of Leydig and Eabl-Eiickhard, and especially Diet!), which 

 might be called the myeloid tissue or substance. 



(1) Ganglion cdls. — These have not, as in 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.J 

 They are, as a rule, much smaller, however, than 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 

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

 the lob.ster 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 ganglion cells of the American lobster 

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

 cell of the lobster's braiu 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-fibers. The 2»<iiJ^tsuhst((Hz, 7narksuhsfaHz or myeloid substance, as we may designate it, dif- 

 fers in its topographical relations 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 fiuely-granular part of the brain, in which 

 granules liave short irregular fibers passing through them. In his Vom Bau flcs thierinchcn Korjyers, 

 p. S!), Leydig thus refers to it: 



lu the brain and ventral ganglia of the leech, of insects, ana in the braiu of the Gastropods (Schuecken) I observe that 

 the stalks (stiele) of the ganglion-cells iu nowise immediately arise as nerve-fibers, but are jdanted in a molecular mass 

 or piiiiktsiibstaii: situated in the ceuter of the gaiigliou, and merged with this substance. It follows, from what I 

 have seen, that there is no doubt that the origin of tlic nerce-fibers first takes place from this central piuiktsuhstaiiz. 



This relation is the rule. But there also occur iu the nerve-centers of the invertebr.ates single definitely situ.ated 

 ganglion cells, whose continuations become nerve-fibers without the intervention of a superadded puuktsubstanz. 



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

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

 fibers. 



We at present consider that by the passage of the continuation of the gaugliou cells into the piinklsKbslanz this 

 continuation becomes lost iu the fine threads, aud on the other side of the piiiiktsubstanz 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 suhstancc as a mixture from the most diverse ganglion cells. 



The myeloid substance in the brain of Asellus is not however difi'ereutiated into distinct spher- 

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



* Fig. 255, Idotwa inorata, and Fig. 25fi, Serolis, drawn by J. S. Kingsley. 



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

 Sph;eroma, in which he figures and describes the brain and nervous system in general of that Isopod. 



; .Second Report United States Entomological Commission, ch. xi. The Brain of the Locust, 1880 (PI. xi. Fig 

 3b-3e). 



