STRUCTURE OF THE BRAIN OP THE SESSILE-EYED CRUSTACEA. 5 



which are so characteristic of the brain of the Decapod Crustacea and the insects; and in this 

 respect there is probably a wide difference between the brain of Decapoda and Edriophthahuata. 



mSTOLOGICAL TOPOGEAPHY OP THE NERVE-TISSUES. 



(1) The ganglion cells. — These cells form a cortical layer enveloping on all or nearly all sides the 

 central myeloid mass. The cells being distinct and more or less loosely arranged readily take a deej) 

 carmine stain, while the much more dense myeloid mass remains white and unstained. 



The ganglion cells are collected into more or less delinite masses, enveloped by connective 

 tissue, the latter as it were forming a mesh, inclosing spherical masses of ganglion cells. In a ver- 

 tical section, such as that represented by Figs. 2 and 3, passing through theanterior and middlepartof 

 the brain and in the horizontal section (Fig. — ), while the ganglion cells are seen to be packed more 

 or less solidly around the central myeloid portion, they are also seen to be disposed in more or less 

 distinct lobular masses, which are inclosed by connective tissue. Seven or more distinct lobes or 

 subspherical masses of these ganglion cells may be distinguished on each side of the brain. 



As seen in Figs. 2 and 3, the upijermost or dorsofroutal lobes are the double sets fllliug the 

 upper or dorsal fissure between the right and left lobes of the brain and marked a and h; b is 

 divided into two snblobes, the upper (i') being small, flattened, and lying on the dorsal and inner- 

 edge of the central lobe. The third set is a double lobe, c c' ; these may be called the dorsolateral 

 set; they are more or less connected with the lateral lobes d d\ and the latter with the externo- 

 commissural set of lobes (e e'). On the dorsal side of the brain near the base of the optic ganglia 

 are two sets, one above and one below (</) the base of the optic ganglion; the exact relation of 

 these to the others is not very plain from our sections, but they are in front of and external to the 

 outer edge of the lobes of the brain. 



The optic ganglion is enveloped by a lobulated mass of ganglion cells exactly like those of the 

 brain proper, and these lobes {h i k, Fig. 27) which envelop the myeloid mass can be distinguished 

 from the outer one at the beginning of the outer division of the nerve fibers sent to the eye from 

 the ganglion cells. 



(2) The nerve fibers. — The fibers arising from the ganglion cells form the commissures which 

 unite the brain with the subcesophageal and succeeding ganglia; and also the commissui'es between 

 the two cerebral lobes. 



One set of fibres arise in the dorsofroutal groui) of ganglion cells (Fig. 3, / b), to become lost in 

 the myeloid substance. The fibers are seen to pass down, and to form a part of the subu?.sophageal 

 commissure, although we did not trace them to the last abdominal ganglion. Judging from Michel's 

 observations on the commissural fibers of Oryctes nasieornis,* there is little doubt but that in all 

 Arthropoda certain nerve-fibers arising in the pro cerebral lobes pass uninterruptedly to the last 

 ventral ganglion. 



It will be further seen by reference to Figs. 2, 3 (Asellus), and especially Fig. 27 (Cecidottea), that 

 the fibers arising from certain of the ganglion cells in lobes c and c' pass into the cerebral lobe in 

 two directions, some connecting the two lobes, forming the transvei'se commissure, while others 

 pass down and run parallel with the fibers from the dorsofroutal lobes and aid in building up the 

 suba'sophageal commissures. The latter commissure is also re-enforced by fibers from the lateral 

 lobes d d\ e e'. 



From what we have seen in the sections represented by the camera sketches referred to (Figs. 

 2, 3, and 27), and from what is known of the cells and fibers of other Arthropods, there is no doubt 

 but that all the ganglion cells give rise to fibers, some of which at least pass directly through or 

 above or arouud the myeloid substance of the cerebral lobes and form the commissures. This 

 independence of the myeloid substance appears to be more general in the Asellidaj, at least this 

 we would infer from Leydig's statements previously quoted. When we look at Fig. 1, which is a 

 composition (drawn, however, with the camera) from the sections represented by Figs. 5 and 8 we 

 see that the two main longitudinal commissures pass above the seven post-cephalic ganglia repre- 

 sented in the figure. Those ganglia are masses of myeloid substance, with a cortical layer of gan- 



* Micliels. Beschreibung ties Nerveusystems von Oryctes nasieornis iu Larven, Puppeus und Kiiferzustand. Zeits. 

 f. wissens. Zoologie., xxxiv, 641-702. 1880. 



