STRUCTUKE OF THE BUAIN OF TUB SESSILE-EYEl) CRUSTACEA. lOl 



\vliii;lj arc so characleri.stic of the brain of the Decapod C'riistacea and tbe iiiscict.s; and in this 

 resi)ect there is probably a wide ilittereuce between the brain of Decapoda aud Edrioplithahnata. 



HISTOLOGICAL TOPOGRAPHY OF THE NEHVK TISSl KS. 



(1) Tht (jaiKjlion nils. — These cells form a cortical layer envelopinj>()ii all or nearly all sides the 

 central myeloid mass. The cells being distinct aud more or less loosely arranged readily take a deep 

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



The ganglion cells are collected into more or less definite mas.ses, enveloi)ed by connective 

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

 tical sectiou, such as that represented by Figs. 2 and 3, passing through the anterior and middle i)art of 

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

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

 distinct lobular masses, which are iuclosed 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 iu Figs. 2 aud 3, the uppermost or dorsofroutal lobes are the double sets filling the 

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

 divided into two sublobes, the upper (/>') being small, flattened, and l.\iug on the dorsal and inner 

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

 set; they are more or less counected with the lateral lobes d rf', aud the latter with the e.xteruo- 

 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; tiie 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, aud these lobes {U i k, Fig. 27) which envelop the myeloid mass cau 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 ncr Pi- fibers. — The ftbers arising from the ganglion cells form the commissures which 

 unite the brain with the suboesophageal and succeeding ganglia; and also the commissures between 

 the two cerebral lobes. 



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

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

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

 observations on the commissural flbers of Orycivs nanicorni-s,* there is little doubt but that in all 

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

 ventral ganglion. 



It will be further seen by reference to Figs. 2, 3 ( Asellus), aiul especially Fig. 27 (Cecidotaia), tliat 

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

 two directions, some connecting the two lobes, forming the transverse commissure, while others 

 ])ass dowu aud ruu parallel with the fibers from the dorsofroutal lobes and aid iu building up the 

 subtesophageal commissui-es. The latter commissure is also re-enforced by fibers from the lateral 

 lobes d d\ e eK 



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

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

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

 above or around the myeloid substance of the cerebral lobes aud form the commissures. This 

 independence of the myeloid substance appears to be more general in the Asellidie, 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 postceiihalic ganglia rei)re- 

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



" Michels. Beschreibung eles Nervensystems von Oryctes nasioornis in Larven, Puppeus uuil liiiferznstand. Zeits. 

 f. wisseus. Zoologie., xx.\iv, 641-702. 1880. 



