34 A. S. PACKARD, JR., ON THE ANATOMY 



To recapitulate and generalize from the foregoing facts : The brain is largely composed of 

 masses of nuclei (nucleogenous bodies), enclosed by a mesh-work of connective tissue ; 

 these bodies nearly fill up the lower part of the brain, i. e., that part below the origin of 

 the nerves. In the upper half or third of the brain whence the nerves originate, the 

 larger and smaller ganglion-oells and bundles of nerve-fibres appear and preserve a more 

 or less definite topographical relation to the entire brain. The nucleogenous bodies at and 

 near the top of the brain are confined to each side of the brain, though masses of large 

 ganglion-cells, associated with smaller ones, and nuclei, one on each side, just behind the 

 middle, pass from below upwards; these groups of cells are more or less spherical as they 

 grow smaller near the under side and at the top of the brain. The ganglion-cells 

 altogether give rise to bundles of nerve-fibres ; though it is probable that many nerve- 

 fibres are without beginnings from cells, but originally developed from nuclei, as the gang- 

 lion-cells probably are in the beginning ; since, in the larval brain, no fibres are to be 

 seen, the brain substance consisting of cells alone. (See plate 3, fig. 3a.) 



Thus the tract of nerve-fibres in each half of the brain is irregularly wedge-shaped, 

 the apex situated near the centre of each hemisphere, and the base spreading out irreg- 

 ularly on the top, thus pushing aside, as it were, and crowding to the walls on each side 

 the seemingly less dynamic portion of the brain, i. e., the masses of nuclei, or undeveloped 

 cells (nucleogenous bodies). At the upper part of the back of the brain, just outside, at 

 the origin of the posterior commissures, are two longitudinal groups of ganglion-cells on 

 each side ; these disappear below with the commissural nerves themselves. 



The asymmetry of the brain, compared with that of other arthropods is remarkable ; 

 the large ganglion-cells are most abundant in the centre behind the middle, extending 

 from that point to the posterior side of the brain ; a median line is only slightly indicated 

 by the arrangement of the fungoid bodies. The tract composed of large nerve-fibres, 

 with scattered ganglion-cells on the left side, is much more extensive than on the right. 



Comparison with the brain of other At^thropods. So wholly unlike in its form, the want 

 of antennal nerves, and its internal structure, is the supra-oesophageal ganglion or brain of 

 Limulus, to that of the higher Crustacea {i. e., Decapoda, the brain of the lower Crustacea 

 not yet having been examined), that it is difficult to find any points of comparison. 



Histologically, judging by my few sections of the lobster's brain which are stained with 

 carmine, the brain of Limulus agrees with that of other arthropods in having similar large 

 and small ganglion-cells, but the topography of the cell-masses essentially differs in the 

 two types of brain. There are in Limulus no BaUensubstanz-m.sisses, so characteristic of 

 other arthropods, — the histological elements constituting these not having yet been dis- 

 covered in Limulus. 



We conclude, therefore, that, topographically, the internal structure of the brain of 

 Limulus is constructed on a wholly different plan from that of any other arthropodous 

 type known, so much so that it seems useless to attempt at present to homologize the 

 different regions in the two types of brain. The plan is simple in Limulus ; much more 

 complex in other arthropods, especially in the brain of the decapodous, and probably most 

 other Crustacea, the Decapoda having two pairs of antennal nerves beside the optic. In 

 external appearance the two types of brain are entirely unlike. The symmetry of the 



