428 RECORD OF CURRENT RESEARCHES RELATING TO 



fourth are smaller, than the rest ; the ganglia of the terminal plexns 

 project somewhat into the last thoracic segment. The histology of a 

 segmental ganglion shows that a connective-tissno sheath containing a 

 mass of large cells, to bo called " fibrogcnous colls," invests the 

 nervous mass, and is strongly pigmented by a brownish granular pig- 

 ment contained in stellate cells, as in other parts of the body ; the 

 pigment cells are contractile. Beneath the neurilemma lies a layer of 

 large polygonal cells with dark nuclei, which stain readily ; beneath 

 these lies the central mass of the ganglion, composed of two apposed 

 oval grey masses in which the outlines of cells are discernible. Of 

 the entering nerve-cord a part passes through the ganglion unaltered, 

 a part is lost in the superficial layer, and the greater part breaks up 

 in the central mass. The structure of the brain and the abdominal 

 ganglion-plexus is somewhat different. In the former the grey central 

 substance prevails, and is present wherever the two lateral halves unite ; 

 it is invested by a thin transparent layer. The fibres which unite the 

 chief brain with the lower ganglia show a chiusma, the commissural 

 fibres of one side being derived from the opposite side of the upper 

 mass. The lower ganglia — antennary, &c. — each contain two hemi- 

 spheres of medullary substance ; one of these is formed by the break- 

 ing up of the entering nerves, the other by the large-celled cortical 

 substance, together with a bridge of grey matter which is in connection 

 with that of the other ganglia. 



The connection of the optic ganglion with the chief brain is only 

 apparent ; the band uniting them is merely connective tissue, and the 

 real connection of the former is with the upper antennary. Of the 

 small abdominal ganglia, the last ditfers little from the segmental 

 ganglia ; the smaller first pair is connected with it by a commissure, 

 and consists chiefly of cortical substance ; the second pair consists 

 entirely of cortical substance, and has no posterior commissure. 



Sense-Organs. — The upper antennary nerve divides into two 

 branches in the third joint — the one for the convex, the other for the 

 concave aspect of the antenna. These branches go on to the end, 

 giving off a number of variously-sized twigs, some to the muscles, some 

 to act as sensory nerves, on the way. A group of these latter are dis- 

 tinguished as cutaneous ; and of these may be seen, running up the side 

 of the antenna, three long, unbranched twigs, containing small granular 

 and apparently nucleated dilatations at short intervals ; they are trans- 

 parent and strongly refractive, and are found besides in various parts 

 of the body ; they have no special end-organs. 



Another set of sense-organs consists of small chitinous elevations 

 on the antenna) ; they stain red with osmic acid and picrocarmine, and 

 in large species contain triangular or bacillar granulations set side by 

 side, among which a fine nerve ends ; possibly they may be primitive 

 visual organs. Further, certain sabre-shaped hairs are found constantly 

 projecting from some wall-like chitinous elevations, all over the body, 

 supplied each by a nerve. On the first pair of antenna) a few thick hairs, 

 each terminated by finer ones, and supplied by a four-fibred nerve twig, 

 perhaps constitute auditory organs. Eight organs similar to these, but 

 in pau'S, occur in the same position. The anteunulc bears some paired, 

 somewhat retractile processes, medially constricted, and containing 



