Morphological Notes. 41 



11. The nervous system. 



The ganglionic chain shows a strongly developed eupra-oeso- 

 phagial, or cephalic ganglion; from its anterior corners extend the 

 very stout and thick optical nerves, dilating apically into an ovate or 

 bean-shaped disc, which is more or less closely pressed against the layer 

 formed by the retina. Between the optical nerves rise two stout nerves 

 from the front margin of the cephalic ganglion, and run to the first 

 pair of antennas; at the base of these antennal nerves are situated the 

 two vesicles which have been interpreted by Claus') as auditory organs. 

 These vesicles are small in the Oxycephalidœ but very largely develop- 

 ed in the Xiphocephalidae. The commissural cords connecting the 

 cephalic ganglion with the infra-oesophagial ganglion are thick and stout, 

 much thicker than for instance in Lanceolidce. Close upon the infra- 

 oesophagial ganglion follow the first two pairs of perœonal ganglia; they 

 are not coalesced with the infra-oesophagial ganglion but in the Oxy- 

 cephalidae they are so closely pressed against it that the commissural 

 ■cords between them are scarcely perceptible. The following four pairs 

 of peraeonal ganglia are equidistant, situated each in the middle of the 

 corresponding segments, the third, fourth, fifth, and sixth. The pair of 

 ganglia supplying the seventh perajonal segment and its appendages 

 with nerves are situated just at the limit between the sixth and seventh 

 segments, close to the preceding pair of ganglia but not coalesced with 

 it. In the hind portion of the body the ganglionic chain shows three 

 pairs of ganglia, one pair situated in the middle of each of the pleonal 

 segments, and the last consisting of two or three pairs, fused together 

 and so supplying the whole urus with nerves. The ganglionic chain is 

 thus composed of twelve pairs of ganglia, some pairs being placed clo- 

 sely together. The commissural cords run close to one another, but 

 are not coalesced. In the Xiphocep halidse the first two pairs of pe- 

 rgonal ganglia are more separated from one another and from the infra- 

 oesophagial ganglion, so that the commissural cords are distinct, but 

 not longer than the ganglia themselves. 



12. The glands. 



Glands are present in most of the joints of the perjeopoda and 

 uropoda of all the species of both the families. They are either simple, 



1) C Claus, 23, p. 150; 36, p. 15. 



Nova Acta Reg. Soc. Sc. Ups. Ser. III. 



