GAMMARUS. 355 



by a layer of protoplasm, which, separates it from the 

 sarcolemma or sheath. 



Nervous System and Sense Organs. 



The nervous system consists of a paired chain of 

 ganglia, each pair of ganglia being united to the next by 

 two distinct cords. The first, or supra-oesophageal 

 ganglion, which forms the brain (PL II., br.), lies imme- 

 diately below the eyes, and is connected with the ventral 

 nerve chain (PI. II., n.c.) by a perioesophageal nerve ring 

 (PL II., feri. n. ?'.). In the brain, as in the other 

 ganglia, there are, as Packard (15) observed in Asellus, 

 three kinds of elements. — (1) Ganglion cells, with large, 

 deeply-stained nuclei, which are grouped round the 

 ganglion in pyramidal masses, the bases of which are 

 applied to the ganglion ; (2) nerve fibres which arise from 

 the ganglion cells and connect the ganglia; (3) myeloid 

 tissue or substance (the " punktsubstanz " of Ley dig), in 

 which the nerve fibres are embedded, and which forms the 

 mass of the ganglion. 



The brain (PL II., br.), Avhich has been very fully 

 described and figured by Koehler (9), is a syncerebrum, 

 consisting of (1) two large superior lobes, to the external 

 extremities of which are attached the optic lobes con- 

 nected with the retina by parallel nerve fibres, (2) two 

 smaller median lobes, (3) two large inferior lobes, named 

 by Koehler the " olfactory lobes," each with a lateral 

 olfactory bulb, which appears to be an aggregation of 

 spherical masses of myeloid tissue. Some fibres from 

 both median and inferior lobes pass to the superior lobes, 

 those from the inferior lobes intercrossing to form a 

 chiasma. Into the inferior lobes run the nerves from the 

 first antennae, and, a little further back, those from the 



