GAMMARUS. 357 



nerve on each side being joined, in the first segment of 

 the appendage by the corresponding non-ganglionated 

 nerve of the more anterior pair. A few fibres are given 

 off at their junction to the longitudinal muscles con- 

 necting the leg to the body-wall. The abdominal ganglia 

 are smaller than those of the last three thoracic legs or 

 peraeopods (PI. II., per. 1, 2, 3). 



The fourth pair of abdominal ganglia (PI. II., abd. 

 g. 4) innervate the last three segments of the body (which 

 almost always move together), giving off two pairs of 

 branches to the first pair of uropods, a pair running 

 directly backwards, and a pair of fine nerves which run 

 upwards to the alimentary canal in the neighbourhood of 

 the point of origin of the posterior diverticula. 



The chief sense-organs connected with the nervous 

 system are the eyes. Many of the setae scattered over the 

 body are probably tactile in function, and those of the 

 antennae, which have a rich nerve supply, may be auditory 

 or olfactory. The calceoli of the second pair of antennae 

 are probably olfactory, and appear to contain a nerve-mass 

 (see above). With regard to the eyes, the external 

 features of which have been already described, the 

 ommatidia or component simple eyes, appear from above 

 to be four-sided masses of black pigment, embedded in a 

 surrounding pigment which appears white by reflected 

 light, though by transmitted light it is pale violet. Each 

 ommatidium consists of a transparent axis, formed distally 

 of a two-celled crystalline cone, proximally of a slender 

 rhabdome, sheathed by five retinular cells. The proximal 

 end of each of the retinular cells contains a nucleus and is 

 drawn out to form a retinal fibre which passes to the optic 

 ganglion. The space between the ommatidia is filled by 

 the light-reflecting pigment, contained in cells of which 



