24 



There are seven pairs of thoracic ganglia {th. g.), the 

 ganglia of each pair being closely connected. The pairs 

 of ganglia are connected by commissures, those between 

 the sixth and seventh pairs of ganglia being very 

 short. Each pair of thoracic ganglia gives off a pair of 

 stout nerves, which split into several parts, and supply the 

 appendages. From the middle of the length of the 

 commissures connectinsr the ffanfflia, nerv'^es arise which 

 innervate the muscles of the body. 



In Ligia the abdominal ganglia are all fused into a 

 single ganglionic mass [(ib. g.) situated in the anterior 

 region of the abdomen. In the Isopoda all stages are 

 found, from the original separate condition of the 

 abdominal ganglia to the fused condition occurring in 

 Ligia. From the abdominal ganglion nerves arise, which 

 supply the appendages and muscles of the abdomen ; a 

 large pair of nerves run from the posterior end of the 

 ganglion to supply the uropoda. 



A small median nerve runs between the commissures 

 connecting the thoracic ganglia from the sub-oesophageal 

 ganglion to the seventh pair of thoracic ganglia. It has 

 been termed the ' sympathetic ' nei^^^e, but there is no 

 evidence that it is of such a nature. 



Sensory Organs. 



The eyes. — As the eyes of the Ligia oceanica are 

 different from the eyes of other Isopods, which have been 

 described by Parker, Beddard and others, their structure 

 will be given in detail. 



They are compound and sessile, occupying almost the 

 vrhole of the lateral region of the head. In the mature 

 animal each eye consists of upwards of 500 ommatidia. 

 The corneal cuticula is facetted. The corneal facets of the 



