20 THE VOYAGE OF H.M.S. CHALLENGER. 



of a number of separate lobes ; a paii- of conuectives unite the cerebral ganglia with 

 tlie ventral chain ; the latter is composed of thirteen distinct pairs of ganglia, some of 

 wliich bear unmistakable evidence of being the result of a fusion between several pairs 

 of primitively distinct ganglia ; this is particularly the case with the anterior of the 

 ganglia, which are seen in longitudinal section to be composed of two fused ganglia. 

 The eight following ganglia belong apparently to the thorax. Of these the three anterior ^ 

 are separated from each other by considerably longer connectives than those which 

 unite the posterior thoracic ganglia ; the latter get closer and closer together (in 

 correspondence with the diminished size of the segments to which they belong), and 

 the last pair of thoracic ganglia are hardly separated from the fused ganglionic mass 

 belonging to the abdomen ; a careful examination of the latter, especially by the help of 

 longitudinal sections, shows that it is in reality composed of six pairs of distinct ganglia, 

 the last of which is considerably larger than the rest, inasmuch as it has to supply the 

 telson as well as the last abdominal segment. The abdominal ganglia occupy only the 

 first three segments of the abdomen. 



In Serolis necera the fusion of the posterior and of the anterior ganglia is even more 

 marked. 



In the cephalothorax there are three pairs of ganglia, of which the posterior is much 

 the largest. This is united by a long connective with the next thoracic ganglia, which is 

 situated at the posterior extremity of the second free thoracic segment ; it is clear, how- 

 ever, that it belongs to the segment in front, since it was quite easy to trace the nerves 

 passing forwards to this segment. The fourth thoracic ganglion is closely connected 

 with the succeeding thoracic and the abdominal ganglia, the whole forming an elongated 

 oval mass lying in the posterior thoracic and anterior ten abdominal segments ; the 

 demarcations between the four anterior pairs of ganglia could be recognised by the stout 

 nerves given off on either side to their respective segments ; of the posterior ganglia, 

 however, it was impossible to discover how many there were ; a vast number of nerve 

 filaments take their origin on either side, and afford no indication of the number of 

 ganglia, two of these passing in a direction parallel to the longitudinal axis of the body 

 from the hinder end of the ventral chain are especially large ; they supply the telson. 



Eye. — With one exception — Serolis antarctica, which is perfectly blind — all the 

 known species of Serolis are provided with a pair of eyes situated upon each side of 

 the cephalic shield at al)out the middle of its antero-posterior diameter ; the eye has 

 usually a reuiform outline, and in general aspect recalls that of the TrUobite, with which 

 it has been compared ; more rarely it is oval {Serolis hromleyana), and in Serolis minuta 

 it is almost circular. 



In all the shallow-water species the eye is relatively small, Ijut very conspicuous from 

 the abundant deposition of pigment ; in all the deep-sea forms, with the exception of 



' This is not clearly shown in the figure owing to an oversight. 



