MOEPHOLOGY OF CYCLOPS. 29 



ill front the very short optic iicrves, aliove and external to which are the two short frontal 

 nerves, and further back the nerves to the antennules, which rise in distinct triano'ular 

 lobes or widenings out, Avith the apex outwards and forwards, marked by the large and 

 distinctly triangular nerve-cells they contain. 



From its upper surface it gives off two superior frontal nerves, and from its ventral 

 angle a median uzygos nerve to the epistoma and labruin, forming an azijgos ganglion 

 before breaking up. The loAver part of the brain is perforated by two pairs of the 

 muscles from the angle of the gullet. 



Farcesophageal Cords. — These pass obliqiiely downwards and backwards ; they are 

 elliptical in section and nucleated on their outer surface, the inner part next the gullet 

 being simply connective *. As is well known, the nerves to the antennaj arc given off from 

 these, rather towards their anterior termination, a point to which I shall revert. Behind 

 and above the antennary nerves, a short pair of cutaneous nerves come off and run a 

 short course obliquely outwards, forwards, and upwards to end in a ganglion below the 

 hypoderm, just behind and dorsad of the antenna. 



rentral Cord (PL IV. figs. 1, 2, 9 ; PL III. fig. 1, «<?.).— This is a cord of ovoid section, 

 extending back from the back of the gullet to the end of the fifth thoracic segment, or 

 beginning of the sixth, where it bifurcates or is continued by two branches which, giving 

 off branches, finally end in the furcal processes. Its depth is greatest immediately behind 

 the gullet, and diminishes posteriorly, so that its lower side is convex in the cephalon. 

 Beyond the second segment it is suddenly flattened from above downwards. It does not 

 narrow much from side to side before the third thoracic segment, where it suddenly con- 

 tracts ; beyond this, it forms a rhomboidal enlargement towards the posterior end of the 

 fourth segment, and another slight one just before its bifurcation. There are no distinct 

 ganglia save these, in which, indeed, I have only sometimes been able to ascertain the 

 presence of cellular elements. The cord is, of course, slightly splayed out laterally at 

 the origins of the nerves, but that is all. The ordinary rounded nuclear elements of the 

 central nervous system are exclusively peripheral and, indeed, chieflj^ confined to the 

 dorsal and ventral surfaces. A few occur on the anterior edge, where the cord abuts 

 against the gullet ; only a few very fine elongated nuclei, possibly representing the neuri- 

 lemma, are found beyond the middle of the second thoracic segment (except a very few, 

 foiu" or five, more rounded, sometimes observalile in the ganglion of the fourth segment), 

 where the dorsiventral flattening takes place, so that in the next segment the cord is 

 tape-like. 



In the third segment are given olT the nerves to the fourth limb, and a nerve to the 

 side of the fifth segment ; from the ganglionic (?) enlargement in the fourth segment 

 are given olf the nerves to the great flexor muscles, to the fifth appendage (first rudi- 

 mentaiy foot), and to the sides of the sixth thoracic segment. From just before the 

 bifurcation of the cord a nerve is given off, of which part goes to the sixth thoracic limb 

 (genital valve) and part to the inner side of the vulva of the female. 



I have found it almost impossible to fully trace the nerves from the anterior part of tlie 



* I use the term cord adnsedly, because ' commissure ' or ' connective ' would imply the absence of ganglionic 

 elements and has, probably, led to much erroneous reasoning. 



