PA RASITIC COPEPODS—GALIGIDJE— WILSON. 



521 



But the fourth pair, even in those species in which they are rela- 

 tively large (Zepeophtheincs longvpes, etc.), are notably defective in 

 muscles. Hence, the} 7 can be but little used by the copepod, and the 

 genera {Pseud ocalig us, Alebion), in which they are reduced to mere 

 rudimentary stumps, can not suffer much inconvenience from their 

 loss. The muscles of the alimentary 

 canal and those used in respiration have 

 been already described. 



NERVOUS SYSTEM. 



The nervous S3 r stcni is composed of 

 two central ganglia and the paired nerves 

 which arise from them. One ganglion 

 lies above the oesophagus and the other 

 below it, but the lateral commissures con- 

 necting the two are so large that it would 

 be more strictly correct to speak of the 

 ganglia as fused together with a small 

 opening through the center for the pas- 

 sage of the (jesophagus (Pickering and 

 Dana, p. 89). The upper, supraoesopha- 

 geal ganglion or brain, is about half as 

 large as the lower and gives off from its 

 anterior end three pairs of nerves (fig. 

 27). 



The first or inner pair (1; are optic 

 nerves and arise from a small optic lobe 

 produced on the dorsal surface of the 

 ganglion. 



They are very short and their roots 

 cross so that each eye is supplied by 

 fibers from both sides of the brain. 



In Lepeophtheirus the second pair (2) 

 arise just outside the first and go to the 

 firsl antennae. They are much larger 

 than the preceding pair and subdivide 

 into a number of branches, which supply 

 both the plumose setae upon the basal F 

 joint and the simple tactile setae upon the 

 terminal joint. From the size of this nerve and the detail with which 

 every seta is innervated it is very evident, as A. Scott has well stated, 

 that these first antennae are important sensory organs. A branch 

 from this second pair of nerves extends inward to the gland which 

 secretes the filament for attachment during the chalimus stage. This 

 branch is not noted by Scott, but is mentioned by Pickering and 



ig. l!7. — Nervous system of a Lepeoph- 

 theirus. (After A. Scott.) 



