420 



Mr. R. Gurney on the 



The outer edge of the joints of the inner ramus is fringed 

 with a row of delicate spines. The numbers of seta? borne 

 by the different joints are shown in the following table : — 



The fourth pair have the outer ramus two-jointed and 

 shorter than the three-jointed inner ramus. 



The fifth pair of legs are simply small knobs bearing a 

 single terminal seta. 



Wilson states (1911, p. 271) that in males and immature 

 females of the Ergasilidge a pair of rudimentary legs are to 

 be found on the genital segment, but I have not found any 

 trace of them in Thersitina at any age or in either sex. 



Internal Anatomy. 



The internal structure, so far as I have been able to study 

 it, does not differ from that of Ergasilus as described by 

 Wilson. The only point in which there appears to be any 

 important difference is in the nervous system. I have not 

 been able to follow its arrangement in the adult female, but 

 in the free-swimming female and in the male the greater 

 part of it is fairly easy to see. Whereas in Ergasilus there 

 is, according to Wilson, a double ventral nerve-cord with 

 ganglia corresponding to each of the swimming-legs and an 

 additional ganglion in the genital segment, in Thersitina the 

 nerves of the first three pairs of swimming-legs arise from 

 the posterior angles of the great postcesophageal ganglion, 

 which also sends off a pair of slender cords which run back 

 parallel and close to each other for a considerable distance 

 before sending off nerves to the fourth pair of legs and con- 

 tinuing on into the genital segment (PI. XIII. fig. e). I 

 have not been able to detect any separate thoracic ganglia. 



