NO. 1788. 



NORTH AMERICAN EROASILID^— WILSON. 



297 



the material at hand. But the numerous nuclei in the swellings at 

 the bases of those nerves would indicate that some of the fibers at 

 least originate there. 



The ■postesophageal ganglion. — This is also club-shaped in lateral 

 view, but strongly flattened dorso-ventralty. It extends from just 

 behind the gullet to the posterior margin of the first thorax seg- 

 ment, and is made up exactly like the ganglion in front of the gullet. 

 There is a swelling at its posterior smaller end from whence arise 

 the nerves that go to the first swimming legs. From the anterior 

 end of the ganglion are given off nerves which supply the mandibles, 

 the two pairs of maxillte and the maxillipeds in the male. While 

 the nuclei are noticeabl}^ increased in number at the posterior swelling 

 that gives rise to the nerves of the first legs, there are only slight 



Fig. 17.— Tr.\nsverse section through third thoracic segment of Ergasilus centraechida- 

 EiTM. eg, Cement gland; g, ganglion of third swimming legs; i. intestine; Im, longitudinal 

 muscles; m, dorso-ventral muscles of third legs; n, nerve of third leg; sr, semen receptacle. 



differences in other parts of the ganglion. Accordingly we may look 

 upon it as a complete fusion of the various ganglia that supply the 

 mouth-parts (n, fig. 16). 



The ventral cord. — This extends from the end of the posterior gan- 

 glion back into the sixth or genital segment, where it divides into 

 two branches which finally end in the anal laminae. 



For the entire distance through the thorax it is evidently made up 

 of two cords lying side by side and partially fused. This is most 

 evident at the ganglionic swellings which occur at the origin of the 

 nerves going to the swimming legs (fig. 17). Here there is plainly a 

 separate ganglion on either side (g), the two being connected by a 



