THALASSEMIA AND ECI1IURUS. 433 



cord is arranged in close undulations, and exhibits no ganglionic 

 enlargements. It consists of a moderately long sheath, in which 

 the nervous matter is contained in a very soft condition. 



The reproductive system consists of four sacs which open 

 on the ventral surface by minute orifices, two immediately 

 behind the genital hooks, the other two about an inch farther 

 back, and both pairs about one-third of an inch from the 

 median line. When the Eehiurus is not in season, they are 

 about one inch and a half long, one-fourth of an inch in 

 diameter, highly transparent, so as to be almost invisible, 

 and possessed of the power of twisting in all directions. 

 When the male is in season they become greatly enlarged, 

 four inches long, half-an-inch in diameter, with one or two 

 contracted portions. The contained fluid is milk-white, and 

 rather consistent ; and when examined under a high power, 

 it is seen to swarm with exceedingly active globular sper- 

 matozoa, which exhibit rapid whirling and dancing motions. 

 The male organs, when in this condition, are remarkably 

 beautiful objects, being covered with large thread-like and 

 transparent scarlet blood-vessels, which are relieved by the 

 dead cream-white of the organs themselves. We have not 

 seen the female sacs fully distended ; when moderately so, 

 the eggs appear to be arranged as in the roe of osseous 

 fishes, and are about the size of millet-seed. Examined under 

 the microscope, the egg appears as a highly - transparent 

 globule, enclosing towards its centre a number of smaller 

 globules or cells. 



The structure of Thalassema n&ptuni is in all respects 

 identical with Echvurus, the only difference, and that an 

 unimportant one, being the less complicated arrangement of 

 the intestinal lube. 



The oral and anal hooks and spines of Echiu/rus, and the 

 oral hooks of the Thalassema, are protruded and withdrawn 

 exactly as the setae and hooklets among the Annelides. 



