418 AuGusTA Rucker, 



is easily broken and consequently more easily regenerated and more 

 liable to Variation. The flagellum is a little more complicated than 

 one mig-lit think at first siglit. Witli longitudinal sections of dis- 

 tended and contracted specimens, on which to make observations, 

 tlie difficulty soon disappears. Macroscopically this appendage is 

 Seen to be made up of from seven to seventeeu Segments according 

 to tlie indivual. Out of sixty complete flagella the number of Seg- 

 ments was as follows. 



1 flagellum had 17 joints 



I 



Thus sixteen is seen to be the number of joints which occurs j 

 oftenest in the flagellum, if we consider the first small segment a | 

 true Joint, which it undoubtedly is. The cylindrical wall of this ' 

 small segment is more heavily chitinized than any of the other 1 

 Segments of the flagellum. There is a thickening around its entire | 

 anterior rim, which thus has the form of a ring ; on the upper surface 

 this thickening is not confiued to the edge but extends slightly forward |l 

 and also backward for almost the whole length of the Joint. In the \ 

 edge of this thickening, in the mid-dorsal line, is a sligth groove i 

 into which fits the point of a chitinized plow-share-shaped projection ^ 

 which extends from the roof of the last segment of the body. This : 

 furnishes a pivot by means of which the heavy muscles situated in 

 the last segment of the body, are able to raise the flagellum. The A 

 distal edge of this segment has smoother setae, two short ones on | 

 the dorsal and two longer ones on the ventral side. It has no whorl | 

 of plumulose setae as have all the other true segments. These plumu- < 

 lose setae are situated about one tliird of the length of the segment i 

 from the distal end. The second, third, fourth, sixtli, eighth and i 

 tenth Segments have in addition prominent smooth setae near their f 

 posterior termination. The terminal Joint usually has near its apex < 

 a second whorl of plumulose setae. That there is great Variation i 

 in the size of this Joint and arrangement of the setae can be seen i 

 in an examination of the following table and of the Agares of plate 22. ^ 

 The flagella of only adiilt vspecimens are considered in the table. 



