Pilidium of Certain Nemerteans. 251 



xxxii). Their position is also marked by the depression, or circular 

 pit, jv^hich eventually appears at this point. This pit seems to be 

 well developed in all species of pilidia. In general it increases in 

 depth with the increase in age of the individual. It is thus very 

 much deeper in mature pilidia found free in the water than in the 

 young specimens raised in the aquarium. 



After the formation of the muscular system strong fibers pass 

 directly from the apical plate to the anterior, lower borders of the 

 body and to the esophagus. By the contraction of these muscles the 

 pit is deepened, and the plate is withdrawn far below the general 

 surface of the body. The movement of the flagellum is probably 

 also aided by these muscles. 



The cilia are longer on the apical plate than elsewhere on the 

 body, and some of them increase so much in size that they come to 

 be recognized as flagella. The number of such flagella is at first 

 considerable (Figs. 1, 2, PL xxxiii), but they rapidly fuse together to 

 such an extent that only a few are present in the early pilidium (Fig. 

 1, PI. xxxiv). These increase in length until they become half as 

 long as the diameter of the body. Fusion goes on as the pilidium 

 increases in size. At the age of three or four days the number has 

 diminished to one or two long, whip-like flagella with several shorter 

 ones (Figs. 5, 6, PI. xxxii; Figs. 2, 3, PI. xxxiv). It usually happens 

 that these eventually fuse together, and the pilidium is provided with 

 a single, thickened flagellum, the length of which is in M. caeca and 

 C. leidyi about equal to that of the body. 



When but a single flagellum is present its compound nature can 

 be recognized at the point where it spreads out on the cells of the 

 apical plate, for a considerable number of cells furnish it support. 

 Biitschli (6) was the first to recognize that the flagellum was a con- 

 solidation of several more slender ones. Aceto-carmine often serves 

 to separate the original constituents. 



In C. marginatus there is usually but a single flagellum as early 

 as the gastrular stage. In addition to this several rudimentary ones 

 may be present (Fig. 6, PI. xxxiii; Fig. 5, PI. xxxv). Wilson figures 

 (32) several flagella for the pilidium of C. lacteus. I think a single 

 one, however, is much more common in all stages after the gastrula. 



It occasionally happens that the flagella of the apical plate are 

 arranged in two distinct clusters, as Wilson shows in his figure of 

 C. lacteus. Likewise the apical plate itself may be divided into two 

 In a few instances two distinct and widely separated plates were 

 present — one at the apex as usual, and the other about half way 



