230 w. G. RIDEWOOD. 



Possibly in some cases the sixth phimes are permanently 

 arrested in their development on one side, or on both sides, 

 or they are not developed at all, for some polypides of 

 Cephalodiscns hodgsoni of full size and with mature 

 g-onads have five pairs of plumes only. In the cases of those 

 polypides which are found with eleven plumes the explana- 

 tion may just as likely be that the sixth plume has failed to 

 develop on one side as that one of the plumes has been lost 

 by injury. 



As regards the development of the pinnules on a plume- 

 axis, about five pairs appear simultaneously, and at the time 

 of their origin they occupy nearly the whole, or, at all events, 

 more than half of the length of the plume-axis behind the 

 end-bulb. The later pinnules are developed a pair at a time 

 at the basal end of the series. 



Development op the Plumes in Buds of Cephalodiscus 

 dodecalophus. 



The bud of Cephalodiscus dodecalophus in its 

 earliest phase (text-fig, 4, a) resembles that of C. hodgsoni 

 except in being somewhat smaller. The first pair of plumes 

 and the buccal shield begin to differentiate simultaneously (b 

 and c). By the time the second plumes appear (e) the first 

 pair are relatively longer than in C. hodgsoni. The third 

 plumes develop before the second pair have grown very 

 large (p and g). 



In buds with three pairs of plumes present the size of the 

 first pair varies considerably, as may be seen by reference to 

 H and G. Although h is a larger bud than g, it is not much 

 more advanced in its development; the second and third 

 plumes are only slightly more developed than in g, and there 

 is no sign of the fourth plumes. Yet the first plumes are 

 very large in n, and have refractive beads in the terminal 

 end-bulbs, whereas the corresponding plumes in g are 



