PLUMES OF CEPHALODISCUS. 237 



shield is of remarkably large size. Compare text-fig. 6, c, 

 with text-fig. 2, d ; and text-fig. 4, e. 



The third plumes appear to the outer side of the second, 

 and are in contact with them at their bases (d, 3). The two 

 plumes of the first pair are widely separated from one 

 another (d, 1). When in buds of Cephalodiscas hodgsoni 

 and Cephalodiscus dodecalophus the third plumes 

 make their appearance (text-fig. 2, e, 3, and text-fig. 4, g, 3) 

 they are directed strictly laterally, whereas in Cephalo- 

 discus uigrescens (text-fig. 6, d and e, 3) they point more 

 anteriorly than laterally. This doubtless is connected with 

 the relatively late origin of the plumes, and the relatively 

 small size of the plumes during the early stages of their 

 development. 



A, B, and D illustrate a peculiarity of many buds and adults 

 of Cephalodiscus uigrescens, the bending forward of the 

 posterior lobe of the buccal shield. The occurrence of the pos- 

 terior lobe in this position may be due to an exceptional con- 

 traction of the muscles of the shield brought about by the 

 irritating properties of the formalin solution in which the 

 animals were killed, but whether this be so or not, it indicates 

 considerable mobility of the organ in question in normal con- 

 ditions of existence. 



Text-fig. 6, p, shows the dorsal view of a bud in which the 

 fourth pair of plumes are making their appearance. The 

 " body " of the bud here figured was placed so far forward as 

 compared with the stalk of the shield that the plumes, in- 

 stead of being set on the anterior edge of the " body " as in 

 D, lie in a deep groove between the "body " of the bud and 

 the dorsal wall of the buccal shield. The plumes are drawn 

 in a dotted line to signify that they would be seen in the 

 positions they occupy in the figure if the " body " were 

 transparent ; as a matter of fact the " body " is so black 

 that the employment of the usual clarifying reagents fails to 

 make it transparent. The bud in question was drawn as a 

 whole, with no plumes visible ; then the " body " was dis- 

 sected off and the characters and positions of the plumes 



