250 



W. G. RJDEWOOD. 



the pre-septal cavity may represent the pair of collar cavities 

 of Balanoglossus and Cephalodiscus.^ The obliquity 

 of the coclomic septum of the Actiuotrocha is exactly similar 

 to the obliquity of the boundary between the collar cavities 

 and trunk cavities of Cephalodiscus (see the upper line 

 of dashes in text-fig. 10), the dorsal part being much more 

 anterior than the ventral. 



A comparison of such published figures of buds of 

 Rhabdo pleura as show developing plumes, with the buds 

 of Cephalodiscus described in the earlier part of this paper, 

 confirms the general impression that the two plumes of the 



P - -- X 



Text-figure 11. — Two buds of llhabdopleura normani, 

 dorsal aspect; copied from Lankcster, ' Uuart. Jouni. Mic. Sci.,' 

 vol. 24, 1884-, pi. 39, fig. 3, buds 5 aud 7. h. Body of the bud, 

 b.s. Buccal shield, p.a. Plume-axis. ;)/. Developing pinnules. 

 s. Stalk. 



adult E-habdopleura are equivalent to the first pair of 

 plumes of Cephalodiscus. The plumes of Rhabdopleura 

 arise as a pair of digit-like processes (see text-fig. 11), close 

 together, and dorsal to the shield ; later on the pinnules 

 develop in pairs, and in the adult the pinnules are directed 

 ventrally towards the shield, exactly as are the pinnules of 



' See Fowler, ' Eestschr. 70ten Geburtstage R. Lcuckarts,' 1892, p. 297, 

 and Ilarmer, 'Ptcrobranchia of the "Siboga" Expedition,' 1905, p. IIG. 



