246 W. G. RIDEWOOD. 



neuropore is not represented, and the collar forms no projec- 

 tion in the median dorsal line above the base of the pro- 

 boscis. Except for this interval the whole of the anterior 

 margin of the collar forms a strongly-developed fold, split 

 up dorsally to form the arms, and ventrally constituting the 

 operculum.'' 



My observations on the development of the plumes and 

 post-oral lamella of buds of Cephalodiscus entirely bear 

 out this contention. While, however, Harmer finds it neces- 

 sary to insist that the post-oral lamella is a derivative of the 

 anterior edge of the collar, and not of its posterior edge as 



Text-figure 10. — Diagram of the collar and adjacent parts of 

 Cephalodiscus as seen in a longitudinal section of the poljpide. 

 b.s. Thick ventral wall of the buccal shield, c.c. Anterior part of 

 collar cavity, c.c'. Posterior part of same. c.p. Position of right 

 collar pore, in distance, e. Torn edge of body-wall of right side. 

 g.s. Internal opening of gill slit. m. Mouth, no. Notochord. 

 p. Opening of the cavity of the plume-axis into the collar cavity. 

 p.a. Axis of first plume of right side. p.c. Proboscis cavity. 

 ph. Dorsal wall of pharynx, pi. Pinnules, po.l. Median part of 

 post-oral lamella, se. Wall or septum between collar cavity and 

 trunk cavity, sp. Space between the posterior flap of the buccal 

 shield and the post-oral lamella, leading into the mouth {m.). 

 t.c. Dorsal part of trunk ccelom. t.c'. Ventral part of same. 



he formerly supposed,' the point does not appear to me to be 



of special consequence. The post-oral lamella is really a 



pair of ventro-lateral flaps united together behind the mouth, 



> ' "Challenger" Reports,' vol. xx, part G2, 18S7, p. 43. 



