248 W. G. RIDEWOOD. 



guided into the mouth and prevented from escaping by the 

 free edge of the post-oral lamella being at the time in close 

 contact all round with the posterior flap of the buccal shield. 

 The lateral lobe of the post-oral lamella thus keeps the food 

 current distinct fi-om the exhalent current through the gill 

 slit and from the water passing in or out of the collar canal, 

 both the gill slit and the collar pore being situated posterior 

 to the lateral lobe of the post-oral lamella. 



If Phoronis is justly to be regarded as a member of the 

 Hemichordata, and the tentacles are collar-structures com- 

 parable with the plume-axes of Cephalodiscus, one must 

 admitj with Harmer,^ that what is in Cephalodiscus known 

 as the post-oral lamella is in Phoronis produced into a row 

 of tentacles instead of a paired flap. On this admission there 

 is no great difficulty in effecting a comparison between the 

 plume-systems of the two. 



The line of the bases of the tentacles of Phoronis runs 

 in the double spiral well shown in Benham's figure of the end 

 view of the animal with the tentacles cut short.^ This figure, 

 if inverted, is closely comparable with text-fig. 8 of this 

 paper. Below the mouth is a row of tentacles, corresponding 

 with the post-oral lamella of Cephalodiscus, continued on 

 each side to the centre of the spiral, which point is equivalent 

 to the uppermost limit of the post-oral lamella in text-fig. 8. 

 Here it becomes continuous with the tentacles of the supra- 

 oral series, as in Cephalodiscus the extremities of the base 

 of the post-oral lamella are continuous with the terminal 

 members of the series of plumes. The notch in the supra- 

 oral series of tentacles, marked x in Benham's figure, is the 

 equivalent of the space between the right and left plumes of 

 the first pair in Cephalodiscus. 



Not only has this correspondence between plumes and post- 

 oral lamella of Cephalodiscus with the two series of 

 tentacles of Phoronis been indicated by Harmcr, but that 



' ' Pterobranchia of the "Siboga" Expedition,' 1905, pp. IIG, 117. 

 2 'Quart. Journ. Mic. Sci.,' vol. 30, 1889, pi. 10, fig. 7. 



