SOME BIPINNARI^ FROM THE ENGLISH CHANNEL. 457 



base has the same position as the base of the dorsal oral pair 

 of arms in the more familiar type of Bipinnaria. The two 

 anterior pairs of dorsal processes in the former represent, 

 therefore^ the single pair of anterior arms of the latter. I have 

 accordingly made a distinction in this case between the terms 

 ciliated processes and ciliated arms ; in the larva under con- 

 sideration the two anterior pairs of dorsal processes clearly 

 represent one pair of bifid arms (figs. 1 and 2, r. a. d. a. and 

 /. a. d. a.). The significance of this distinction will be seen 

 shortly. 



Before proceeding to compare the Plymouth larva with 

 Bipinnaria asterigera I must point out a discrepancy 

 which has been revealed by a study of the literature upon 

 the latter form. MiiUer^s account of the Norwegian larva 

 difi'ers from those of Sars, Koren, and Danielssen with regard 

 to the number of lateral processes. It has already been stated 

 that Sars characterised his Bipinnaria by the possession of 

 twelve arms round the mouth. It is clear from the later 

 descriptions that these appendages correspond with the whole 

 series of paired arms in other Bipinnarise; although in Sars's 

 larva they are crowded together in a group at the posterior 

 end of the body. Koren and Danielssen agree with Sars in 

 saying that there are six pairs of appendages, and they give a 

 figure showing the arrangement of tlie arms. The most ante- 

 rior pair corresponds exactly with the pair of anterior ventral 

 arms of other Bipinnariee, as it arises as a pair of processes 

 from the prse-oral ciliated ridge. The remaining five pairs 

 arise from the dorsal and posterior sides of the larva, and are 

 clearly processes from the post-oral ciliated ridge ; they are 

 therefore comparable to the five pairs of processes from the 

 same ridge in the Plymouth larva. Muller, however, not only 

 mentions that there are six pairs of processes from the post- 

 oral ridge, but he also gives a figure of them. He does not 

 contrast this number with that given by the Norwegian natu- 

 ralists, so that it is quite possible that he was mistaken. It is 

 certainly not likely that any difference in this respect really 

 existed ; for the two specimens examined by Miiller were 



