REMARKS ON PEDALION, 341 



mollusca. It is not an extravagant supposition that the 

 Rotifers of to-day are the nearest representatives of the 

 common ancestors of Mollusca, Annuloida, and Arthropoda. 

 Supposing that the Arthropoda, or at any rate the Crus- 

 tacea, were developed from such a form, we should 

 not expect the laterally paired limbs of the Nauplius 

 form to be at once evolved, limbs in various positions 

 would obviously be possible, and after such various dis- 

 positions of limbs as might arise had had their chance (we 

 are supposing that conditions occurred which favoured the 

 development of such protuberances of the integument as we 

 see in the male of Asplanchna, and the adaptation of mus- 

 cular bands to these protuberances) natural selection pro- 

 ceeded along the definite line of paired ventrally placed 

 appendages, which has resulted in our present Arthropods. 

 Pedalion Avould then be a representative of one of the un- 

 successful candidates in limb-arrangement — he remains much 

 as that unsatisfactory ancestor of his was — whilst the forms 

 which we must suppose did exist with trochal discs and 

 limbs arranged more like those of Nauplius are — are non- 

 existent to-day on account of the very fact that they lie in 

 the direct line of ancestry ; their descendants are the Crus- 

 tacea. No doubt, by speculations similar to the foregoing 

 Pedalion might be accounted for, as a link in a supposed 

 retrograde evolution of the Rotatoria from Crustacea (though 

 it is then hard to explain the water-vascular system of the 

 Rotatoria), or other hypotheses might be framed by regarding 

 the median unpaired dorsal and ventral limbs, as potentially 

 lateral and duplicate, because the ' calcar' in some Rotatoria 

 is median and azygos and, in others, is paired. As matters at 

 present stand we really have no criterion in most cases where 

 speculation as to genealogy has of late been applied, and can 

 only attach the value of suggestions to such exercises of inge- 

 nuity which are, nevertheless, not to be condemned as illegiti- 

 mate. It will be interesting to see how Pedalion Avill be dealt 

 with by those naturalists who are constructing genealogies. 



The two caudal ciliated lappets of Pedalion are very 

 remarkable, not as indicating any affinities, but as being 

 quite excejDtional amongst Rotatoria. No other Rotifer pos- 

 sesses externally placed cilia in addition to those of the 

 cephalic region. 



Dr. Hudson does not mention the presence of the water- 

 vascular system, and 1 may say that I failed to detect any 

 indication of it — even when working with a No. 10 a im- 

 mersion of Hartnack.^ It is very easy to miss such a struc- 



' In a letter Dr. Hudson states that, last year, in one specimen he dc- 



