CHAPTER XI. 



Order IV. SCIRTOPODA. 



Swimmlnrj ivllh their ciliarij wreath, and shipijimj with Artliropodous limbs; foot 

 absent. 



The fourth order, Scirtopoda, although it contains but two Rotifera, each in a genus 

 by itself, is one of no little importance, as it is that in which the Rotifeka and Crus- 

 tacea most nearly touch. The true position of the Rotifeea in the animal kingdom 

 has long been a matter of keen dispute, and the chief authorities have taken opposite 

 sides : Professor Owen, Dr. Leydig, and others, rank them among Crustacea ' ; while 

 M. Milne-Edwards, Dr. Colin, Professor Huxley, with the majority, would consider them 

 as VEEMES. Nor is this surprising ; for the Rotifeka possess many characters that are 

 common, in various degrees, to ARTHROrODA and VERMES alike ; and, so far as their nutri- 

 tive, reproductive, or nervous systems are concerned, might with little difficulty be placed 

 in either sub-kingdom. But there were three respects, before the discovery of Pedalioii and 

 Hexarthra, in which the Rotifeea appeared to differ from ARTHROPODA, and to resemble 

 VEEMES. These are, first, that they do not possess pairs of jointed appendages, articu- 

 lated to the body, with muscles prolonged into their interior ; and on this point great 

 stress was laid. Secondly, that they swim by means of ciliary wreaths ; and thirdly, 

 that they possess a vascular system, with ciliated tags, whose chief function is probably 

 a respiratory one. 



But the discoveries oi Pedalion and Hexarthra have shewn that Rotifera exist whose 

 internal structure is perfectly normal, and which yet possess three pairs of unquestion- 

 ably Artliropodous limbs ; and these discoveries have in consequence disposed of the chief 

 objection to the ranking of the Rotifeea among ARTHROPODA. It may, on the other hand, 

 be fairly urged that the balance of argument even now inclines towards those who are 

 in favour of the opinion that the Rotifeea, as a class, are nearer to VEEMES : yet no 

 one, I think, who has studied both Pedalion and the NaupHus larva of one of our 

 fresh-water Entomosteaca, would feel satisfied with their being placed in two distinct 

 sub -kingdoms. 



In conclusion, I see no reason why the Rotifeea should be assigned solely to VEEMES 

 or ARTHROPODA ; and I would propose to consider them as a class that links these two 

 sub-kingdoms together.^ 



Family XX. PEDALIONID^. 



Arthropodous limbs six ; head truncate; corona of two concave lobes ; ciliary wreath 

 as in Philodinadm ; trophi malleo-ramate. 



The family contains two genera, Pedalion and Hexarthra, each containing only 

 a single species. The two are much alike '■' in the possession of six Arthropodous limbs 



' [I consider the Rotifeka a class of the sub-kingdom ARTHEOPODA, co-equal in rank with 

 Insecta and Crustacea. — P.H.G.] 



- Of course I am here treating the matter simply as one of formal classification ; and from this 

 point of view it is enough to say that if we knew none but the humbler forms of the Eotifeea, we should 

 call them VERMES ; whereas if Pedalion and Mcvarthra were our only examples, we should call them 

 the lowest forms of ARTHROPODA. 



^ Mr. Oulien Deby, in the /. Roy. Micr. Soc. lS7fl, p. 381, has suRgestod that the two Rotifera are 



K 2 



