113 



XII. 



OBSERYATIOXS ON ROTIFERS, WITH STECIAL REFERENCE TO 

 THOSE FOUND IN THE NEIGHBOURHOOD OF HERTFORD. 



By F. W. Phillips. 



Head at Hertford, 23rd March, 1880. 

 Plate II. 



A RoTiFEK may be briefly described as an animalcule, having one 

 or more lobes at the anterior extremity of the body, which bear 

 upon their edge a row of long, vibratile cilia, which, when in 

 motion, appear, by an optical illusion, like revolving cog-wheels, 

 and from these most striking and peculiar organs the class derives 

 its name, Eotifera, or wheel-animalcules. 



Of all the classes of animalcules, the Rotifera, or rotifers as they 

 are more commonly called, have ever excited the greatest interest 

 and wonder in the minds of both learned and unlearned. Much 

 discussion has arisen at vai'ious times as to what place they should 

 occupy in the animal kingdom ; their affinities being divided between 

 the classes Crustacea and Annelida, but perhaps the greater part of 

 tlieir characteristics leans towards the worm side of the question. 



They are very small, the maximum size which they attain being 

 about -3-0 th of an inch, and some of them are as small as ^i^o^th of 

 an inch. Yet the majority may be easily distinguished by the aid 

 of a pocket lens, sometimes sailing through the water with an easy 

 rolling motion, like a small boat upon the waves, sometimes quietly 

 browsing amongst the green threads of the Algae, or else attached 

 by the tail to a p»iece of weed. 



For the sake of convenience we will divide the rotifers into two 

 groups, viz. those which attach themselves by the foot to a weed 

 and live in a case or tube, and those which habitually swim freely 

 through the water ; this latter division comprising by far the larger 

 number. 



The number of species is, however, so great, that in giving an 

 anatomical sketch, we must confine ourselves to one rotifer belong- 

 ing to the latter class, which we will take as a type of the others, 

 viz. Rotifer vulgaris. First of all I must call your attention to the 

 so-called "wheels," which consist of two disc-like lobes or pro- 

 jections furnished all round the margins with long cilia, which move 

 in such rapid and well-timed succession as to appear like revolving 

 wheels ; and so perfect is this illusion that you cannot but feel 

 how appropriate is their name. The cause of this illusion is that 

 the forward stroke of each cilium is so quick that it cannot be per- 

 ceived, while the backward stroke is much slower, and therefore 

 perceptible. Other opinions on the subject have been given by 

 Dujardin, Dutrochet, and Ehrenberg ; but I am inclined to accept 

 the theory of Faraday which I have given you, because I have re- 

 peatedly observed this motion with a high power and by the aid of 



VOL. I. — PART III. 8 



