164 



On The Rotifers, by Dark Field Illumination. 

 By C. T. Hudson, M.A., LL.D., &c. 



At the ordinary meeting, on February 28th, Dr. Hudson 

 exhibited, in the Mathematical Theatre of University College, a 

 large number of very beautiful transparent drawings, . which 

 being strongly illuminated from behind by lamps placed in a 

 closed box, gave most faithful representations of the various rotifers 

 as they appear under the microscope when illuminated with the 

 paraboloid. The drawings were fully described, and the following 

 is an abstract of Dr. Hudson's remarks. 



Dr. Hudson commenced his observations by remarking upon the 

 difficulties which beset any one at the outset who attempted to give 

 a true definition of what a Rotifer was. This class of creatures 

 had been variously described in different text books, but there had 

 been no definition yet produced which would not be at once torn to 

 pieces by the critic who attempted to make it apply to the whole 

 group ; for though it was easy to describe typical forms, the 

 further one got from the central figure the more the definition had 

 to be shorn of its positive assertions, until so many of the leading 

 features were at length dropped that the only part of the original 

 definition remaining might be that of " small creatures which laid 

 eggs ;" a distinction which, however, equally applied to hens and 

 snakes. Instead, therefore, of attempting any universally appli- 

 cable description, he proposed to show them a diagram of a typical 

 Rotifer, and to explain briefly its distinctive structure. The room 

 was then darkened, and Dr. Hudson proceeded to show as trans- 

 parencies a series of admirably executed coloured diagrams as 

 follows : — (1) Brachionus pala, as a typical form of Rotifer, 

 showing cilia, water vascular system, alimentary system, repro- 

 ductive organs, &c. In connection with this, it was pointed out 

 that the whole family might be grouped into — (a) Those that swim 

 and creep ; (&) those that swim only ; (c) those that are rooted for 

 life to one spot ; (d) those that both skip and swim. This plan 



