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XL — A Nci'j Type of Infusorian : Arachnidiopsis paradoxa. 



By E. Penard, Sc.D. 



{Read June 19, 1918.) 

 One Plate. 



Though our title indicates a type of Infusorian yet unknown, it is 

 quite probable that we owe to Saville Kent the first notice about 

 the very interesting group of fresh-water Protozoa of which we are 

 going to treat. 



On page 637 of his " Manual of Infusoria," that author 

 describes in the following terms the genus which he created for this 

 small group : — 



" Arachnid i am, gen. Tiov. Animalcules free-swimming, ovate 

 or spherical ; oral aperture terminal, central, surrounded by a 

 circular wreath of large, flexible, tentaculiform cilia, which consti- 

 tute the only locomotive or prehensile organs ; endoplast and 

 contractile vesicle conspicuously developed. Inhabiting salt and 

 fresh water. 



"This new genus holds a position between Mesodinium and 

 Sti'omhidium, in having the central mouth and even oral circle of 

 cilia characteristic of the former, but wanting, as with the latter, 

 its special supplementary leaping setse. This single oral wreath 

 of cilia at the same time attains a much greater development, and. 

 assumes a character completely distinct from either of the above- 

 named genera ; these cilia, indeed, resemble rather small flexible 

 tentacula than the ordinary cilia or setae of the normal representa- 

 tives of this group. In their more ordinary condition these organs 

 are recurved gracefully towards the posterior extremity of the 

 body, and when in active use exhibit a perfectly independent 

 motion. The spider-like aspect of the animalcules of this genus, 

 with their rounded bodies and straggling tentacle-like cilia, has 

 suggested the title for their distinction. More correctly they may 

 perhaps be likened to the i'ree-swimming Coelenterate genus 

 Arachnachtis." 



Kent then describes in this genus Arachnidium two species : 

 A. globosum (PI. 32, figs. 48 and 49), which he found, in 1874, in 

 a pond at Stoke Newington, represented by a few examples only ; 

 and A. convoluhtm (PI. 32, fig. 41), a single specimen of which was 

 met with at Bognor, on the Sussex coast, in September, 1872. 



At the same time Kent refers to this new genus, as a third species, 



u 2 



