RECENT MEMOIRS ON FRESHWATER RHIZOPODA. 67 



Resume of Recent Contributions to our Knowledge 

 of " Freshwater RnizoroDA." Part III. Heliozoa 

 {Desmothoraca) and Monotpialamia {Monostomata) . 

 Compiled by W. Archer, F.R.S. 



Desmothoraca, Hertvvig and Lesser. 



In this group tlie skeleton is formed of one single solid 

 piece enclosing an inner cavity and with peripheral openings ; 

 at the same time it is distinguished from the single-chambered 

 test of the Monothalmia by its homaxial fundamental form. 



There is not any ground to regard the Desmothoracous 

 skeleton to be, as it were, composed of individual pieces once 

 isolated and then fused into one hollow-globular entirety, 

 but rather it must have been so ab initio, and in it the irregu- 

 larly arranged apertures must have been preformed. Both 

 the known forms (one of Avhich is now for the first time 

 described by Hertwig and Lesser) appertaining here possess 

 a more or less elongate and slender stipes, on the upper end 

 of which is borne the hollow perforate skeleton containing 

 the living body-mass, and the lower end of which is attached 

 to some external support. 



Hedriocystis pellucida, Hertwig and Lesser (this Journal, 

 vol. xvi (1876), PI. XXII, figs. 21 and 22), 



is characterised by the possession of a stipitate, single^ 

 cavitied skeleton. The stipes (0-05 — 0*075 mm. long) (fig. 

 21, s) is attached by its dilated base to algal filaments and 

 such like foreign bodies, whilst the upper end passes without 

 any sharp limitation into the contour of the skeleton (fig. 

 21, t). The latter is ovate-elongate and in the direction of 

 the stipes, like it delicate and pellucid, and so constituted 

 that its wall, at all the places through which the pseudopodia 

 pass, projects as so many pointed prominences, the whole 

 lending to it the aspect of a knobbed club with stellate pro- 

 jections. Except at the apices of these projections, which are 

 perforate and destined to allow the passage of the pseudo- 

 podia, the wall is otherwise closed. 



The formation of the skeleton begins with the production 

 of the stipes (fig. 22, s), thereupon succeeded by the deposi- 

 tion of the skeleton, and both by the agency of the protoplasm. 

 When fully formed the round body-mass is freely poised 

 within the cavity of the skeleton, as if suspended by the 

 pseudopodia. It consists of a finely granular protoplasm, 

 and contains an oval nucleus (with nucleolus) (figs. 21 and 



