Ml P.IMTISH FRESHWATER HEL10ZOA. 



silicious scales arranged tangentially and the bases of 

 radial silicious spines of various forms ; no mucilaginous 

 investment present ; nucleus and endoplasm placed 

 eccentrically. 



There are eight species belonging to this genus that have 

 not been recorded from the British Isles ; the absence of any 

 record of A. aculeata from our fauna is certainly remarkable 

 as it is oue of the commonest species on the continent of Europe. 

 A brief description of these eight species is here given. 



A. aculeata H. & L. (A. flava Greeff). Envelope of firm 

 consistency, formed of several layers composed of the bases 

 of the radial spines and tangentially arranged scales; the 

 radial spines stout, tapering, sharply pointed, straight or 

 slightly curved, the bases enlarged and nail-headed in form ; 

 the tangential scales of various irregular forms ; plasma grey 

 in colour; contractile vesicle single; pseudopodia tenuous, 

 long, granuliferous. Diameter of envelope 35 /u to 40 ^ ; radial 

 spines about one-third of the diameter of the envelope in length. 



A. mimetica Penard. Body small ; the envelope composed 

 of tangentially arranged scales and the bases of the radial 

 spines; the radial spines are truncate with bases pin-head in 

 form and are invisible in the living animal ; zoochlorella? 

 often present in the plasma ; pseudopodia very long, tenuous, 

 retractile and granuliferous. Diameter of envelope 12 ju to 

 20 ju; length of radial spines 7 ju to 12 ,u. 



A. spinifera Greeif, Penard emendat. This is apparently 

 an autonomous species and quite distinct from the true A. 

 spinifera Greeff. The envelope is formed in the usual way ; 

 the radial spines are not tapering, are truncate and of two 

 different kinds, one longer than the other. Diameter of 

 envelope 40 /j. to 50 n ; the length of the longer spines equal 

 to-about three-quarters of the diameter of the envelope and 

 that of the shorter spines equal to about one-third the diameter. 



A. rubella Penard. The envelope formed in the usual way ; 

 the radial spines are long, tenuous, pointed and invisible in 

 the living animal ; they normally form the axes for the pseudo- 

 podia; the plasma reddish in colour; no contractile vesicle 

 observed. Diameter 23 /m to 27 p. 



A. longiseta Penard. Envelope rather thick, of the usual 

 structure; the radial spines tapering but not sharply pointed, 

 about two-thirds of the diameter of the envelope in length ; 

 contractile vesicle usually absent. Diameter of envelope 

 about 40 fj.. 



A. pantopoda Penard. Body small; envelope 'formed of 



