Structure of Tents of Fresh-water Mhizopoda-. 109 



about 1 /j, to 2 fi in total thickness, a magnification of x 1500 or 

 more is necessary for theii examination, which presents a good 

 many technical difficulties. Cushman and Henderson* give micro- 

 photographs of Areella tests, together with a description of their 

 methods of procedure ; there is, however, room for further investi- 

 gation. The test of A. mitrata Leidy offers perhaps the least 

 difficulty in examination owing to its exceptional thickness and 

 large cells ; the species is, however, rare in Britain, although plen- 

 tiful in the United States. (See PL III, figs. 2-4; 



Centrofyxis aculeata (Ehrenb.) Stein has a stout chitinous test 

 more or less overlaid with particles of extraneous material. Under 

 a moderate magnification the envelope, where visible, appears 

 punctate, but these markings are not really perforations but small 

 plates or disks embedded in the chitin ; by treating the test with 

 a solvent, such as caustic potash of carefully regulated strength, 

 they might possibly be isolated and their exact nature ascertained. 

 These tests are furnished with a varying number of hollow chitinous 

 spines, the small extremities of which are frequently plugged by 

 minute splinters or pointed grains of silica, a peculiarity which is 

 even more pronounced in the case of the spines of Dijfiuyia echinu- 

 lata Penard (6), the only known species of that genus bearing 

 spines on its test. 



Many tests possess what appear to be pores, e.g. around the 

 apertures of several species of Arccllie and on the necks of some 

 Xebelm, but if the test be broken and examined in section it will 

 often be found that these " pores " are in reality minute hemi- 

 spherical nodules on either the outer or inner surface (PI. Ill, fig. 5) ; 

 true pores can be recognized as a rule in broken sections, or may 

 be revealed by minute bubbles of gas escaping through them when 

 the tests are treated with acid or otherwise manipulated. Trans- 

 ference from water to oil of cloves sometimes reveals them. 



Group 2 contains a large proportion of the fresh-water species 

 of Ehizopoda ; the tests are similar in principle to those of the 

 familiar caddis-worm, being constructed of particles of sand or 

 other material cemented on to a pellicle, and the form of the 

 finished structure is characteristic in each species. 



A drop of sediment from any pond or stream, or a squeezing 

 from a moist piece of moss, is nearly sure to provide examples of 

 one or more species of Dijflugia, which may be taken as typical of 

 this group ; they are easily kept alive for observation if supplied 

 with fresh water now and then, and with patience an observer may 

 have the opportunity of seeing the process of construction of a 

 new test ; in any case, if powdered glass of various colours be 

 provided, any new tests will be seen to have these coloured grains 

 incorporated in them. 



* Amer. Nat., xl. (1906). 



