OF THE AECELLINA. 



555 



A. vulgaris and other species a contractile vesicle has been perceived. The 

 processes are longer, as a rule, than those of Difflugia, fibrous, and more 

 branched. The shells are very commonly compressed, and have a discoid 

 figure ; and in none are they soft and beset with extraneous particles, as in 

 Difflugia, but are chitinous and elastic. 



" The Arcellce (says Dujardin) seem to differ among themselves by the 

 intimate structiu'e of their lorica, which sometimes appears membranous, 

 at others finely striated, reticular, or with granules disposed in spiral lines. 

 Some Arcellce have also spinous prolongations from the border of their lorica. 

 Pressure fractures their lorica like a brittle substance. The contained sub- 

 stance escapes through the cracks so formed, in the fonn of contractile expan- 

 sions like those of Amoehce. I have seen one larger lobe almost separated, 

 as if about to become an independent being. M. Peltier has observed con- 

 tact to take place between the expan.sions of neighbouiing Arcellce without 

 any union being effected, while the processes of the same Arcellce united and 

 became blended together. 



" The lorica in young Arcellce is extremely diaphanous ; and granulations 

 or striae are to be seen only in those of larger size : hence it may happen, with 

 respect to some species, that they represent but different stages of existence 

 of the same animal." 



Ajicella vulgaris (xxi. 7, 8, 9). — 

 Lorica roimd and bell-shaped, with a 

 hemispherical or turgid back; smooth, 

 but with rows of minute gi'anules ; colour 

 yellow or ^eddish-bro^\^l. Abimdant 

 amongst Lemnae and aquatic plants. 

 1-570" to 1-240."^ 



A. aculeata. — Yellowish, hemisperical, 

 though often mis-shapen, and spinous 

 throughout, or only around one-half of 

 the margin ; the shell is not readily de- 

 stroyed by heat, and is covered with 

 short spiciila. 1-210". 



A, dentata. — Membranous ; of a he- 



mispherical or polygonal form -, margin 

 dentated; colour yellow or green. 

 Amongst Conferva. 1-570" to 1-240". 



A. (?) hyalina. — Membranous, smooth, 

 elliptical or globular, smaller than the 

 preceding, thin and soft, colourless. 

 Found in debris at the bottom of pond- 

 water, along with Cypliidium aureolum, 

 &c. 1-1150 to 1-570". The sheU is 

 not quite symmetrical, one side being 

 more convex than the other. Apertm-e 

 sometimes iiTegular. Ehrenberg was not 

 certain that this species is not a Difflugia. 

 It is indeed very like many specimens of 

 D. Enchelys. 



A. Americana. — Oblong ; aperture 

 small, round, not in the median line. 



A. comtricta. — Ovate; slightly con- 

 tracted about the foramen, which'is ver}^ 

 large and to one side. 



^ A. clisphcsra. — Oblong, almost di- 

 vided into two by a central constric- 



tion 



large foramen. 



one-half nearly occupied by the 

 This is a veiy doubtful 

 Arcella, and contrary in form to the 

 character of monothalamous cells. A 

 comparison of Ehreuberg's account with 

 his figures leads us to believe this sup- 

 posed species to be no other than a young 

 Rotalia of two cells (xx. 41), or other 

 incomplete pol}i:halanious shell. 



A. ecornis. — Large; hemispherical, not 

 areolar ; apertm-e roimd, large, placed to 

 one side ; entire. 



A. lunata. — Subglobose, large ; with 

 a wide semi-lunar opening, seated to one 

 side. 



A. Nidus-penchdm. — Ovate-oblong, 

 hyaline, loosely areolated; aperture in 

 front, oblong, margin entire. 



A. Pileus. — Hemispherical, depressed, 

 reddish, minutely and elegantly areolar ; 

 aperture central, circidar. 



A. ? Globulus. — Subglobose ; with 

 loosely reticular lines, appearing granu- 

 lar; aperture large, simple. 1-730". On 

 moss at Berlin, Potsdam, kc. 



A. granulata. — Oblong, hyaline. Has 

 the habitat and size of A. hyalina, with a 

 granular instead of a smooth surface. 

 1-940". On moss in Hercynia, &c. 



A. caudicola. — Ovate, oblong, rounded 

 at each end, hyaline, very delicately 

 hispid, not areolar; aperture anterior, 

 round, large. 1-840". Habitat of A. 



Nidus-pendulus. In Venezuela, on roots 

 of plants, such as ferns, &c. 

 A. Okenii (Perty) (xxi. 15). 



Genus CYPHIDIUM (XXII. 24-27).— Has only one dilated variable 

 process, and a lorica of the form of a pitcher, with protuberances issuing 



