THE ANIMAL KINGDOM. 



ment of short spines set on a thin membrane, rounded gaps here and there 

 giving exit to the pseudopodia. The marine Amoeba obtecta inhabits a 

 cup-shaped gelatinous theca. The test of those Amoebina which possess 

 one is always monothalamous, when chitinoid either colourless, yellow or 

 brown, according to thickness. It has a single aperture, except in three 

 genera, Diplophrys, Ditrema and Amphitrema^ with two at opposite poles. 

 Pores at the base of the test for the escape of water have been observed in 

 two instances, in Hyalosphenia papilio and Nebela bursella. The test is 

 cup-shaped in Cochliopodium, more or less hemispherical, with the aperture 

 limited by an inwardly turned rim in Arcella, usually globular, oval or 

 pyriform. Owing to compression, or to obliquity of the mouth, it may 

 become bilateral ; in Difflugia spiralis and Pkurophrys (= P seitdodif- 

 flugia?} Helix it is usually said to form a half-coil. As to the former, 

 however, Leidy states that it is retort-shaped, and an internal septum 

 stretched across the neck of the retort gives the appearance of a spiral 

 twist. The simple chitinoid shell may be very thin, flexible and closely 

 applied to the body (Pamphagus^ Diplophrys!}, thicker, but not rigid 

 (Cochliopodium\ stiff (Ditrema]^ and the body not in complete contact 

 with it (Hyalosphenia^ Platoum], as is nearly invariably the case in the 

 genera to be mentioned. In Quadrula it consists of transverse rows of 

 squarish chitinoid plates, and in Arcella of two membranes, an inner thin, 

 homogeneous, and an outer, composed of hexagonal prisms, placed ver- 

 tically, and filled with water. Difflugia, Amphitrema, &c., have foreign bodies 

 of very various kinds (diatom-frustules, sand-grains, &c.) attached to the 

 outer surface of the chitinoid membrane, or held together by a cement, 

 either gelatinous or siliceous (?). In Euglypha and its allies the plates 

 composing the test appear to be silicified ; they are probably held together 

 by a chitinoid cement. The plates of the row surrounding the mouth are 

 pointed and serrated, and that aperture has consequently a margin strongly 

 denticulated in Euglypha ; it is beaded in Trinema, but as a rule it is 

 smooth. The remaining plates of the test are oval, or hexagonal, disposed 

 in regular longitudinal lines and sometimes overlapping, exceedingly 

 minute in Cyphgderia, and implanted in a chitinoid membrane 1 . The 

 test is sometimes furnished with processes, or tubercles, with a crest or 

 with spines, coarse or fine, and in Placocista moveable. 



The protoplasm varies much in density in the naked Amoebina or 

 Nuda. When it is fluid the granules, &c., which it contains are distributed 



* Nebela (Leidy) is remarkable for the very varied shape of the siliceous elements of its test : 

 they are round or oval discs, or rod-like bodies, varying in length and size, sometimes intermingled, 

 imbedded in a chitinoid membrane (Taranek). They were supposed by Wallich (A. N. H. (3), xiii. 

 pp. 233-4) to be derived possibly from metamorphosed diatom-frustules. The cementing material 

 between the sand-grains, &c. of the test in Diffltigia sometimes stains readily. There appea'rs to be 

 a diversity of opinion as to the chitinoid or siliceous nature of the plates in some instances, e. g. in 

 Quadrula. 



