222 GENEEAL HISTORY OF THE INFUSORIA. 



differences supply specific and generic characters of much vahie by reason of 

 their constancy. Where the mouth has an even uninterrupted margin, it is 

 said to be " entire." Its normal figure may be considered circular (XXI. 9). 

 However, in many instances it is irregular (XXI. 15), or a projecting portion 

 encroaches on it (XXI. 6). In Dijflugia depressa and I), gigantea it is uneven ; 

 in Arcella lunata, semilunar ; in Difiugia ampulla, ovate ; in Splienoderia, so 

 contracted as to be linear. Still more frequently the margin of the aperture is 

 dentated or spinous : examples occiu' in Difflugia denticulata, D. Jmvigata, D. 

 oligodon, D, acantlwpliora (XII. 64), and D. ciliata, in Arcella dentata and 

 in EughjpJia. The symmetrical position of the mouth is wanting in several 

 species ; and Schlumberger elevated this variation to the importance of a ge- 

 neric distinction. The obliquity of the aperture — its position out of the median 

 line — is noticed in Arcella Americana, A, constricta, A. ecornis, and in A. lu- 

 nata, also in the genus Trhiema (Duj.) and in Cijplioderia (Schlumberger). 

 WTien the mouth appears formed by the mere incompleteness of the outline of 

 the shell, and is without a neck or deep margin, it is often said to be trimcate 

 — in fact, the oral end of the shell is truncated or abruptly cut ofi" by the 

 orifice. 



The shells of A^xellina may be fractured by pressure when the contained 

 sarcode matter escapes through the fissures, extending itself in lobe-like pro- 

 longations, which take on the characters of ordinaiy expansions (XXI. 7). 

 Since the opacity of the shell is generally an impediment to the observation 

 of the contained matter, its ruptui-e by pressiu'e, or its partial solution by 

 some reagent, as sulphui'ic acid, which acts upon the chitinous basis, must 

 be resorted to in order to discover the nature of the animal mass within. 

 With or without such preparation, it is not unfrequently seen that the living 

 mass is not uniformly adherent to the inner surface of the shell, but is, on 

 the contrary, detached at different parts, leaving interspaces between it and 

 the testa, varying in size and number. These vacuities may possibly arise 

 from the detachment of the soft matter by reason of the quantity poinded out 

 fi'om the mouth of the shell, or other^vise from the formation of vacuoles at 

 those points, just as often happens on the surface of an Amceha. 



h. Shells of Polythalamia or Foraminifera. — These have a great diver- 

 sity in figure and size, and are often veiy beautifully coloured and sculptm^ed. 

 From the resemblance of many to the shells of Cephalopoda, especially to those 

 of Nautili (XXI. 28), they were for a long time ranged along -^ith those 

 highly- developed Mollusca. The shells of Polythalamia consist of a greater 

 or less number, according to age and species, of communicating chambers or 

 cells, aggregated together or superposed on one another in different ways, 

 the mode of disposition, however, varying within certain limits even in the 

 same species. Thus Dr. Carpenter, speaking of Orhitolites, says {Proceedings 

 Royal Society, 1855), — " Starting from the central nucleus, w^hich consists of a 

 pear-shaped mass of sarcode nearly surrounded by a larger mass connected 

 with it by a peduncle, the development may take place either on a simple 

 or upon a complex type. In the former (which is indicated by the circular 

 or oval fonns of the cells, which show themselves at the sui-face of the disk, 

 and by the singleness of the row of marginal pores), each zone consists of but 

 a single layer of segments, connected together by a single annular stolon of 

 sarcode, and the nucleus is connected with the first zone, and each with that 

 which siuTounds it, by radiating peduncles proceeding from this annulus, 

 which, when issuing from the peripheral zone, will pass outwards through the 

 marginal pores, probably in the form of pseudopodes. In the complex type, 

 on the other hand (which is indicated by the narrow and straight- sided form 

 of the supei-ficial cells and by the multiplication of the horizontal rows of 



