52 THE FORAMIN1FERA 



The nuclear characters and modes of reproduction of the Foramini- 

 fera are considered below. 



The Structure of the Test. The test is found in its most rudiment- 

 ary condition in Myxotheca, where it consists of a gelatinous layer, 

 which may form the whole covering or may contain grains of sand. 

 The shape follows the changing contour of the protoplasm, the 

 pseudopodia break through at any point, and no definite and 

 permanent orifice is formed. In Hyalopus (Fig. 15) the test is 

 more resistent, and may have an oval form and a definite orifice ; 

 but here again the shape varies with that of the contained proto- 

 plasm, becoming arborescent when growing among the crowded 

 stems of algae, and the number of orifices may be indefinitely in- 

 creased (Schaudinn, 43). In most of the Gromiidea the test is 

 chitinous and flexible ; but it has a definite shape, and one or two 

 permanent orifices. 



Another type of test is found in some Gromiidea, and in all the 

 Astrorhizidea and Lituolidea. The tests are here formed of foreign 

 particles, such as fragments of sand, sponge-spicules, the shells of 

 other Foraminifera, etc., fastened together by a cement, which may 

 be firm or flexible, and consist of chitin or calcium carbonate or 

 ferric oxide. In the Astrorhizidae the walls are thick and soft, con- 

 sisting of mud, or of only slightly cemented sand (Fig. 17, a), while 

 in other cases, as in Saccammina (Fig. 17, &), the particles are 

 united into a rigid structure. 



In some species of Textularia, Quinqueloculina, and other genera, 

 though the tests are chiefly calcareous, a large proportion of foreign 

 arenaceous material is contained in them. 



A very remarkable feature of the tests of arenaceous Foramini- 

 fera is the evidence they appear to offer of a selective power exercised 

 by certain species, in collecting materials. In some cases, no 

 doubt, the nature of the test depends on the constituents of 

 the sea bottom in which the animal lives, but in others certain 

 elements alone are selected. Thus in the same dredgings may be 

 found the tests of Pilulina and Saccammina, the former composed 

 of a close felt of siliceous sponge-spicules, laid together to form a 

 wall of uniform thickness ; the latter of coarse grains of sand united 

 by cement (Fig. 17, b and c). In both the test consists of a single 

 spherical chamber, and the size attained is about the same in both. 

 The cylindrical tests of Bathysiphon filiformis, Sars, are composed of 

 a felt of sponge-spicules, covered externally by a layer of fine sand. 

 The same sample of Pteropod ooze supplies representatives of 

 species of the family Lituolidae, characterised by the coarseness of 

 the sand grains of which the tests are composed ; and of Trochammi- 

 nina distinguished by fine-grained tests. 



The type of test found in Euglypha (Fig. 3) and its allies is very 

 exceptional. It is formed of rounded or hexagonal plates, of siliceous 



