114 PROTOZOA. 



vitreous types, while in the latter are comprised the " porcellanous," 

 " arenaceous," and " chitinous " types. It has been shown, however, 

 that this division is not strictly natural, since various arenaceous 

 types possess a test which is more or less extensively porous. It 

 would appear, in fact, that the composition of the shell is liable to 

 variation, in accordance with the nature of the material obtainable 

 by the organism at any particular station, and that it is therefore 

 possible to attach too great value to this character in framing a 

 classification of the Foraminifera. 



Again, as regards the form of the shell, great differences exist 

 among the Foraminifera, and as concerns the mere external con- 

 figuration, this is so variable that little or no value can be attached 

 to it in classification. Moreover, in the two great series of the 

 Perforate and the Imperforate Foraminifera it is common to find 

 parallel or " isomorphic " groups. That is to say, we meet with two 

 series of forms, repeating each other's peculiarities and variations in 

 form, but the shell in the one series being perforate, while in the 

 other it is imperforate. 



The simplest form among the Foraminifera is that of a single 



spheroid of sarcode, capable of secreting for itself a hard covering, 



as in the flask-shaped Lagena (fig. 29,/) or the 



globular Orbulina (fig. 22). Forms such as these 



are said to be "unilocular" or " monothalamous," 



the test consisting of but a single chamber, not 



subdivided by partitions or " septa." In the more 



complex Foraminifera, the sarcode of the body 



Fig. 27~^0rbuiina undergoes a subdivision into partially separated 



universa. a simple segments, produced by constrictions in the srow- 



Foraminifer from the . ° , , J . . . ° . 



Pliocene strata (Sub- mg protoplasm, and each 01 these segments be- 

 naiy. nin (D'OrHgny)! f comes more or less completely divided off from its 

 neighbours, or enclosed by a wall of shell. In 

 these " multilocular " or " polythalamous " Foraminifera, therefore, 

 the shell ultimately comes to consist of a series of chambers, 

 separated by partitions of the test, and filled with sarcode. The 

 partitions, or " septa," between the different chambers, are, however, 

 perforated by one or more apertures, through which pass connecting 

 bands, or " stolons," of sarcode ; so that the sarcode occupying the 

 different chambers is united into a continuous and organic whole. 

 Each segment may give out its own pseudopodia through perfora- 

 tions in its investing wall (fig. 18), or the pseudopodia may be 

 simply emitted from the mouth of the shell by the last segment 

 only. In any case, the direction in which the segments are devel- 

 oped is governed by a determinate law, and differs in different 

 species, the form ultimately assumed by the shell depending wholly 

 upon this. The forms assumed by the shells of Foraminifera are, 



