66 PROTOZOA chap. 



cases the part of the previously existing chamber next the 

 pylome serves as the hinder part of the new chamber, and the 

 old pylome becomes the pore of communication. But in some of 

 the " Perforata " each new chamber forms a complete wall of its 

 own (" proper wall," Fig. 13, 8b), and the space between the two 

 adjacent walls is filled with an intermediate layer traversed by 

 canals communicating with the cavities of the chambers 

 (" intermediate skeleton "), while an external layer of the same 

 character may form a continuous covering. The shell of the 

 Perforata may be adorned with pittings or fine spines, which 

 serve to increase the surface of support in such floating forms as 

 Globigerina, Hastigerina, and the like (Fig. 17). In the 

 " Imperforata " the outer layer is often ornamented with regular 

 patterns of pits, prominences, etc., which are probably formed 

 by a thin reflected external layer of protoplasm. In some of 

 the " Arenacea " a " labyrinthine " complex of laminae is formed. 

 A very remarkable point which has led to great confusion 

 in the study of the Foraminifera, is the fact that the shell on 

 which we base our characters of classification, may vary very 

 much, even within the same individual. Thus in the genus 

 Orbitolites the first few chambers of the shell have the character 

 of a Milioline, in Orbiculina of a Peneroplis. The arrangements 

 of the Milioline shell, known as Triloculine, Quinqueloculine, and 

 Biloculine respectively, may succeed one another in the same 

 shell (Figs. 14 4, 15). A shell may begin as a spiral and end 

 by a straight continuation : again, the spherical Orbulina 

 (Fig. 16 l) is formed as an investment to a shell indistinguish- 

 able from Globigerina, which is ultimately absorbed. In 

 some cases, as Ehumbler has pointed out, the more recent 

 and higher development shows itself in the first formed 

 chambers, while the later, younger chambers remain at a lowlier 

 stage, as in the case of the spiral passing into a straight 

 succession ; but the other cases we have cited show that this is 

 not always the case. In Zagena (Fig. 13 2) the pylome is pro- 

 duced into a short tube, which may protrude from the shell or be 

 turned into it, so that for the latter form the genus Entosolenia was 

 founded. Shells identical in minute sculpture are, however, found 

 with either form of neck, and, moreover, the polythalamial shells 

 (Nodosaria, Fig. 13 3), formed of a nearly straight succession of 

 Zagena-like chambers, may have these chambers with their com- 



