798 MICROSCOPIC FORMS OF ANIMAL LIFE 



to such an extent that they are scarcely, or not at all, visible 

 externally, as is the case in Cristellaria (fig. 17), Polystomella (fig. 23), 

 and Nonwnina. The turbinoid spire may coil so rapidly round an 

 elongated axis that the number of chambers in each turn is very 

 small; thus in Globigerina (figs. 20, 21, Plate XIX) there are 

 usually only four; and in Valvulina the regular number is only 

 three. Thus we are led to the ftiaeriaZ arrangement of the chambers, 

 which is characteristic of the textularian group (fig. 8, a, 6, and 9. 

 Plate XVIII), in which we find the chambers arranged in two rows, 

 each chamber communicating with that above and that below ii 

 on the opposite side, without any direct communication with the 

 chamber of its own side, as will be understood by reference to fig 



FIG. 607. Discorbina globularis (RosaU)ia varians, Schultze), - 

 with its pseudopodia extended. 



622, A, which shows a ' cast ' of the sarcode-body of the animal. On 

 the other hand, we find in the nautiloid spire a tendency to pass 

 (by a curious transitional form to be presently described) into the 

 cyclical mode of growth ; in which the original segment, instead of 

 budding forth on one side only, developes gemmae all round, so that 

 a ling of small chambers (or chamberlets) is formed around the 

 primordial chamber, and this in its turn surrounds itself after the 

 like fashion with another ring ; and by successive repetitions of the 

 same process the shell comes to have the form of a disc made up of 

 a great number of concentric rings, as we see in Orbitolites (fig. 609) 

 and in Uydodypeus (fig. 627). 



These and other differences in the plan of growth were made by 



