48 MANUAL OF ZOOLOGY. 



the permanent condition of a simple species. They commence 

 their existence, therefore, as Monothalamia, and are converted 

 into Polythalamia merely by a process of ' vegetative' or ' irre- 

 lative repetition.' As their development proceeds, the primitive 



fe' 



w v 



a b e d 



^ugosa. b. A*. 

 tpidi 



Pig. 5. Polythalamous Foranrunifei'a. a. Nodosaria rugosa. b. A*, longisiikata. 

 c. A\ spinicosta. d, A*, hispida. 



mass of sarcode, or ' primordial segment,' throws out fresh 

 segments in the form of buds according to a determinate 

 law ; and it is upon the direction in which these segments are 

 evolved that the ultimate form of the shell depends. The 

 more important variations in this respect are as follows : If 

 the additional segments are added to the primordial chamber 

 in a linear series, so as to form a straight or slightly curved 

 line, we obtain respectively &Nodosaria (fig. 5) or a, Dentalina. 

 When the new chambers are added in a spiral direction, each 

 being a little larger than the one which preceded it, and the 

 coils of the spiral lying in one plane, then we get the ' nauti- 

 loid' shell, so. common amongst the Foramimfera (fig. 6 a). 



a 



Fir. 6. a. Robulina echinata, a ' nantiloid Foramimfei-.' &. Globigerina bnlloides, 



This type of shell is so closely similar to the shape of the pearly 

 nautilus, that the older naturalists were long in the habit of 

 classing these forms along with the Cephalopoda, or Cuttle- 



