206 ANIMAL PEDIGREES 



the chambers in communication with one another ; 

 and the outermost or marginal chamber communi- 

 cates with the outer world through a series of holes 

 round the edge or rim of the disc. During life all 

 the cavities are filled with a slimy protoplasm, very 

 similar to that of which an Amoeba consists ; 

 through the perforations in the septa the protoplasm 

 of one chamber communicates freely with that of 

 the neighbouring chambers ; and through the mar- 

 ginal apertures at the rim of the shell pseudopodia 

 can be protruded and food captured and digested. 



An Orbitolite grows by addition of new chambers 

 round the margin of the shell. The protoplasm, 

 becoming too abundant to be contained within the 

 cavities of the shell, protrudes as a rim all round 

 the margin of the shell, and by deposition of 

 calcareous matter gives rise to successive new 

 chambers to the shell. 



The discoidal shape of shell, so characteristic of 

 Orbitolites, is very unusual amongst Foraminifera, 

 and it is a matter of great interest to determine in 

 what way it has been acquired. Bearing in mind 

 what has just been said as to the mode of growth 

 of the shell, it is clear that the oldest part i.e., that 

 which alone was present in the young animal is 

 the central portion, and that the successive concen- 

 tric rings are younger and younger as we pass 

 outwards towards the circumference, the marginal 

 chamber being the youngest and latest formed of 

 the whole series. Now if we look at the central 

 or oldest part of an Orbitolite we find that it has 

 not the concentric arrangement of the peripheral 



