86 



THE FORAMINIFERA 



with a chitinous envelope, together with siliceous particles pre- 

 viously taken into the protoplasm. 



ORDER Miliolidea. 



On coming to the Miliolidea we have a large body of evidence 

 on dimorphism, thanks in great measure to the careful investiga- 

 tions of Schlumberger, by whom, either alone or in conjunction 

 with Munier-Chalmas, the foundations of our knowledge on the 

 dimorphism of the tests of Foraminifera have been laid. The tests 

 will first be described, the nuclear characters and such details of 

 the life-history as are to hand being given at the end. 



Family Miliolinidae. Before considering the phenomena of 

 dimorphism in this family, it is necessary to describe the character- 

 istic structure of the test in certain forms. 



FIG. 20. 



Comuspira involvens, Keuss. a, the megalospheric 

 form, x 90. b, the microspheric form, x 50. (From 

 Brady, Parker, and Jones, Trans. Zool. Soc. vol. xii. PI. 

 40, Figs. 1 and 2.) 



FIG. 21. 



Splroloculina limbata, d'Orb., 

 x 30. (After Brady.) 



The simplest type is met with in Comuspira. 



The whole of the test except the central chamber (which pre- 

 sents a well-marked difference in size in the two forms, Fig. 20) 

 consists of a continuous tube, gradually increasing in diameter as 

 it is followed away from the centre, but without any constrictions 

 dividing it into separate chambers. In both forms it is disposed 

 in a closely-wound spiral lying in one plane, so that a section in 

 this plane would divide the test symmetrically. 



In the genus Spiroloculina the arrangement is somewhat similar, 

 but here the tube is divided into distinct chambers, each of which 

 ends in a contracted mouth with an everted lip. The chambers 

 increase successively in length, and are so disposed that each 

 occupies half a turn of the spiral. It results from this arrange- 

 ment that the mouths of the chambers are directed alternately in 

 opposite directions, and each chamber is applied to that which is 

 next but one before it in the series. A straight line, which passes 



