SECT. n. STRUCTURE OF FORAMINIFERA. 29 



added to it, each chamber being in continuity with its 

 predecessor, so that the whole shell will be straight 

 and rod-like, the last opening being the mouth. If the 

 original shell be globular, and if all the successive 

 gemmse given out be equal and globular, the shell 

 covering and uniting them will be like a number of 

 beads strung upon a straight wire. Sometimes the 

 successive gemmse increase in size so that each chamber 

 is larger than the one which precedes it ; in this case 

 the compound shell will have a conical form, the pri- 

 mary shell being the apex, and the base the last formed, 

 the aperture of which is the mouth of the whole shell ; a 

 great many Foraminifera have this structure. The 

 spiral form is very common and much varied. A series 

 of chambers increasing in size may coil round a longi- 

 tudinal axis, like the shell of the snail ; but if each of 

 the successive chambers, instead of being developed 

 exactly in the axis of its predecessor, should be directed 

 a little to one side, a curved instead of a straight axis 

 would be the result; there is a regular gradation of 

 forms of Foraminifera between these two types. The 

 convolutions are frequently flat and in one plane, but 

 the character of the spiral depends upon the successive 

 enlargement or not of the consecutive chambers ; for 

 when they open very wide and increase in breadth, 

 every whorl is larger than that which it surrounds ; but 

 more commonly there is so little difference between the 

 segments after the spiral has made two or three turns, 

 that the breadth of each whorl scarcely exceeds that 

 which precedes it. 



However varied the forms may be, the mouth of the 

 last shell is the mouth of the whole, either for the 

 time being or finally. For all the chambers are con- 

 nected by narrow apertures in the partitions between 

 them. Each chamber is occupied by a segment of 

 the gelatinous sarcode body of the animal, and all the 



