XII] OF THE FORAMINIFERA 861 



successive increment thereof, taken in connection with the fluid or 

 semi-fluid nature of the protoplasmic substance, is enough to suggest 

 that the molecular forces, and especially the force of surface-tension, 

 must exercise a controlling influence over the form of the whole 

 structure; and this suggestion, or behef, is already implied in our 

 statement that each successive increment of growing protoplasm 

 constitutes a separate drop. These "drops," partially concealed by 

 their successors, but still shewing in part their rounded outhnes, 

 are easily recognisable in the various foraminiferal shells which are 

 illustrated in this chapter. 



Fig. 428. Orbulina imiversa d'Orb. 



The accompanying figure represents, to begin with, the spherical 

 shell characteristic of the common, floating, oceanic Orbulina. In 

 the specimen illustrated, a second chamber, superadded to the first, 

 has arisen as a drop of protoplasm which exuded through the pores 

 of the first chamber, accumulated on its surface, and spread over 

 the latter till it came to rest in a position of equihbrium. We may 

 take it that this position of equilibrium is determined, at least in 

 the first instance, by the "law of the constant angle," which holds, 

 or tends to hold, in all cases where the free surface of a given Uquid 

 is in contact with a given sohd, in presence of another liquid or a gas. 

 The corresponding equations *are precisely the same as those which 

 we have used in discussing the form of a drop (on p. 466); though 

 some slight modification must be made in our definitions, inasmuch 

 as the consideration of snriace-tension is no longer appropriate at 

 the solid surfaces, and the concept of snviace-energy must take its 



