50 THE FORAMINIFERA 



surface, as though borne by a stream, and two streams of granules 

 flowing in opposite directions, centrifugal and centripetal, are to be 

 seen on opposite sides of the same filament. Sometimes a mass of 

 protoplasm forms a swelling on the surface, and is carried along for 

 some distance before it thins out and merges in the substance of the 

 filament. 



The tip of a pseudopodium may be seen to be alternately ex- 

 tended and retracted according as the centrifugal or centripetal 

 stream gains the ascendency. A mass more solid than the rest of 

 the protoplasm may be seen to be carried to the tip, turn and pass 

 back for some distance with the return current, then to be caught 

 in the centrifugal stream, and again carried to the tip. One is 

 reminded of a cork at the summit of a jet of water, under the 

 contending forces of the upward flow of the jet and of gravity. 

 But the cause of the streaming movement in the pseudopodia of 

 the Foraminifera, like the ultimate cause of movement in all 

 contractile tissues, is still beyond the limits of our knowledge. 



The minute structure of the protoplasm has been carefully ex- 

 amined by Biitschli, who finds in the coarser pseudopodia, and in 

 the membranous expansions between them, the alveolar structure 

 which is present in so many protoplasmic structures. In the fine 

 pseudopodia, however, this is absent, and they appear under the 

 highest powers as homogeneous threads of extreme tenuity, with the 

 small granules scattered along their surface. From the peculiar way 

 in which, in the living pseudopodium, the granules course along the 

 threads, sometimes leaving them for a moment to pass apparently 

 through the open water, Biitschli is inclined to the view, suggested by 

 Max Schultze, that an invisible hyaline layer may invest the visible 

 threads of the Foraminifera in the same manner as the more visible 

 hyaline ectosarc invests the axial endosarc of the pseudopodia of the 

 Heliozoa. 



In addition to this intrinsic streaming movement there is a 

 movement of the reticulum as a whole. New pseudopodia shoot 

 out from the central mass, others are shortened and retracted, and 

 the whole system is in a condition of tension and constant move- 

 ment, as becomes evident at once when an attempt is made to draw 

 any part by camera lucida. When a strand of the reticulum gives 

 way a momentary collapse of the neighbouring strands is seen, 

 followed by the rapid lengthening of some strands and shortening 

 of others, resulting in renewal of the tension. 



The food of Foraminifera consists largely of diatoms and algae, 

 either alive or in a state of decay. In some cases, however, it is of 

 animal nature, for Khumbler finds that the Globigerinae capture and 

 digest the Copepods which abound at the sea surface, and that the 

 pelagic Pulvinulinae contain the skeletons of Radiolaria as well as 

 of diatoms, which have been taken as food (38, pp. 1 and 2). 



