3532 



THE LOWEST ANIMALS 



Polymorphina. 



a, , c. From different aspects (magnified). 



circles of chambers, the pores for the-emission of pseudopods being situated on the 

 edge of the disc. The different species of Orbitolites form an interesting series, illus- 

 trating transitions from a simple to a most complex type. In a vertical section of 

 the disc of the figured species the innermost chambers exhibit the simple type, 

 later formed ones the intermediate, and the outermost series the highest type and 

 the greatest differentiation of structure. 



The shells of the perforate Foraminifera may be constructed of only one, but 

 most commonly of many, chambers, arranged according to various plans. The 

 simplest type with one chamber {Lagena} is shaped like a Florence oil flask; in 

 many-chambered forms the segments may succeed one another in a straight line or 

 in a spiral, and the coils of the spiral may or may not be in the same plane; or, 

 again, the segments may form alternately on each side of a long axis. 



In Polymorphina communis the segments are combined in a somewhat 

 obscurely spiral arrangement. In the Foraminifera group a number of forms 



have arisen exhibiting an extensive series of 

 variations on a few simple types, and show- 

 ing transitions between forms which at 

 first seemed distinct. The majority of 

 species live at the bottom of the sea, but 

 some are pelagic, and occur in abundance on 

 the surface. Among the latter, Globigerina 

 is one of the most widely distributed. 

 Its shell is about one-fortieth of an inch in 

 diameter, and usually composed of seven globular chambers, arranged spirally in 

 such a manner that all are visible from above, but only the last four from below. 

 Each chamber opens by a creseentic orifice into the depression in the middle of the 

 loWer surface. Perfect specimens bristle with long slender spines. The pores 

 afford passage to the pseudopods which stream along the spines. In life, the 

 shell is sunk in the midst of a 

 bubbly sphere of protoplasm, 

 which serves as a float. The in- 

 vestigations of deep-sea expedi- 

 tions have brought to light the 

 fact that the floor of the ocean, 

 at depths between five hundred 

 and two thousand five hundred 

 fathoms over vast areas, be- 

 tween 110 north and south 

 of the Equator, is formed of a 

 pinkish- white mud, containing 

 on an average about 60 per 

 cent, of carbonate of lime. 

 The presence of this material 

 is mainly due to shells of 

 Foraminifera, especially Glo- SHELI^S OF Globigerina (much magnified). 



