THE SARCOBINA 231 



II. FORAMINIFERA. 



General Characters Shell-Structure. The characteristic features 

 of this group are the possession of reticulose pseudopodia and of a 

 shell or test. The Foraminifera are typically creeping forms, 

 moving slowly, and using their net-like pseudopodia chiefly for food- 

 capture. Certain genera, however, such as Globigerina, have taken 

 secondarily to a pelagic existence, and float on the surface of the 

 ocean, spreading their nets in all directions around them. On 

 the other hand, some forms have adopted a sedentary life, attaching 

 themselves firmly to some object. An example is seen in the genus 

 Haliphysema (Fig. 17), once believed to be a sponge, and in the 

 remarkable genus Polytrema and allied forms, recently monographed 

 by Hickson (282) organisms which in many cases have a striking 

 and deceptive resemblance to corals. 



The test may be secreted by the animal itself, and then is usually 

 either chitinous or calcareous, rarely siliceous or gelatinous (Myxo- 

 theca) ; or it may be made up of foreign bodies cemented together, 

 as in Haliphysema (Fig. 17), and is termed generally " arenaceous," 

 but the materials used may be of various kinds, and the organism 

 sometimes exhibits a remarkable power of selection (see p. 34, supra). 



The typical form of the shell, as in the Amcebaea Testacea, is a 

 chamber with a w'de aperture sometimes more than one 

 through which the pseudopodia are extruded, as in Gromia (Fig. 21). 

 In addition to the principal aperture, the wall of the shell may be 

 perforated by numerous fine pores, through which also the protoplasm 

 can stream out to the exterior. Hence the shells of Foraminifera 

 are distinguished primarily as perforate and imperforate, the former 

 with, the latter without, fine pores in addition to the principal 

 opening. 



Whether perforate or imperforate, the shell remains a single 

 chamber in the simple forms, as in the Amcebsea Testacea. In some 

 cases, when the animal reproduces itself by binary fission, the proto- 

 plasm streams out through the principal aperture to give rise to 

 the body of the daughter-individual, which forms a shell for itself, 

 and, when the division is complete, separates completely from the 

 mother, which retains the old shell. Division of this type is seen 

 in Euglypha (Fig. 59). But in many species, when the animal out- 

 grows its original single-chambered shell, the protoplasm flows out 

 and forms another chamber, which, however, is not separated off 

 as a distinct individual, but remains continuous with the old shell, 

 so that the animal, instead of reproducing itself by fission, remains 

 a single individual with a two-chambered shell. By further growth , 

 third, fourth, . . . nth chambers are formed successively, each newly- 

 formed chamber being, as a rule, slightly larger than that formed 



