MORPHOLOGY AND TAXONOMY OF THE SARCODINA 453 



formed with growth and budding division until the mature indi- 

 vidual again results. Thus there is a typical alternation of genera- 

 tions in the life history of a foraminif eron ; the microspheric indi- 

 vidual starting from a zygote, with its production of amebulae is 

 an asexual generation while the macrospheric individual starting 

 from an asexual spore is the sexual generation giving rise to gametes. 

 In Polystomellina the relative abundance of macrospheric and micro- 

 spheric shells is 38 to 40 of the former to 1 of the latter (Rhumbler, 

 1923). 



Test dimorphism has led to much confusion in classification, and 

 this difficulty, as pointed out by Cushman (1928), is enhanced by 

 Hofker's (1927) discovery of trimorphic types amongst fossil 

 foraminif era. 



Fossil forms are known from the paleozoic to recent times. These 

 have played a conspicuous part in geologic formations and are 

 useful today in economic ways. A complete classification must 

 take such forms into consideration. This is well done in Cushman's 

 system of classification in which 45 families and 411 genera, living 

 and fossil, are keyed and described. As with the Radiolaria it is 

 inexpedient to repeat such keys here and the reader is referred to 

 Cushman's excellent treatise (1928) for family and generic diagnoses. 



Sub-class IV. AMOEBAEA. 



When rhizopods are mentioned the mental picture in most cases 

 is Ameba or some of its close relations amongst the Amoebaea. It 

 is not the largest group of rhizopods but some of the forms included 

 here are amongst the most common types of Protozoa, while their 

 apparent simplicity and enigmatic movement have given them the 

 popular position of the lowest forms of animal life and the phrase 

 "from Ameba to man" is familiar to everyone. They are present 

 in all stagnant, fresh and brackish water; in damp moss or leaves; 

 abundant in the superficial soil, and also abundant as commensals 

 or parasites in all kinds of animals. 



In all of the naked forms there is a well-marked differentiation of 

 the protoplasm into endoplasm and ectoplasm. The latter is more 

 dense, the former more fluid and with typical cyclosis. In the 

 shelled types there is frequently a characteristic zonal differentiation. 



Pseudopodia are never myxopodia or axopodia. Naked forms 

 have blunt finger-form processes or lobopodia formed by an outflow 

 of ectoplasm and endoplasm. Shelled forms in the majority of 

 types have pseudopodia, composed apparently of ectoplasm only. 

 These have considerable power of movement apart from the usual 

 ameboid type of flowing substance, and may sway or move inde- 

 pendently with vigor. In the naked forms pseudopodia may be 

 thrown out from any part of the ceil, but in shelled types they are 



