THE SARCODINA 217 



forms of simple structure, however, in which no skeleton is present, 

 the subdivisions are defined entirely by the characters of the proto- 

 plasmic body and the pseudopodia, which furnish distinctions of 

 very doubtful validity. Not only may the characters of the 

 pseudopodia vary in different phases of the life cycle, as already 

 stated in the case of Amoeba proteus, but even in the same phase 

 under the influence of different media. Thus, no two forms of 

 amoeba could appear more distinct at first sight than the Umax 

 and radiosa forms, originally regarded as distinct species. In the 

 Umax-form the whole body flows forward as a single pseudopodium, 

 gliding along like a slug ; in the ra^'osa-form the spherical body 

 becomes star-like, sending out sharp-pointed pseudopodia on all 

 sides. Nevertheless, Verworn showed that the one form could be 

 changed into the other by differences in the medium (Fig. 85). 

 Doflein (238) obtained similar form-changes in Amoeba vespertilio. 

 and showed that the body-form and character of the pseudopodia 

 were quite inadequate features for distinguishing the species of 

 amoeba, depending as they do upon the conditions of the environ- 

 ment and the nature of the medium. Compare also Gruber (246) 

 on form- varieties of Amoeba proteus. 



In view of the protean nature of these organisms, it is not sur- 

 prising that much diversity of opinion prevails as to the arrangement 

 of the groups and the exact position of some of their members. It is 

 usual to put a number of primitive organisms together in a group 

 termed Proteomyxa, the members of which probably have more 

 affinities with various members of other groups than with one 

 another. On the other hand, the more highly organized Sarcodina 

 are classified without difficulty into well-characterized orders ; 

 such are the Foraminifera, Mycetozoa, Radiolaria, and Heliozoa, 

 though even in these groups there are forms near the border-line and 

 of doubt "ul position. 



The classification adopted here is mainly that of Biitschli (2), 

 with the addition of some forms not included in his great work, as 

 follows : 



A. SUBCLASS RHIZOPODA. Typically creeping forms with 

 branched, root-like pseudopodia. 



I. Order Amoebcea. Amoeboid forms of simple structure ; 

 skeleton lacking or in the form of a simple shell. 



1. Suborder Reticulosa (Proteomyxa}. With filose 



or reticulose pseudopodia, without shell. 



2. Suborder Lobosa. With lobose pseudopodia. 



(a) Section Nuda, without shell or skeleton. 



(b) Section Testacea, with shells. 



II. Order Foraminifera. With reticulose pseudopodia and 

 shells. 



