THE PROTOZOA THE DAWN OF LIFE 7 



or orders of the Rhizopoda the order Lobosa, of which Amoeba 

 is one of the simplest. 1 



Some of the Lobosa differ from Amoeba in the possession of a 

 comparatively dense outer coat, forming a shell or test enclosing 

 the protoplasm. One of these, called Difflugia, is fairly common 

 in fresh water, and collects minute sand grains to form its flask- 

 shaped test, the particles being agglutinated together. While the 

 bulk of the protoplasm of Difflugia is contained within the interior 

 of this curious test, the organism pushes out comparatively long 

 pseudopodia. 



Equally interesting, and also a commoner member of this 

 group, is Arcella, an organism which has a convex, transparent 

 shell of a tough material, said to be chitinoid from its resemblance 

 to chitin, a horny substance such as we find the integument or 

 outer skin of insects composed of. This transparent shell of 

 Arcella is convex on one side, flat on the other, and in the middle 

 of the flat surface there is a rounded opening through which the 

 pseudopodia are protruded. 



All the other Rhizopods differ from the Lobosa in having 

 their pseudopodia in the shape of long, slender threads. The 

 Foraminifera have these thread-like pseudopodia, and form a 

 most interesting order, the members of which chiefly inhabit the 

 sea, and, with a few exceptions, are provided with tests of the 

 most varied and exquisite shapes, composed of carbonate of lime, 

 or of cemented particles of sand. D'Orbigny first described the 

 Foraminifera as minute cephalopods or cuttlefish, from the re- 

 semblance of the shells of some species to the Pearly Nautilus, 

 and used the name Foraminifera to express the fact that the 

 chambers of their shells communicated by pores, and not by a 

 tubular siphon as in the nautilus. Their true nature as Pro- 

 tozoa was elucidated later, thanks to the careful and laborious 

 investigations of Williamson, Carpenter, Dujardin, and Max 

 Schultze. 



That the Foraminifera are of very ancient origin we realise 



1 The Animal Kingdom is divided into phyla (sub-kingdoms), the phyla into 

 classes, the classes into orders, the orders into families, the families into genera, the 

 genera into species, while the species themselves are assemblages of individual 

 animals agreeing with one another in certain constant characters. Thus, in terms of 

 classification, we should describe the Amoeba as follows : Kingdom animalia. 

 Phylum (sub-kingdom) protozoa. Class rhizopoda. Order lobosa. Family 

 amoebae. Genus amoeba. Species amoeba proteus. 



