9 



was first acquired may readily be understood by passing from 

 the common Amoeba to forms where the pseudopodia are 

 restricted to one part of the surface ; and then to Difflugia, 

 where the rest of the body is enclosed by a case formed of 

 small sand grains, picked up and attached by the protoplasm ; 

 and Arcella, where a delicate shell of a chitinous nature is 

 secreted by the surface layer of protoplasm. In such forms 

 the pseudopodia are still short and thick, as in Amoeba, but 

 the change from these to the higher Foraminifera, where 

 the pseudopodia are long and delicate, is a slight one, which 

 can be readily understood. Finally, the Foraminifera have 

 branched out into a large number of modifications which 

 difier comparatively little from one another. 



In tracing the history of the Kadiolaria, we start from 

 much the same point as that occupied by the ancestral 

 Foraminifera, but diverge in a difi'erent direction. If we 

 imagine an Amoeba-like form becoming more and more regu- 

 larly spherical in shape, while the pseudopodia get longer 

 and thinner and more regularly arranged, and the ectosarc 

 becomes more clearly distinguishable from the endosarc, we 

 shall have it gradually passing into an ancestral Radiolarian 

 or a Heliozoon (such as Actinos'phcermm) , for Haeckel has 

 shown* that the four existing groups of Kadiolaria, the 

 Acantharia, the Spumellaria, the Nassellaria, and the Phaeo- 

 daria, may be traced back to a common ancestor, which 

 agrees in all particulars with an ancestral Heliozoon in 

 which the endosarc has become separated from the ectosarc 

 by a membrane, thus forming a central capsule. This form, 

 to which Haeckel has given the name Actissa (fig. 5), is so 

 closely allied to Actinosphcerium that it is certain the two 

 forms must have had a common ancestor not far back, and 

 only differing from Actissa in having no capsule membrane. 

 The Heliozoa of the present day split off at this point, and 

 * See Nature, vol. xxix, pp. 274 and 296. 1884. 



