REPORT ON THE RADIOLARIA. XCV 



becoming spherical and by fine pseudopodia protruding all round instead of a single 

 flagellum ; the nucleus at the same time assuming a central position. 



144. TJie Sphcerastrum-Stage. — The Actinophrys-stage of the young Radiolaria, 

 which proceeds immediately from the flagellate zoospore, is probably connected with 

 the Actissa-stsige by an intermediate form, which may be regarded as a simple skeleton- 

 less Heliozoon with a jelly- veil ; a well-known example of such a form is S-phcBrastrum 

 (in the solitary, not the social condition) and Heterophrys. This important intermediate 

 form has arisen from the simple Actino'phrys-si&gQ by the excretion of an external 

 structureless jelly-veil, such as is formed in many other Protista {e.g., in the encystation 

 of many Infusoria). The young Radiolarian in this second Heliozoon-siagQ becomes a 

 simple cell with pseudopodia radiating on aU sides ; its body consists of three concentric 

 spheres, the central nucleus, the protoplasmic body proper, and the surrounding calymma 

 or jelly-veil. When a firm membrane is developed between the last two spheres this 

 Sphcerastrum-stage passes over into the Actissa. 



The gap in our empirical knowledge which still exists between the flagellate stage (§ 142) 

 and the simplest Eadiolarian stage {Actissa, § 145), can be fiUed hypothetically only by the assump- 

 tion of several Hdiozoon-slB-^QS, following one upon another. It is possible also that the capsule- 

 membrane is not formed between the endoplasm and exoplasm (as here supposed), but that the 

 membrane was formed first outside the cell and the extracapsulum subsequently secreted around it. 



145. The Actissa-Stage. — The first Spumellarlan genus, Actissa, is not only the 

 simplest form actually observed among the Radiolaria, and the true prototype of the 

 whole class, but also the simplest form under which the Radiolarian organisation can be 

 conceived. It is therefore extremely probable that Actissa not only forms the common 

 stem-form of the whole class in a phylogenetic sense, but is also its common ontogenetic or 

 germinal form. Probably in all Radiolaria the Sphcerastrum-stnge develops immediately 

 into the typical Actissa-stage, by the formation of a firm membrane between the proto- 

 plasmic body of the spherical Hehozoan cell and its jelly- veil. Thus arises the 

 characteristic central capsule, which is wanting in the nearly related Heliozoa. It is 

 further probable that all Radiolaria in their early stage w^ill so far conform to the state 

 of things in Actissa as to have the capsule-membrane of the spherical skeletonless cell 

 perforated everywhere by fine pores. This structure is retained in all Spumellaria, 

 whilst in the other three legions those structural relations of the capsule which are 

 characteristic of each develop from the Actissa-stnge. 



146. TJie Ontogeny of the Spumellaria. — In the simplest case the individual develop- 

 ment in the Spumellapja ceases with the ^c^z'ssa-stage. In all other genera of this legion 

 diverging forms proceed from this, of which the diflerent growth of the three dimensive 



