THE PROTOZOATHE DAWN OF LIFE n 



The Radiolaria are marine Protozoa, well known to most 

 amateur microscopists on account of their extremely beautiful 

 skeletons, formed of silica, which are very favourite objects for 

 exhibition under the microscope. While present in all seas, and 

 in every latitude, living at the surface, at varying depths, and 

 near the bottom, the Radiolaria are most numerous in tropical 

 seas, and their siliceous 1 skeletons form a deposit or Radiolarian 

 ooze at depths of 2,000 to 3,000 fathoms. Like the foraminifera, 

 their history can be traced far back in the records of the rocks, 

 their fossil skeletons occurring so remotely as the Cambrian strata, 

 and in each succeeding geological epoch. 



The living Radiolarian has the protoplasm divided by a per- 

 forated membranous sac (called the central capsule), into a central 

 mass surrounding the nucleus, and an outer layer from which 

 the slender thread-like pseudopodia are protruded. The nucleus 

 contained in the enclosed or intra-capsular protoplasm, as it is 

 termed, is always at first single, but later may divide again and 

 again. The skeleton, usually composed of silica, may be a globu- 

 lar, conical, star-shaped, or disk-shaped perforated shell, frequently 

 supported by spines radiating out from the centre, or may con- 

 sist of loosely woven needle-like spicules; while some Radiolaria 

 have the skeleton composed of a chitinoid substance called acan- 

 thin. Reproduction takes place by simple binary fission, and 

 in some species by spore formation, in which the protoplasm con- 

 tained in the central capsule breaks up into small masses which 

 become flagellate spores or flagellula. 



When we examine a living Radiolarian under the microscope, 

 we shall see some minute yellow cells embedded in the proto- 

 plasm, which are living unicellular plants. These minute, 

 simple plants are microscopic algae, called Zooxanthella, and 

 multiply by binary fission in the protoplasm of the Radiolarian, 

 to whom they are of considerable benefit. In fact, we have here 

 an example of that intimate association between two living 

 organisms, to which the scientific term symbiosis 2 is applied. 

 Now, during the process of respiration the Radiolarian is continu- 

 ally taking up oxygen from the surrounding water and giving off 



1 Latin, silex, quartz or flint ; used for the kind of mineral forming the siliceous 

 spicules of sponges, of Radiolarians, and the frustules of diatoms. 

 8 Symbiosis, from the Greek, meaning " living together." 



