PROTOZOA : RADIOLARIA. 



59 



often does not entirely fill the shell. The pseudopodia are 

 emitted through the foramina in the test, and are long, ray-like 

 filaments, which display a slow movement of granules along 

 their borders. 



The Polycystina are all microscopic, and are all inhabitants 

 of the sea, having a very wide distribution. They are also 

 found abundantly in certain Tertiary deposits, being often er- 

 roneously described as Diatomacea. 



III. FAM. THALASSICOLLIDA. The Thalassicollida have been 

 defined as being Rhizopoda which are " provided with structureless 

 cysts containing cellular elements and sarcode, and surrounded 

 by a layer of sarcode, giving off pseudopodia, which commonly 

 stand out like rays, but may and do run into one another, and so 

 form networks" (Huxley.) 



The three best-known genera of the family are Sphcerozoum, 

 Collosphcera, and Thalassicolla. They are all marine, being 

 found floating passively at the surface of most seas ; and they 

 vary in size from an inch in diameter downwards. Sphcerozoum 

 consists essentially of a number of spherical sarcode-bodies 

 (sometimes called " cellaeform bodies ") with distinct nuclei, 

 surrounded by a zone of siliceous spicules, the whole being 

 embedded in a common gelatinous matrix. The centre of the 

 mass is vacuolated, sometimes to such an extent that it be- 

 comes a hollow sphere. 



In Collosphcera, the spherical body which is very like that 

 of the preceding form is enclosed in a transparent siliceous 



Fig. 7. Morphology of Radiolaria. a Siliceous fenestrated test of Collosphtzra 

 Huxleyi; b Tkalassicotta morum, showing cellseform bodies, compound groups 

 of spicules, and radiating pseudopodia. 



envelope, which is perforated by numerous rounded apertures 

 or " fenestrae." This form, therefore, approaches very closely 

 to the Polycystina, especially to those in which the foramina 

 are so large that the test is reduced to a mere reticulate frame- 

 work (fig. 7, a). 



