PEOTOZOA: RADIOLARIA. 



53 



branous layer, or * ectosarc,' and an internal granular layer, 

 or ' endosarc.' The siliceous spines are hollow, being grooved 

 at the base by a gutter which is continued further up the 

 spine by a canal terminating at the apex of the spine by a 

 distinct aperture. The spines in consequence of this struc- 

 ture are able to serve for the transmission of the pseudopodia, 

 which gain the exterior by running through the canals and 

 escaping at their apices. Many of the pseudopodia, however, 

 do not occupy the canals of the spines. 



II. FAM. POLYCYSTINA. The members of this family are 

 closely related to the Forarninifera, differing from them chiefly 

 in the fact that their shells are 



composed of flint instead of car- 

 bonate of lime, as in most of the 

 latter. They possess a body of 

 sarcode, which is enclosed in a 

 foraminated siliceous shell, which 

 is often furnished with spine-like 

 processes, and is usually of great 

 beauty (Jig. 7). The sarcodic 

 substance of the body is olive- 

 brown in colour with yellow- 

 globules, and often does not en- 

 tirely fill the shell. The pseudo- 

 podia 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 micro- 

 scopic, and are all inhabitants of 

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

 found abundantly in certain Tertiary deposits, being often 

 erroneously described as Diatomacece. 



III. FAM. THALASSICOLLIDA. The Thalassicollida have been 

 defined as being Rhizopoda which are ' provided with structureless 

 cysts containing cellular elements and sarcode, and surrounded 

 ly a- layer of sarcode, giving off pseudopodia, which commonly 

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

 form net-works.' (Huxley.) 



The three best known genera of the family are Sphcerozoiim , 

 Collosphcera, and Thalassicolla. They are all marine, and vary 

 in size from an inch in diameter downwards. Sphcerozoiltn 

 consists essentially of a number of spherical sarcode bodies 

 > (sometimes called ' cellas-form bodies') with distinct nuclei, 

 surrounded by a zone of siliceous spicules, the .whole being 



Fig. 7. Polycystina. Rhopalocanium 

 ornatum, showing the pseudopo- 

 dial foramina. 



