PROTOZOA : RADIOLARIA. 8 1 



family sometimes attain a considerable size, and are found 

 floating near the surface in most seas. 



IV. FAM. THALASSICOLLIDA. This family, as now restricted, 

 comprises floating marine organisms, which are in many re- 

 spects closely allied to the preceding, but in which the intra- 

 capsular sarcode contains a complex nucleus. The skeleton 

 may be wanting (as in Thalassicolla and Thalassolampe), or it 

 may be present in the form of spicules or spines developed in 

 the extra-capsular sarcode. 



HELIOZOA. 



The Heliozoa may be denned as Rhizopoda, which possess a 

 contractile vesicle, and are devoid of a central capsule. The body 

 is naked, or is provided with skeletal structures of a variable 

 nature, but sometimes siliceous. The pseudopodia stand out like 

 rays, but may anastomose with one another. 



In their radiant pseudopodia and in the occasional presence 

 of siliceous spicules, the Heliozoa are allied to the typical 

 Radiolarians ; but the absence of a central capsule and the 

 presence of a contractile vesicle approximate them to the 

 Amcebea; while the absence of "yellow cells," as also of a 

 gelatinous outer investment to the sarcode, distinguish them 

 further from the true Radiolarians. They must therefore be 

 regarded as an inosculating group, related on the one hand to 

 the Amoebea, and on the other to the Radiolaria.% 



Most of the Heliozoa are inhabitants of fresh water, and we 

 may select as a type the common " Sun-animalcule " (Actin- 

 ophrys sol), in which no hard structures are developed. In 

 this animalcule (fig. 21), the body consists of a spherical mass 



Fig. 21. Actinophrys sol, showing the radiating pseudopodia. 

 One specimen has swallowed a Diatom. 



of sarcode, about 1-1300 of an inch in diameter, and usually 

 covered with long, radiating, filamentous pseudopodia, which 



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