Upon first reading this I said 'huh?'. Even for Webster, this one is a bit on the cryptic side. Thus, for your reading enjoyment - here is what a Heliozoan is in plain English.

rhizopod
A creeping protozoan (such as an amoeba) that has a rootlike pseudopodia.

Bear with me, everything will be explained

pseudopodium
pseudo (fake) + pod (foot). A retractable footlike thing made of cytoplasm (the jello-like stuff that fills a cell) that is used for movement or food gathering. Think amoeba. Note that not all animals that are capable of pseudopodium are amoebas. The slime mold has this ability and is not an amoeba.

The members of Heliozoan are the freshwater 'cousin' to the marine radiolarian well known for their silicate skeletons. There are two divisions of Sarcodina, the class Actinopodea and the class Rhizopodea. The Actinopodea have the tendency for their pseudopods to radiate out from a central glob or shell. The Rhizopodea include the well known amoebas, some shelled some 'naked'. The most well known amoeba is that of Protozoa Sarcodina Rhizopodea Amoeba proteus.

One description of some Heliozan:

In Madog River, we find Sarcodines, those animalcules that move by means of pseudopods. At first, all we could see were the little ones with the burst of rays sticking out, the Heliozoa. These are incredibly gorgeous creatures. One Heliozoan looks like a grainy green disc surrounded by colorless, clear cytoplasm, from which the axopodia (rays) emerge. I think that one is Acanthocystis. Another one, small with a colorless, bubbly-looking interior, is Actinophrys. A larger, more irregular, colorless, bubbly-looking one is possibly Actinosphaerium. Recently we have been finding the stalked heliozoan-like sarcodine, Clathrulina, whose cytoplasm is encased in a lattice composed of silica.

http://www.heptune.com/madog.html

Please note there is some confusion in the places looked regarding the classification. Some refer to the phylum protist class Sarcodina, others refer to kingdom protist, and phylum Sarcodina. I use the later.

He`li*o*zo"a (?), n. pl. [NL., fr. Gr. the sun + an animal.] Zool.

An order of fresh-water rhizopods having a more or less globular form, with slender radiating pseudopodia; the sun animalcule.

 

© Webster 1913.

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