History of Protozoology - shell, central, capsule, fertilization, unicellular, skeleton, organisms and usually

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3. Order Foraminifera. Relatively large Rhizopoda; the body is enveloped in a many-chambered porous calcareous shell. The pseudopodia, usually branched, make their way through the pores. They are free-living, heterotrophic, and feed on living organisms; entirely marine. Inland they appear only in salt-lakes —residues of earlier seas—as in Transylvania. The shells of many Foraminifera are very complex in structure but the prin ciple is always the same. The germ-cell (agamete or zygote) surrounds itself with a shell and usually resembles a Thecamoeba; after some time, protoplasm makes its way out of the opening in the shell, spreads over a part of the shell-surface and forms a new shell; this makes the second chamber. This process goes on until the number of chambers in an adult animal may exceed a hundred. Only rarely are all the chambers arranged in a row, usually they are rolled up to form a spiral like a snail-shell, (fig. 13). Very frequently each chamber is completely built around by the one following. The shell-material is either the chalk that the animal excretes or extraneous material (grains of sand, etc.). There are also Foraminifera with unchambered shells and others whose shells are imperf orate.

Most foraminiferans live on the bottom of the sea, only a few are plankton forms. All are unicellular and most uninucleate. Reproduction is by multiple division and gamete formation. Fertilization is effected by flagellate gametes. An alternation of generations is established. The Foraminifera, especially some fossil forms, are the largest unicellular organisms. They are of practical importance as stone-formers. Important types: Rotalia, Polystomella (figs. 13, 14), Globigerina (plankton, see GLOBIGER INA) , Miliolina, Nummulites (fossil).

4.

Order Heliozoa. Relatively small Rhizopoda characterized by the possession of so-called axopodia. Entirely free-living, heterotrophic, feeding on living organisms. Found as a rule in fresh water. A small group ; the only natural sub-order is the Centrohelidia, ball-shaped protozoans with a so-called central granule ; this is a cytoplasmic structure capable of division, which lies in the middle of the cell and serves as the point d'appui of the axopodium. It was earlier thought to be a true centrosome, but it really, is nothing of the kind. Some primitive forms (Dimorpha) have a true centrosome which functions both as a basal granule and as the insertion-point of the axopodium. Many species have a peculiar shell made of loosely packed silica needles. They are unicellular and chiefly uninucleate. Reproduction is by simple fission or gemmation; the gemmae grow into flagellate "swarming" individuals. Fertilization processes unknown. Im

portant types :—Acanthocystis, Dimorpha (always provided with two flagella), Wagnerella (sessile, with a stalk; marine).

The remaining Heliozoa are probably related neither with one another nor with the Centrohelidia. Actinophrys (fig. I I) is unicellular, uninucleate, in fresh water, fertilization paedogamous. Actinosphaerium is multinuclear, the fertilization process as in Actinophrys; every individual breaks up into numerous gamonts, each one of which behaves like an Actinophrys pre paring for fertilization. Clathrulina elegans has a ferruginous lattice-work shell : fertilization process unknown. Nuclearia is like Clathrulina without a shell; reproduction by simple fission.

5. Order Radiolaria. Relatively large Protozoa. The body of the protoplasm is divided into two parts by an internal membrane, the central capsule; the so-called intra-capsular cytoplasm con tains the nucleus or nuclei and in the extra-capsular cytoplasm the food is digested. They are usually provided with a silica skeleton and also with a gelatinous sheath. The pseudo podia are intermediate in form between the Rhizopodia and Axopodia. Always free-living, heterotrophic, eating living organ isms, marine plankton organisms. Some are capable of devouring multicellular animals as big as a water-flea. Many Radiolarians contain symbiotic Dinoflagellates, so-called Zooxanthellae. For the most part these forms are unicellular and uninucleate. Reproduction by simple fission and multiple agamete formation; the agametes are flagellate and may sometimes show a strong resemblance to dinoflagellates ; some contain peculiar crystals. The life-cycle of Radiolarians is still very imperfectly under stood, part of it being carried through in the deep sea. The Radiolaria are classified according to their skeleton and the form of the central capsule into four sub-orders : (a) Sub-order Spumellaria. Central capsule pierced in all direc tions by fine pores. Skeleton may be wanting or may be composed simply of fine, loose needles. Some species form colonies. Impor tant types : Collozoum, Thalassic()lla.

(b) Sub-order Acantharia. Central capsule pierced in all direc tions; skeleton consists generally of 20 needles sticking together in the central capsule. Myonemes are attached to the needles and their contraction stretches the gelatinous integument and makes the animal lighter. Important types : Acanthometra, Xiphacantha.