Outlines of zoology . presented by spicules. The (6) Foraminifera or Reticularia include an interesting seriesof shelled forms in which the peripheral protoplasm forms branchinginterlacing threads. A few simple forms occur in fresh water ; the greatmajority occur on the floor of the sea at varying depths; some SYSTEMATIC SURVEY. 109 families are abundantly represented on the surface. The shell isusually calcareous, more rarely arenaceous or chitinous. There issometimes dimorphism. Multiplication occurs by fission, or by theformation of swarm-spores (amceboid or flagellate). Foraminifera arecom
Outlines of zoology . presented by spicules. The (6) Foraminifera or Reticularia include an interesting seriesof shelled forms in which the peripheral protoplasm forms branchinginterlacing threads. A few simple forms occur in fresh water ; the greatmajority occur on the floor of the sea at varying depths; some SYSTEMATIC SURVEY. 109 families are abundantly represented on the surface. The shell isusually calcareous, more rarely arenaceous or chitinous. There issometimes dimorphism. Multiplication occurs by fission, or by theformation of swarm-spores (amceboid or flagellate). Foraminifera arecommon as fossils from Silurian rocks onwards, and at the present dayare very important in the formation of calcareous ooze ; in this respectGlobigerina, with a chambered shell, is especially important. Speciesof Gromia are found in both fresh and salt water; Haliphysema, aform utilising sponge-spicules to cover itself, was once mistaken for aminute sponge. Most kinds of chalk consist mainly of the shells of Foraminifera. Fig. -A Foraminifer (Polystomella) showing shell andpseudopodia.—After Schultze. accumulated on the floor of ancient seas; Nummulites (Fig. 17) andrelated fossil forms were as large as shillings or half-crowns. More complex are the (7) Radiolaria, which are divided by a chitinoidmembrane into an inner central capsule (with one or more nuclei), andan outer portion, gelatinous and vacuolated, giving off radiating thread-like pseudopodia, which very rarely interlace. There is usually askeleton in the form of a siliceous lattice-work or regularly disposedspicules outside the central capsule, but in some cases the shell isformed of a horn-like substance called acanthin, which is probably acomplex silicate. Radiolarians multiply by fission, which sometimesincludes a halving of the skeleton, and by spores, which in some casesare dimorphic. Most Radiolarians include unicellular Algae (yellow no PHYLUM PROTOZOA THE SIMPLEST ANIMALS. cells), with which they live in
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Keywords: ., bookcentury1900, bookdecade1910, bookidcu3192, booksubjectzoology
