Explanation: The Müllerian law precisely details the geometric arrangement of acantharian spines, positioning them at the intersections of specific lines of latitude and longitude.

Explanation: The endoplasm houses organelles and nuclei, while the ectoplasm contains cytoplasmic extensions for prey capture and food vacuoles for digestion, confirming the statement.

Explanation: The periplasmic cortex is composed of twenty distinct microfibril plates, not a single continuous membrane, and is connected to the spines by contractile myonemes.

Explanation: Myonemes in Acantharians are contractile structures that primarily function in buoyancy control by expanding and contracting the ectoplasm, not nutrient absorption.

Explanation: The microfibril mesh capsular wall separates the endoplasm from the ectoplasm, while the periplasmic cortex surrounds the ectoplasm, not separated by the capsular wall.

Explanation: Cytoplasmic extensions in the ectoplasm are primarily utilized for prey capture, whereas contractile myonemes are responsible for buoyancy control.

Explanation: Acantharians typically range in size from approximately 200 microns up to several millimeters, which is significantly larger than a few nanometers or micrometers.

Explanation: Acantharians are uniquely distinguished among radiolarian protozoa by their skeletons, which are composed of strontium sulfate.

Explanation: Contractile myonemes in Acantharians are crucial for buoyancy control, enabling the ectoplasm to expand and contract, thereby regulating the cell's volume and position in the water column.

Explanation: Acantharians are marine microplankton that typically range in size from approximately 200 microns in diameter up to several millimeters.

Explanation: The periplasmic cortex, which surrounds the ectoplasm, is composed of twenty distinct microfibril plates.

Explanation: The Müllerian law specifically describes the highly precise and geometric arrangement of spines on an Acantharian, defining their positions based on lines of latitude and longitude.

Explanation: Food vacuoles, located within the ectoplasm, are specifically responsible for the digestion of captured prey, processing the nutrients for the acantharian.

Explanation: Acantharian skeletons are uniquely composed of strontium sulfate, not calcium carbonate, distinguishing them from many other marine plankton.

Explanation: Acantharian skeletons do not fossilize because strontium sulfate is scarce in seawater, causing the crystals to dissolve after the organism's death.

Explanation: Celestine, the strontium sulfate mineral in Acantharian skeletons, is the heaviest mineral in the ocean, and its high density acts as mineral ballast, facilitating rapid sinking to bathypelagic depths.

Explanation: The common name for the strontium sulfate mineral found in Acantharian skeletons is celestine, not gypsum.

Explanation: The mineral form of strontium sulfate (SrSO4) that constitutes Acantharian skeletons is celestine.

Explanation: Acantharian skeletons do not fossilize because strontium sulfate is very scarce in seawater, leading to the dissolution of the crystals after the organism's death.

Explanation: The high density of celestine, the strontium sulfate mineral in Acantharian skeletons, functions as mineral ballast, ensuring rapid settling of the organisms to bathypelagic depths.

Explanation: Acantharian skeletons are primarily and uniquely composed of strontium sulfate, distinguishing them from other radiolarians that typically have silica skeletons.

Explanation: The Holacanthida order is characterized by 10 diametric spicules without a central junction, whereas the description of 20 radial spicules loosely attached at the center of the cell applies to the Chaunacanthida order.

Explanation: While morphological classification generally agrees with molecular phylogenetic trees, the morphologically defined groups are often polyphyletic, meaning they do not perfectly align as monophyletic groups.

Explanation: Molecular data suggest that Holacanthida evolved first, followed by Chaunacanthida, while Arthracanthida and Symphacanthida, with their more complex skeletons, are believed to have evolved more recently.

Explanation: Acantharians are indeed classified under the Subphylum Radiolaria, and the Class Acantharia was established by Haeckel in 1881, later emended by Mikrjukov in 2000.

Explanation: The Class Acantharia was initially classified by Haeckel in 1881 and later emended by Mikrjukov in 2000, reversing the order presented in the statement.

Explanation: The Symphyacanthida order is indeed defined by its 20 radial spicules that converge to form a tight central junction within the cell.

Explanation: The Holacanthida order is characterized by 10 diametric spicules that are simply crossed without forming a central junction.

Explanation: Molecular data suggest an evolutionary progression from Holacanthida (earliest) to Chaunacanthida, and then to Arthracanthida and Symphacanthida (most recent and complex skeletons).

Explanation: The Arthracanthida and Symphacanthida orders are associated with molecular clades E and F, respectively.

Explanation: The Class Acantharia was initially classified by Haeckel in 1881, with an emendation later provided by Mikrjukov in 2000.

Explanation: The Chaunacanthida order is characterized by having 20 radial spicules that are loosely attached at the center of the cell.

Explanation: A primary characteristic used for the taxonomic classification of Acantharians is the specific manner in which their spines are joined at the center of the cell.

Explanation: Within the biological hierarchy, Acantharians are classified as the Class Acantharia, a classification established by Haeckel in 1881.

Explanation: Acantharians are heterotrophic marine microplankton, and many are mixotrophic due to hosting photosynthetic endosymbionts, meaning they are not exclusively autotrophic.

Explanation: While many acantharians host single-celled algae, particularly in certain clades (some B, all E and F), it is not true that *all* species are known to do so.

Explanation: Photosymbiosis is indeed hypothesized to enable acantharians to flourish in oligotrophic oceanic environments and to supply the metabolic energy required for the upkeep of their strontium sulfate skeletons.

Explanation: It is not definitively known if the symbiotic relationship is always mutually beneficial; there is a possibility that acantharians exploit their algal symbionts, potentially digesting them after utilizing their photosynthetic products.

Explanation: Clade E and F acantharians primarily host symbionts from the haptophyte genus *Phaeocystis*, whereas diverse dinoflagellate genera are typically hosted by symbiotic Holacanthida acantharians.

Explanation: The mismatch between internal and external symbiont communities in Clade F acantharians actually suggests that these organisms are *selective* in choosing and maintaining their symbionts, rather than being non-selective.

Explanation: Acantharians that host photosynthetic endosymbionts are considered mixotrophs, as they acquire energy through both heterotrophy (consuming prey) and autotrophy (from their symbionts' photosynthesis).

Explanation: The proposed nutrient exchange involves acantharians providing nitrogen and phosphorus to their algal symbionts, while the algae, in turn, produce sugars through photosynthesis for the acantharians.

Explanation: In symbiotic Acantharian species, the algal symbionts are maintained within the endoplasm, which is separated from the ectoplasm by a capsular wall.

Explanation: Hypothesized advantages of photosymbiosis for acantharians include thriving in low-nutrient regions and acquiring energy for skeleton maintenance, but increased resistance to predation is not mentioned as a benefit.

Explanation: Clade E and F acantharians primarily host symbionts from the haptophyte genus *Phaeocystis*, although *Chrysochromulina* symbionts are also sometimes observed.

Explanation: The observed mismatch between internal and external symbiont communities in Clade F acantharians suggests that these organisms are selective in their symbiont acquisition and maintain these symbionts for extended periods.

Explanation: Symbiotic Holacanthida acantharians host diverse dinoflagellate genera such as *Pelagodinium*, *Heterocapsa*, and *Azadinium*, but *Phaeocystis* is primarily hosted by Clade E and F acantharians.

Explanation: Adult Acantharians are typically multinucleated, possessing multiple nuclei within their cells, rather than being uninucleated.

Explanation: The fusion of Acantharian swarmer cells to form a new acantharian has not yet been observed, indicating a gap in the complete understanding of their reproductive cycle.

Explanation: The primary obstacle to studying Acantharian life cycles is the inability to 'close the lifecycle' and maintain them in culture through successive generations, rather than their size making observation impossible.

Explanation: Acantharians belonging to the Holacanthida and Chaunacanthida orders, characterized by diametric or loosely attached radial spicules, respectively, are indeed capable of encystment.

Explanation: Reproduction in Acantharians is indeed believed to involve the formation of swarmer cells, which have been observed to be released from both cysts and non-encysted cells under laboratory conditions.

Explanation: Releasing swarmer cells in deeper water is hypothesized to *improve* the survival chances of Acantharian juveniles, possibly by offering a more stable or protected environment, rather than decreasing them due to pressure.

Explanation: Adult Acantharians are characterized by being typically multinucleated, meaning their cells contain multiple nuclei.

Explanation: The primary obstacle to a comprehensive study of Acantharian life cycles is the persistent inability to 'close the lifecycle' and successfully maintain these organisms in culture through successive generations.

Explanation: The main challenge in observing the full life cycle of Acantharians is the inability to 'close the lifecycle' and successfully maintain these organisms in laboratory cultures through successive generations.

Explanation: Reproduction in Acantharians is believed to involve the formation of swarmer cells, which can be released from both cysts and non-encysted cells, although the fusion of these cells has not yet been observed.

Explanation: Observations in ocean basins like the Iceland Basin and Southern Ocean confirm that high settling fluxes of acantharian cysts contribute substantially to the vertical transport of organic carbon to the deep sea.

Explanation: The frequent discovery of acantharian cysts in sediment traps indicates their crucial role in the vertical transport of biomass and organic matter to deep-water environments.