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===Axopodia=== [[File:Axopodium Mikrotubuli.jpg|thumb|Cross-section of the double spiral [[microtubule]] structure in an axopod]] The most distinctive characteristic of the actinophryids is their axopodia. These axopodia consist of a central, rigid rod which is coated in a thin layer of ectoplasm. In ''Actinophrys'' the axonemes end on the surface of the central nucleus, and in the multicellular ''Actinosphaerium'' they end at or near nuclei.<ref name="Anderson1960" /> The axonemes are composed of microtubules arranged in a double spiral pattern characteristic of the order.<ref>{{cite book|author1=Gast, R.J.|editor1-last=Archibald|editor1-first=J.|editor2-last=Simpson|editor2-first=A.|editor3-last=Slamovits|editor3-first=C.|editor4-last=Margulis|editor4-first=L.|editor4-link=Lynn Margulis |editor5-last=Melkonian|editor5-first=M.|editor6-last=Chapman|editor6-first=D.|editor7-last=Corliss|editor7-first=J.|title=Handbook of the Protists|pages=1β17|date=2017|publisher=Springer International|location=Cham, Switzerland|isbn=978-3-319-32669-6|doi=10.1007/978-3-319-32669-6_28-1|chapter=Centrohelida and Other Heliozoan-like Protists}}</ref> Due to their long, parallel construction, these microtubules demonstrate strong birefringence.<ref name="Tilney1967">{{cite journal|last1=Tilney|first1=L.|last2=Porter|first2=K.|title=Studies on the microtubules in heliozoa II. The effect of low temperature on these structures in the formation and maintenance of the axopodia|journal=Journal of Cell Biology|date=July 1967|volume=34|issue=1|pages=327β343|pmc=2107222|pmid=6033539|doi=10.1083/jcb.34.1.327}}</ref><ref name="Suzaki1994">{{cite journal|last1=Suzaki|first1=Toshinobu|last2=Ando|first2=Motonori|last3=Inai|first3=Yoko|last4=Shigenaka|first4=Yoshinobu|title=Structure and function of the cytoskeleton in heliozoa|journal=European Journal of Protistology|date=November 1994|volume=30|issue=4|pages=404β413|doi=10.1016/S0932-4739(11)80215-4}}</ref> These axopodia are used for prey capture, in movement, cell fusion and perhaps division.<ref name="Suzaki1980" /><ref name="Ando1989" /> They are stiff but may flex especially near their tips,<ref name="Barrett1958" /> and are highly dynamic, undergoing frequent construction and destruction. When used to collect prey items, two methods of capture have been noted, termed axopodial flow and rapid axopodial contraction.<ref name="Suzaki1980" /> Axopodial flow involves the slow movement of a prey item along the surface of the axopod as the ectoplasm itself moves, while rapid axopodial contraction involves the collapse of the axoneme's microtubule structure.<ref name="Suzaki1994" /> This behavior has been documented in many species, including ''Actinosphaerium nucleofilum'', ''Actinophrys sol'', and ''[[Raphidiophrys contractilis]]''.<ref name="Suzaki1994" /><ref name="Kinoshita2001">{{cite journal|last1=Kinoshita|first1=E|last2=Shigenaka|first2=Y|last3=Suzaki|first3=T|title=The ultrastructure of contractile tubules in the heliozoon Actinophrys sol and their possible involvement in rapid axopodial contraction.|journal=The Journal of Eukaryotic Microbiology|date=2001|volume=48|issue=5|pages=519β26|pmid=11596916|doi=10.1111/j.1550-7408.2001.tb00187.x}}</ref><ref name="Kinoshita1995">{{cite journal|last1=Kinoshita|first1=Eiji|last2=Suzaki|first2=Toshinobu|last3=Shigenaka|first3=Yoshinobu|last4=Sugiyama|first4=Masanori|title=Ultrastructure and Rapid Axopodial Contraction of a Heliozoa, ''Raphidiophrys contractilis'' sp. nov.|journal=The Journal of Eukaryotic Microbiology|date=May 1995|volume=42|issue=3|pages=283β288|doi=10.1111/j.1550-7408.1995.tb01581.x}}</ref> The rapid axopodial contraction occurs at high speed, often in excess of 5mm/s or tens of body lengths per second.<ref name="Shigenaka1982">{{cite book|last1=Shigenaka|first1=Y.|last2=Yano|first2=K.|last3=Suzaki|first3=T.|title=Biological functions of microtubules and related structures|date=1982|publisher=Academic Press|location=Tokyo|pages=105β114|chapter=Shigenaka, Y., Yano, K., Yogosawa, R. and Suzaki, T., 1982. Rapid contraction of the microtubule-containing axopodia in a large heliozoan Echinosphaerium}}</ref> The axopodial contractions have been shown to be highly sensitive to environmental factors such as temperature and pressure<ref name="Tilney1967" /><ref name="Tilney1969">{{cite journal|last1=Tilney|first1=Lewis G.|last2=Byers|first2=Breck|title=Studies on the Microtubules in Heliozoa V. Factors Controlling the Organization of Microtubules in the Axonemal Pattern in Echinosphaerium nucleofilum|journal=The Journal of Cell Biology|date=1 October 1969|volume=43|issue=1|pages=148β165|pmc=2107851|issn=0021-9525|pmid=5824062|doi=10.1083/jcb.43.1.148}}</ref> as well as chemical signals like Ca<sup>2+</sup> and colchicine.<ref name="Kinoshita2001" /><ref name="Tilney1968">{{cite journal|last1=Tilney|first1=L.|title=Studies on the microtubules in heliozoa. IV. The effect of colchicine on the formation and maintenance of the axopodia and the redevelopment of pattern in Actinosphaerium nucleofilum (Barrett).|journal=Journal of Cell Science|date=December 1968|volume=3|issue=4|pages=549β62|doi=10.1242/jcs.3.4.549 |pmid=5707852}}</ref>
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