▁tub es . ▁In ▁some ▁parts , ▁the ▁tub es ▁have ▁even ▁col lapsed , ▁and ▁it ▁is ▁impossible ▁to ▁distinguish ▁them . ▁The ▁wall ▁of ▁the ▁tub es ▁is ▁thin , ▁compr ising ▁an ▁irregular ▁mesh work ▁of ▁tri act ines , ▁t et ract ines ▁and ▁a ▁few ▁tri pod s , ▁which ▁are ▁located ▁only ▁on ▁external ▁tub es . ▁Project ing ▁into ▁the ▁interior ▁of ▁the ▁tub es ▁are ▁the ▁ap ical ▁act ines ▁of ▁the ▁t et ract ines . ▁Sp ic ules ▁are ▁equ i angular ▁and ▁equ ir adi ate ▁tri act ines , ▁t et ract ines , ▁and ▁tri pod s . ▁Tri act ines ▁are ▁the ▁most ▁abund ant ▁sp ic ules . ▁The ▁size ▁of ▁the ▁tri act ines ▁and ▁t et ract ines ▁is ▁uniform . ▁The ▁act ines ▁of ▁the ▁tri act ines ▁are ▁con ical ▁or ▁cyl ind rical , ▁while ▁the ▁t et ract ines ▁are ▁always ▁con ical . ▁They ▁are ▁straight , ▁with ▁a ▁bl unt ▁tip . ▁The ▁ap ical ▁act ine ▁of ▁the ▁t et ract ines ▁is ▁con ical , ▁sharp , ▁straight ▁and ▁th inner ▁and ▁shorter ▁than ▁the ▁fa cial ▁ones . ▁In ▁the ▁dist al ▁part , ▁before ▁the ▁tip ▁of ▁the ▁act ine , ▁there ▁are ▁sharp ▁sp ines ▁arranged ▁in ▁ 3 - 4 ▁rows . ▁These ▁sp ines ▁are ▁directed ▁toward ▁the ▁tip ▁of ▁the ▁act ine . ▁Some ▁ap ical ▁act ines , ▁particularly ▁those ▁of