PORIFERA 



"5 



Fig. 1 02. Various types of sponge spicules. From Woods, a-e^ From Demo- 

 spongiae. /, From a hexactinellid. g and h, From extinct groups of sponges, 

 y, From Calcarea. a, With one axis (monaxon). b and c, With four axes 

 (tetraxon: 6 is a "calthrops", c a "triaene" spicule: tetraxon spicules 

 are found in the Tetractinellida ; the Monaxonida have monaxons only). 

 d and e. Irregular. /, With three axes (triaxon; four six-rayed spicules 

 united as part of a continuous skeleton by additional deposits). 7, A three- 

 rayed compound spicule formed by the union of monaxons. 



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