122 



THE INVERTEBRATA 



Fig. 103. Various types of sponge spicules. From Woods, a- e, are from Demo- 

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

 J, from Calcarea. a, With one axis (monaxon). b and c, With four axes 

 w-^u^^'u"' ^ '^ ^ "calthrops", c a "triaene" spicule), d and e, Irregular. /, 

 With three axes (triaxon ; four six-rayed spicules united as part of a continuous 

 skeleton by additional deposits), j, A three-rayed compound spicule formed 

 by the union of monaxons. 



