In sponges, spicules are secreted by amoeboid cells.

Spicules

Slender skeletal spikes made of calcium carbonate or silica; found in sponges and some other animals.

Amoeboid Cells

Cells that move or feed by forming extensions of their cytoplasm called pseudopodia, typical of amoebas.

Sponges

Simple aquatic animals belonging to the phylum Porifera, characterized by their porous bodies through which water flows.