Six-rayed spicules, called hexactines, give the hexactinellids their name. However, some early members of the group only possess spicules with four rays that intersect at right angles in a single plain, called stauractins. These may represent reduced hexactines or the primitive spicule type for hexactinellids. Hexactinellid megascleres have hollow canals in their center that are square in cross section.
As seen in the Antarctic sponge above, viewed from the side and from above, the overall body shape of hexactinellids is more regular than that of other sponges. However, no clear symmetries exist. Often, long tufts of spicules grow at the base of the sponge body acting as an anchor to the substrate. These tufts are presumably important in allowing hexactinellids to live on soft sediments. The body of hexactinellids is vase-like and contains both syncitial and cellular tissues. Syncitia predominate as all surfaces that contact water are syncitial. Syncitia with collars and flagella (choanosyncitia) line the chambers where flagellar beating creates water flow through the body.