Sponges (Hexactinellida)

The Coelenterata are an obsolete taxon used to group together the Cnidaria, Ctenophora and Porifera.  The Hexactinellida are thought to be a basal group of the Porifera because of their simple, mainly asconoid, cup shape (1 – 7).  Other evidence for this hypothesis are their simple, siliceous, four or six pointed spicules (a & h), the presence of a syncitial epidermal layer (indiscrete cells characterised by a large cytoplasm and many nuclei) and their inability to retract the feeding choanocytes in response to stimuli.  Many of the other spicules pictured here (letters besides a & h) are probably broken or malformed spicules as environmental conditions play a large role in spicule formation.