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.

Sponges (Lithistida)

The Coelenterata are an obsolete phylum which encompassed sponges and Cnidarians (with ctenophores within Cnidaria).  The  order Porifera is now classified as a phylum containing all filter feeding sponges.  The most basal of the eumatetazoan phyla, Porifera are derived from the sister taxon to eumetazoans, single celled choanoflagellates.  This wall chart shows representative body forms (1-4), internal structures (5-8) and spicule shapes (7 & 9) of an obsolete class now distributed throughout Porifera.  The group Lithistida has been retained as an order whose members are mainly known as fossils but still have some extant representatives.