Above is a lovely illustration of various examples of the Cystoidea. The bottom left and right creatures are Edrioasteroids. Drawing by Ernst Haeckel. Image taken from Wikipedia.
There are two main groups of living Echinoderms; the Pelmatozoa; which include only Crinoids today, have a dorsal stem (opposite the mouth) extending from the theca (main body) of the organism. This usually acts as a holdfast. Extensions of the water vascular system (arms in crinoids) arise from around the mouth and gather food. The terms dorsal and ventral are fairly arbitrary decisions when talking about echinoderms and there is little clarity in how their axis correspond to those of other phyla; dorsal seems to mean opposite the mouth, ventral corresponding to the mouth surface; but the terms oral and aboral are also used. Extinct classes of Pelmatozoans are generally lumped in the paraphyletic clade; Cystoidea (Cambrian to Permian). Cystoids are basically any Pelmatozoan that is not a member of the surviving superclass; Crinozoa, which almost certainly arose from them. They possess specialized respiratory structures in their theca, these may be pores in between plates in the theca (for example the epispires of Eocrinoids), or external structures on the surface of the thecal plates (rhomboid canal systems of the Rhombifera), pores whithin the plates themselves (diplopores and humatipores of the Diploporita) or foldings of the theca (Blastoidea). Many Cystoids possessed extensions of the ambulacral series (plates associated with the water vascular system and tube feet, usually involved in gathering food and passing it to the central mouth) called brachioles which are derived from the cover plates of the ambulacrum and contain extensions of the water vascular system. The arms of Crinoids may appear similar, but these are direct extensions of the theca and contain extensions of the main body cavity (additionally the gonads, which are within the theca of Cystoids are present in the arms of Crinoids) as well as just the water vascular system. They are also uniserial as opposed to biserial, with the ambulacral system held externally on the surface of the brachials (arm plates).
The above image shows representatives of the 5 Eleutherozoan classes alive today.
The second group; the Eleutherozoa is much more diverse today and includes animals in which the oral surface is either very large and encompasses most of the surface of the organism (Echinoids and Holothuroidaea) or about half of the surface of the organism (e.g. Asteroids and Ophiuroids). Eleutherozoans are generally free living creatures without a stem. They include the extinct Edrioasteroidea which had the mouth and ambulacra facing upwards as in Pelmatozoans. Stellate forms with the ambulacra and mouth facing downwards include carnivorous Asteroids (starfish) and Ophiuroids (brittle stars). The Echinozoa includes groups in which the ambulacra extend over most of the body surface and include today the Echinoids (sea urchins) and the Holothuroidea (sea cucumbers).
However the above groups are by no means the entire diversity of Echinoderms that has ever existed. All modern Echinoderms are crown-group echinoderms with a pentaradial symmetry. In this post I will cover the controversial and enigmatic stem group Echinoderms. I have decided to ignore the Helicoplacoids for now. These early Cambrian Echinoderms seem to show indications of a radial symmetry and have unambiguous ambulacra, indicating that they are the most crownward of the Homalozoa.
Above is a simplified cladogram of the Deuterostomes as they are currently understood. From (http://biology.fullerton.edu/biol261/ch/ch25.html).
Above are various representatives of the Vetulicolia from the Cengjiang biotia in China. Image taken from Wikipedia. Artist: Santon.F. Fink.
Above are shown 2 specimens and interpretative drawings of Vetulocystis from the Chengjiang biotia in China. Image taken from http://www.palaeocritti.com/by-group/echinodermata/vetulocystis.
Above is a diagram showing plate homologies in basal Homalozoans from the recent paper by Zamora, Rahman and Smith 2012. Image taken from http://scientificillustration.tumblr.com/post/25279474245/diagram-showing-inferred-homologies-between.
Above is a diagram of the Cinctan Trochocystites in dorsal (left), ventral (right) and anterior (bottom). Image taken fromhttp://geologie.vsb.cz/paleontologie/paleontologie/zoopaleontologie/DEUTEROSTOMIA/Homostelea.htm.
Above is a diagram of the dorsal aspect of Cothurnocystis; a Cornute. On the top left of the theca can be seen a row of possible gill slits (cothurnopores). The mouth is believed to be on the bottom right hand side of the theca in this image. To the left can be seen the tripartite stele. Image taken from Wikipedia.
Above; the mitrate Renocystis in what is generally believed to be dorsal (left) and ventral (right). Image taken from http://www.fossilien.de/artikel/bundenbach/111.htm.
Above is the solute Syringocrinus. Image taken from http://palaeos.com/metazoa/deuterostomia/homalozoa/soluta.html. This is a website well worth visiting for more information.
Lingula in feeding position.
Ubaghs (1975). Early Palaeozoic Echinoderms.
A.B.Smith (2005). The pre - radial history of early echinoderms. Geol. J. 40: 255-280.
http://palaeos.com/metazoa/deuterostomia/deuterostomia.htm (yet again; well worth a look!)
2005 Vol. 50 No. 20 2342—2354 Shu Degan. On the Phylum Velulicolia. Chinese Science Bulletin
Zamora S, Rahman IA, Smith AB (2012) Plated Cambrian Bilaterians Reveal the Earliest Stages of Echinoderm Evolution. PLoS ONE 7(6): e38296.
A.B.Smith (2005). Fossil Invertebrates/ Echinoderms (other than Echinoids). 334-341.
D.-G. Shu et al (2004). Ancestral echinoderms from the Chengjiang deposits of China. Nature.
John W. Murray (1985). ATLAS OF INVERTEBRATE MACROFOSSILS. LONGMAN.
Ruta, M. 2003.Aspecies−level supertree for stylophoran echinoderms.
Acta Palaeontologica Polonica 48 (4): 559–568.