Invertebrate paleontology
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All living echinodermata have:
Despite their very peculiar structure, they are classified with the Chordates! How is this possible?
Some groups have developed bilateral symmetry at least during parts of their lives; others remain 5-sided symmetric. Despite the adaptive value of bilaterality for free-moving animals, and the merits of radial symmetry for sessile animals, echinoderms confounded the rules by becoming free-moving but radial. That they evolved from a bilateral ancestor there can be no doubt, for their larvae are bilateral. They undergo a bizarre metamorphosis to a radial adult in which there is a 90o reorientation in body axis, with a new mouth arising on the left side and a new anus appearing on the right side. This constellation of characteristics is unique in the animal kingdom. It has both defined and limited the evolutionary potential of the echinoderms. Some forms use their water vascular system exclusively to move around with, other use muscles (sea cucumbers, brittle stars). Several forms of star fish can extrude their stomach through their mouth, and digest their food partially outside their body.
Traditionally, living Echinodermata have been subdivided into 5 groups:
The position of many fossils forms (which were all dumped into the group of the Crinoidea) made many people unhappy, because these forms were morphologically very different from stalked sea lilies. Nowadays, the fossil forms are divided into 20 to 25 different classes, quite a few of which do not have five-sided symmetry. See figure 1 for a simplified cladogram.
Simplified cladogram of Echinoermata; see also Echinoderm systematics on the Berkeley webpage.
Crinoids are pentamerous, stalked echinoderms with a cuplike body bearing five usually branched and commonly featherlike arms. Stalked crinoids, or "sea lilies", lived attached to the bottom, and filter food particles from the currents. Living crinoids are the only remaining attached suspension-feeding echinoderms. Crinoids have no head, no brain, although they do have a (not extensive) nervous system. They use a hydraulic system (the water vascular system) to move and catch food. The water vascular system is connected to the world outside the animal by a sieve plate (or madrepore plate).
Most of a crinoid's body consists of an endoskeleton composed of numerous calcareous pieces, called plates or ossicles. The guts of the crinoid animal is encased in the cup (calyx) that is typically composed of 2-3 circlets of plates. The mouth and anus are usually on the upper or oral surface of the animal, but some crinoids have the anus on their side, outside the arms.
The plates in the cup directly below an arms are called radial plates. Plates in a ring below the radial plates (and alternating with them in position) are called basal plates; there may be more than one ring of basal plates and the lower ones are then called infrabasal plates. In some crinoids the arms themselves are grown together to form the cup; such grown-together arms are called fixed brachial plates. Plates in between these grown-together arms are called interbrachials.
Each arm is an articulated series of plates (ossicles) extending outward from the body. Arms contain extensions of coelomic, nervous, water vascular, and reproductive systems and bear an ambulacral groove bordered by fingerlike tube feet (terminal extensions of the water vascular system), used in suspension feeding and respiration. Arms may branch or not. All living crinoids are pinnulate, that is, they bear a small side branch (pinnule) on alternating sides of successive ossicles along the arm. In living crinoids, the pinnules bear the food gathering tube feet. Pinnules arose in several lineages during the Paleozoic and are characteristic of all post-Palaeozoic crinoids.
All crinoids are passive suspension feeders, an ancient way of feeding (not energy intensive). They produce no feeding or respiratory current but, rather, rely on ambient water movement. In living crinoids, each featherlike arm that radiates from the central body bears an open ambulacral groove bordered by groups of three fingerlike podia, or tube feet, which are terminal extensions of the water vascular). The longest tube foot in group of 3 is held at a right angle and flicks passing food particles into the groove, The shortest tube foot wraps it in mucous secretions; ciliary tracts on the groove floor then transport it toward the mouth. Food particle size ranges from about 50 to 400 µm. Present crinoids eat protists (e.g., diatoms and other unicellular algae, foraminiferans, actinopods), invertebrate larvae, small crustaceans, and detrital particles.
The major nervous system consists of a nerve ring around the mouth and a major concentration of nerve cells opposite the mouth, from where nerves run through the ossicles in the arms, which sends nerves.
More recently (see figure below), it was realized that Echmatocrinus is related to, and possibly ancestral to, Crinoids, but that it can not be classified as a true crinoid (Rhombifera). More importantly, it was realized that the Inadunata are a polyphyletic group (remember cladistics??), and they are now split into the Cladida and the Disparida.
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