The Crinoid, belonging to Crinoida class and Echinodermata phylum is a marine creature, which resembles a flower grown under the water. It is found both, at depth as well as in shallow water. Today, it is not at all abundant and cannot be found commonly, but it was dominant in the Paleozoic period. Fossils of Crinoid, believed to be at least 450 million years old has been found. Many limestone beds found in the Paleozoic era consisted of skeletal Crinoid fragments. Some paleontologists consider the species Echmatocrinus as the oldest specimen derived from the fossil site, Burgress Shale in British Columbia. This species belongs to the Middle Cambrian era.
The word "Crinoid" is derived from 'Krinon' which means "a lily," in Greek. The Crinoid has a long and colorful history of more than 450 million years. The first fossils found were from the Paleozoic era. At that period, it was so abundant that the ocean bed was densely carpeted with Crinoid. During the Mississippian era, their marine environment was widespread in most of the continents, which led to the enormous growth of Crinoids. The Fossils of Crinoid are commonly found in the Permian and Pennsylvanian rocks in eastern Kansas. The abundantly found limestone beds in Europe and America, was made up of fragments of Crinoid. By the end of the Permian period, it was on the verge of extinction and this extinction was the largest extinction in the world history of life.
The Crinoid mainly has 3 body parts, namely, the stem, the calyx and the arms. The stem is similar to the plant stem as it holds and anchors the animal to the substrate or the ocean floor. The stems have disc -like bits arranged on the top portion, which vary in shape. It can be found in elliptical, round or in a star shape and has a hole in the center. The calyx is situated just above the stem, in a cup shape, which consists of the mouth, an anus and the digestive system. In the lower part of the calyx, the plate like structures that have five sides is arranged in 5 rows radically, actually forming the base of the third part. The third part is the arms that help to gather food. It is segmented and has hair like structures or cilia on the grooves. This helps to capture the food particles flowing in the water and propel it to the mouth. Usually the number of arms seen in the old fossils is only 5, whereas the modern species are found with many arms up to 200 and it is always in multiples of 5.
The Crinoids have separate female and male individuals. The Pinnules consist of the genital canals that produce gametes. The Crinoids do not possess any gonads. The sperms and the eggs are released from the ruptured Pinnules into the surrounding water. Later the fertilized eggs, upon hatching, discharge free flowing Vitellaria larvae. This free flowing period of larvae continues only for a few days, and later, it attaches itself to the sediments or any underlying substrate with the help of a gland that has an adhesive property. This gland is seen on the ventral surface of the larvae. Thus, it metamorphoses and turns into a young stalked Crinoid.
The Crinoid arms filter the small particles of food flowing in the water and consume it. Sticky mucus present on the tube feet captures any food particles passing by and flicks it inside the groove. Inside the groove, the cilia push the mucus to carry the food particles to the mouth. It does not have a stomach, and hence the mouth is connected to a small esophagus. This esophagus is then connected to the intestine, which is seen inside the calyx designed as a single loop. The end part of the intestine is joined to a small rectum, which directs the waste particles towards the anus.