Crinoid

Crinoids are living fossils of which there are about 600 recognized species. Crinoidea is a small class compared to the others in the phylum Echinodermata. They are commonly known as sea lilies because they resemble plants, and many have considered them plants, but they are definitely animals. 

Anatomy
Crinoids have pentameral symmetry. They have tube feet operated by a water vascular system. They also have plates made from calcite.

The arms of a crinoid extend from the calyx (the cup-like part of the organism that contains the internal organs of the crinoid). They are used to grab and filter out any plankton that comes near. Some crinoids have a lot of arms, while others have very few. The number of arms varies greatly so it's hard to put an estimate on the number of arms a crinoid will have. One of the explanations for this is possibly due to how much food is available for the crinoid. If the water has a lot of plankton, then the crinoid doesn't need as many arms to catch food, while if there is not a great supply of food, the crinoid will need a larger number of arms to comb and filter the plankton from the water.

All of the arms on the crinoid have tiny pin-like projections. They are called pinnules. The purpose of the pinnules is to fan out the arms by increasing the surface area in order to catch its food floating in the water more easily.

Crinoids also have stalks. Some crinoids have longs stocks (up to a few meters long), while others have short stocks (only a few inches long). The stocks are connected to the holdfast or stem, which is used to anchor the crinoid to the ocean floor. Some crinoids are stemless. By moving their arms up and down through contraction and relaxation of muscles, crinoids are able to swim slowly through the water.

The skeleton of the crinoid is made up of hundreds of tiny plates that usually fall apart when the animal dies.

Reproduction
The basic idea of crinoid reproduction is that the male releases his sperm into the water, while the female releases her eggs into the water. That is where the eggs are fertilized.

The embryos of the new crinoids develop in the water column. After awhile, they eventually form themselves on the reef wall.

Ecology
Crinoids are most common in the deep ocean. They attach themselves to the floor or reef walls, and bury their stems into a substrate to anchor themselves down, and prevent them from floating away. Many crinoids are primarily nocturnal but they are seen during the day with their arms rolled up.

Their primary source of food is plankton or other microscopic organisms. They catch them with their tube feet attached to the pinnules on the arms, which are covered by a mucus substance. The captured prey gets pushed into tiny grooves along the arms, where cilia pull the food to the crinoid's mouth. Both their mouth and their anus are situated on the upper side.

Fossil Record


Crinoids are a living fossil existing virtually unchanged from the ancestors in the fossil record. Crinoids are first found in the geologic age known as the Ordovician Period, but there are many more fossils formed from the Paleozoic Era. It is believed by uniformitarian geologists that they were very abundant during that time period because of shallow sea water setting. They are usually found in limestones, but sometimes they are found in other sediments.

They are vital for forming sediment because limestone, where crinoids are very abundantly found, is often composed of the remains of the exoskeletons of crinoids. 

Related References

 * Crinoidea University of Michigan Museum of Zoology
 * class Crinoidea Earth Rochester
 * Crinoidea Geocities
 * Fossil Record Earth Rochester
 * Sea lily article on Encyclopedia.com