Morphological relationship between sea urchins and starfish

Echinoderms (starfish, brittle star, sea urchin, feather star, sea cucumber)

Sea stars or starfish (Echinoderms like Valvatida, Comb Sea Star, Spinulosida, .. Many animals live in symbiotic relation with sea urchins. Sea-urchins, sand-dollars, brittle-stars and starfish are the . echinoids appear odd because of their range of morphological disparity characterized reason to think that the difference between extant and extinct echinoderms. Story: Starfish, sea urchins and other echinoderms. A feather star. The stars that live in the ocean External links and sources The sea urchin (or kina) has lots of spines and looks like a small hedgehog. Sand dollars have.

Some brittle stars and sea stars can reproduce asexually by breaking a ray or arm or by deliberately splitting the body in half.


Each half then becomes a whole new animal. Echinoderms are protected through their spiny skins and spines. But they are still preyed upon by shells like the triton shellsome fish like the trigger fishcrabs and shrimps and by other echinoderms like starfish which are carnivorous.

Echinoderms serve as hosts to a large variety of symbiotic organisms including shrimps, crabs, worms, snails and even fishes. Sea stars starfish Characteristics of sea stars or starfish Sea stars are characterized by radial symmetry, several arms 5 or multiplied by 5 radiating from a central body.

Mouth and anus are close together on the underside, the anus is at the center of the disc together with the water intake madreporite.

The upper surface is often very colorful. Minute pincer-like structures called pedicellaria are present. These structures ensure that the surface of the arms stay free from algae. The underside is often a lighter color. There are a few starfish that have 6 or 7 arms, for example Echinaster luzonicus or Protoreaster, some even more like the elven-armed sea star Coscinasterias calamaria. Others normally have 5 arms but now have more arms, because after an injury an arm divided and grew into two arms.

Ecology and range or sea stars The starfish lives everywhere in the coral reef and on sand or rocks. Behavior of sea stars The ability of an organism to grow a body part that has been lost Autotomy The spontaneous self amputation of an appendage when the organism is injured or under attack. The autotomized part is usually regenerated. Budding Is asexual reproduction in which an outgrowth on the parent organism breaks off to form a new individual Fission Self-division into two parts, each of which then becomes a separate and independent organisms asexual reproduction The majority of sea stars are carnivorous and feed on sponges, bryozoans, ascidians and molluscs.

Some starfish are specialized feeders, for example the crown-of-thorns that feeds on life coral polyps. Starfish have no hard mouth parts to help them capture prey. The stomach is extruded over the prey, thus surrounding the soft parts with the digestive organs.


Digestive juices are secreted and the tissue of the prey liquefied. The digested food mass, together with the stomach is then sucked back in.

Specialised muscles control the protrusion of the apparatus and the action of the teeth, and the animal can grasp, scrape, pull and tear. The periproct contains a variable number of hard plates, five of which, the genital plates, contain the gonopores, and one is modified to contain the madreporitewhich is used to balance the water vascular system. The entire chewing organ is known as Aristotle's lantern from Aristotle 's description in his History of Animals.

The urchin has, also, five hollow teeth inside, and in the middle of these teeth a fleshy substance serving the office of a tongue. Next to this comes the esophagusand then the stomachdivided into five parts, and filled with excretion, all the five parts uniting at the anal vent, where the shell is perforated for an outlet In reality the mouth-apparatus of the urchin is continuous from one end to the other, but to outward appearance it is not so, but looks like a horn lantern with the panes of horn left out.

D'Arcy Thompson However, this has recently been proven to be a mistranslation. Aristotle's lantern is actually referring to the whole shape of sea urchins, which look like the ancient lamps of Aristotle's time. Instead, the mouth is surrounded by cilia that pull strings of mucus containing food particles towards a series of grooves around the mouth.

At the top of the lantern, the pharynx opens into the esophagus, which runs back down the outside of the lantern, to join the small intestine and a single caecum. The small intestine runs in a full circle around the inside of the test, before joining the large intestine, which completes another circuit in the opposite direction. From the large intestine, a rectum ascends towards the anus. Despite the names, the small and large intestines of sea urchins are in no way homologous to the similarly named structures in vertebrates.

An additional tube, called the siphon, runs beside much of the intestine, opening into it at both ends. It may be involved in resorption of water from food. Radial canals lead from here through each ambulacral area to terminate in a small tentacle which passes through the ambulacral plate near the aboral pole. Lateral canals lead from these radial canals, ending in ampullae.


From here, two tubes pass through a pair of pores on the plate to terminate in the tube feet. This coelomic fluid contains phagocytic coelomocytes, which move through the vascular and hemal systems and are involved in internal transport and gas exchange. The coelomocytes are an essential part of blood clottingbut also collect waste products and actively remove them from the body through the gills and tube feet.

These thin-walled projections of the body cavity are the main organs of respiration in those urchins that possess them. Fluid can be pumped through the gills' interiors by muscles associated with the lantern, but this does not provide a continuous flow, and occurs only when the animal is low in oxygen. Tube feet can also act as respiratory organs, and are the primary sites of gas exchange in heart urchins and sand dollars, both of which lack gills. The inside of each tube foot is divided by a septum which reduces diffusion between the incoming and outgoing streams of fluid.