Question

Phylum Echinoderms

Classes: Crinoidea (sea lilies), Asteroidean (sea stars), Ophiuroidea (brittle stars), Exhninoidea (sea urchin & san dollars), Holothuroidea (sea cucumbers).

In short, compare and contrast traits of these classes such difference and similarity of their anatomy, water vascular system, gas exchange, body plan, development features, larval stages.

Thank yooooou!

Answer 1

Echinoderms are invertebrates that have a pentaradial symmetry,a spiny skin,a water vascular system,and a simple nervous system.

Crinoida(sea lilies)

Sea lilies and feather stars are example of Crinoidea.Both of these species are suspension feeders.They live both in shallow water and depths as great as 6000 meters.Sea lilies refers to the crinoids which,in their adult form,are attached to the sea bottom by a stalk.Crinoids are charactarized by a mouth on a top surface that is surrounded by a feeding arms.They have a U-shaped gut,their bottom is located next to the mouth.Although the basic echinoderm pattern of five-fold symmetry can be recognized,most crinoids have many more than five arms.crinoids usually have a stem attached to themselves to a substrate,but many live attached only as juvaniles and become free swimming as adults.

Asteroidean(sea stars)

The most well known echinoderms are members of class Asteroidea or sea stars.They come in a large veriety of shapes,colors and sizes with more than 1800 species known so far.The key characteristics of sea stars that distinguishes them from other echinoderm classes includes thick arms(ambulacra,singular,ambulacrum) that extend from a central disk where organs penetrate into the arms.Sea stars use their tube feet not only for gripping surfaces but also for grasping prey.Sea stars have two stomachs,one of which can protrude through their mouths and secrete digestive juices into or onto prey,even before ingestion.This process can essentially liquefy the prey,making digestion easier.

ophiuroidea(brittle stars)

Brittle stars belong to the class ophiuroidea.Unlike sea stars,which have plump arms,brittle stars have long,thin arms that are sharply demarcated from the central disk.Brittle stars move by lashing out their arms or wrapping them around objects and pulling themselves forward.Of all echinoderms,the ophiuroidea may have the strongesttendency towards 5-segment radial(pentardial) symmetry.ophiuroids are generally scavengers or detritivores.Small organic particles are moves into the mouth y the tube feet.ophiuroids may also prey on small crustaceans or worms.Some brittle stars,such as the six-armed members of the family Ophiactidea,are fissiparous(divide through fission),with the disk splitting in half.Regrowth of both the lost part of the disk and the arms occur,yielding an animal with three large arms and three small arms during the period of growth.

Exhninoidea(sea urchin and sand dollars)

Sea urchins and sand dollars are examples of Echinoidea.These echinoderms do not have arms,but are hemispherical or flattened with five rows of tube feet that help them in slow movement,tube feet are extruded through pores of a continuous internal shell called s test.Like other echinoderms,sea urchins are bilaterans.Their early larvae have bilateral symmetry,but they develope five fold symmetry as they mature.This is most apparent in the regular sea urchins,which have roughly spherical bodies,with five equally sized parts radiating out from their central axes.Several sea archins,however,including the sand dollars,are oval in shape,with distint front and rear ends,giving them a degree of bilateral symmetry.In these urchins,the upper surface of the body is slightly domed,but the underside is flat,while the sides are devoid of tube feet.This irregular body form has evolved to allow the animals to burrow through sand or other soft materials.

Holothuroidea(sea cucumber)

Sea cucumbers of class Holothuroidea are extended in the oral-aboral axis and have five rows of tube feet.These are the only echinoderms that demonstrate functional bilateral symmetry as adults because the uniquely-extended oral-aboral axis compels the animal to lie horizontally rather than vertically.Like all echinoderms,sea cucumbers have an endoskeleton just below the skin,calcified structures that are usually reduced to isolated microscopic ossicles joined by connective tissue.In some species these can sometimes be enlarged to flattened plates,forming armor.