CHARACTERISTICS

The phylum Echinodermata gets its name from the Greek, literally meaning "spiny skin", and includes sea stars, urchins, brittle stars, and sea cucumbers.

Echinoderms are characterized by their PENTAMEROUS RADIAL SYMMETRY, that is, the body can be divided into five parts (or appendages) which point outward from the center of the body.

starfish gif


Characteristic of all echinoderms is the presence of a water vascular system and internal CALCIUM CARBONATE SKELETON covered with spines and skin. The skeleton varies with the type of echinoderms.

In sea stars and brittle stars, the skeleton consists of multitudes of small calcareous plates that move with one another, forming flexible joints. Sea urchins and sand dollars have ossicles that are fused, forming a rigid skeletal shell (test). In sea cucumbers, the calcareous plates have degenerated and are buried in the fleshy body

 This phylum exists exclusively in the sea

 
sea urchin skeletal shell

Sea urchin skeletal shell

 

 

ORGANIZATION


The WATER VASCULAR SYSTEM is a hydraulic network of canals runs throughout the body, usually ending in a series of tube feet. By examining the ventral side of a sea star, for example, one will be able to see hundreds of tiny feet on each appendage of the star. These are called TUBE FEET. By varying the internal water pressure, the echinoderm can extend and contract its tube feet for locomotion, food collection and respiration.


starfish tube feet
Starfish tube feet

 

 


REPRODUCTION


Most species of echinoderms are diecious, meaning there are separate male and female individuals. Although reproduction is usually SEXUAL, involving EXTERNAL FERTILIZATION of eggs by spermatozoa, several species of echinoderms, such as sea stars and sea cucumbers, can also reproduce asexually.

ASEXUAL reproduction in echinoderms usually involves the division of the body into two or more parts (known as FISSION) and the reproduction of missing body parts. Many sea stars can regenerate a lost portion only if some part of the central disk is present.

 

 

CLASSIFICATION

There are about 7,000 species of echinoderms.

Here we show a small classification to know the class to that an echinoderm belongs:
 
  Whith a "test" (rigid calcarean skeleton)
Class ECHINOIDEA
  

Star-shaped body flexible arms
Class OPHIUROIDEA
  
Non flexible arms
Class ASTEROIDEA
  
  Worm-like and usually soft-bodied
Class HOLOTHUROIDEA
  
1
 A cup-shaped head (calyx) to which are attached long, and branching arms
Class CRINOIDEA
  
 


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