Starfish

**Starfish**, or **sea stars**, are Echinoderms of the Class Asteroidea. They have five or more arms and can be quite large. The Sunflower sea star (*Pycnopodia helianthoides*) is the largest: fully grown, its arm-span is about a metre. This is a bit larger than the famous Crown-of-thorns starfish, which lives on coral polyps.

Although starfish are invertebrates, they do have a kind of skeleton. The bodies of starfish are composed of calcium carbonate plates, known as 'ossicles'. These form the endoskeleton, which takes on a variety of forms such as spines and granules.

They have a primitive nervous system, but not a brain. They also do not have blood, instead they use sea water to pump things around their bodies.

There are over 1500 different species of sea star. Most sea stars are predators. They eat mussels, clams, and other bivalves. Occasionally, they catch small fish.

## Feeding 
: note the tube feet]]

### Senses 
The movement of starfish is guided by their senses of touch and sight. There are five 'eyes', light-sensitive cushions, one at the end of each arm. These and the tube feet are connected to nerve fibres, so these animals are more complex than might appear.

### Tube feet 
Although starfish started off as filter-feeders, they evolved to become major predators of shell-fish (the brachiopods and bivalves). They can also eat small crustacea and fish. Their tube feet developed suckers, perhaps originally to improve movement. Later, they were used to open shell-fish.
:"Suckered tube feet may not have been present in any Palaeozoic sea star".

### Feeding methods 
The shells of brachiopods and bivalves are held together by strong muscles. What the starfish does is clamp hold of them on either side with its tube feet, and apply a steady pull. The starfish, with its muscles and hydraulic system, can pull for much longer than any bivalve muscle can withstand. Apparently, ten minutes are usually enough to open the shell a bit. Then the starfish slips its stomach inside the shell. The stomach can get through a slot as narrow as 0.1mm. The starfish then dissolves the mollusc where it lives, absorbing the nutrients. This digestion process takes much longer than opening the shell, perhaps a couple of days.

Some species swallow the shell whole, and dissolve the contents inside their stomach, then push out the shell afterwards.p45

The ability of starfish to eat brachipods and bivalves developed in the Mesozoic, especially in the Jurassic and Cretaceous. This was part of the *Mesozoic marine revolution*, which transformed the sea-floor fauna. Weakly defended and static shellfish disappeared, and more heavily armoured or more mobile shellfish flourished.

## Regeneration 
Starfish, like many sea creatures, are able to *regenerate* (grow back) parts of their bodies. Starfish are better at regeneration than most other creatures. Not only can a new ray grow when a ray is torn off, but if the torn off ray has even a small piece of the central disk still attached, a whole new starfish can grow from the one ray.p35

Because starfish like to eat clams and oysters, fishermen who gather shellfish have tried for years to get rid of them. To kill the starfish, fishermen would catch them, slice them right in half, and throw them back in the ocean. However, because starfish can grow back parts of their bodies, they were actually  the number of starfish.

## References 
## Other websites 
* Jan Parmentier: Growth of a starfish: the development of the larva of an echinoderm. Micscape Microscopy and Microscope Magazine
* BBC http://www.bbc.co.uk/nature/wildfacts/factfiles/418.shtml 
* from the Tree of Life project 
* Classification of the Extant Echinodermata