Starfish

Lecture Notes

Phylum Echinodermata...spiny skin because most members of the group have defensive spines on the outside of their body. They are found only in the sea and as adults are either attached or can crawl around on the bottom. The 5 different groups include 1 sea lilies (attached)2. starfish, 3. brittle stars, 4. sea urchins, 5. sea cucumbers. For animals relatively high on the evolutionary scale, it is remarkable that a head has never been developed.. Their weird symmetry is called pentamerism, a form of radial symmetry with the body arranged around the mouth. This 5 point symmetry is displayed by most of the modern day echinoderms possibly making a stronger skeletal framework, but their larva have bilateral symmetry.(so did their primitive ancestors).

The skeleton is made up of many crystals of calcite (calcium carbonate). It supports the body wall or test and the reinforced structure may be soft, sea cucumbers or hard, sea urchins but is not a shell because it is covered by living tissue (sand dollar).

The drifting larva also has a skeleton which serve to support their delicate swimming processes and another feature is their water vascular system.

Water Vascular System

The branched tentacles, tube feet are arranged in a double row along the upper side of each arm bounding a food groove and along the branched arms (pinnules). The tube feet can be extended by hydraulic pressure from within the animal and much of the water vascular system is internal. They are supplied with fluid from a radial water canal which runs down the center of each arm just below the food groove and which sends a branch into each pinnule. The radial water canal of each arm connects with that of its fellows via a circular canal running along the gullet of the animal. Pressure is generated inside the system by the contractions of some of the tube feet and also by special muscles in the canal itself which generate local pressure increases to distend the neighboring tube feet.

The activities of the tube feet relate to gas exchange and food gathering. The feet are equipped with mucus glands and when a small fragment of drifting food collides with one, the fragment sticks to the foot which flicks it to the food groove and passes to the central mouth. The double rows ensure efficient feeding..

Some echinoderms also use the tube feet for locomotion (stars, urchins and cucumbers).

Suckered tube feet occur in all sea urchins and many sea cucumbers. Sea stars inhabiting hard substrates also have these feet and use them for locomotion and catching prey.

The fluid in the water vascular system is essentially sea water mixed with cellular and organic material. Apart from driving the tube feet, the fluid is responsible for transporting food and wastes, transporting CO2 and O2 and contains many cells, amoeboid coelomocytes which play a role in excretion, wound healing, and repair and regeneration. No excretory organs have been identified in echinoderms.

The nervous system is strange, as there is no head or aggregation of nerve organs, the only real sense organs are the rudimentary eyes of starfish, chemosensory receptors of sea urchins, and balance organs(stratocysts) in sea cucumbers, and that's it! There are simple receptor cells widely spread over the surface of the animal responsive to touch and chemicals in solution. There is a nerve cord running down each arm close to the radial canal and control the tube feet and body wall muscles. Echinoderms are sensitive to gravity and respond when turned upside down.

Prey The common European star feeds on mussels and oysters while the crown of thorn is well known for its selection of certain reef building coral. Some burrowing stars ingest their prey whole while invert their stomachs into the prey and digest it within the bivalve. Explain how it pulls open prey.

Sea urchins browse on algae on the surface and irregular urchins, sand dollars are more specialized and live partially buried in the sand using modified spines and tube feet to collect particles of detritus for food.

Sea cucumbers use their specialized tube feet around the mouth and sweep the surface of the mud for detritus.

Sexes are separate with a few being hermaphrodite, passing through a male stage before becoming functional females.

Phylum Chordata

These are higher animals possessing a single hollow, dorsal nerve cord and body cavity (vertebrates, ) However a number of lowly chordates display the phylums characteristics and are aquatic. The two sub-phlya are Urochordata (sea squirts) and Cephalochordata( lancelets).

Sea squirts are bottom dwellers growing attached to rocks or other organisms, and are encased in a thick protective tunic composed of a kind of cellulose or tunic (thus Lamarck's name, tunicate). The tubular heart pulses first in 1 direction and then in an opposite direction, changing every few minutes. . At the top is the inhalant siphon and on the side the exhalant siphon. There is no head. Water is pumped through the body, the gill collecting respiratory material and acts like a filter for food particles. Wastes are liberated from the anus near the external siphon. They may be solitary or colonial. Larva have an important role in selecting a settlement point and distributes the species. It is also the larva which displays the chordate notocord characteristic. Economically they are important as fouling organisms as well as evolutionary and zoological interest. They are the 1st animals in which alternation of generations was discovered. Another interesting, but unproven theory, (no fossils), is that vertebrates arose from tunicate tadpoles swimming up rivers where they could exploit the rich organic detritus coming down the river after the freshwater and land plants had become established. In such an environment the development of a backbone to aid in swimming would confer a great advantage. A mutation which produced NEOTENY (attaining sexual maturity in the larval stage) would eliminate the sessile adult.

Three classes occur...Ascidiacea are sessile, living singly or in complex colonial aggregations attached to submerged rocks, wharfs, ships etc. Some resemble black, velvety bananas, others are rounded and pink called sea peaches, others look like scarlet thumbs while others have the appearance of gnarled potatoes. One species, Botryllus, grows as a glistening black or purple mat studded with rosettes of yellow and pink openings like the petals of a flower.

The second class is Thaliacea comprising pelagic, usually glass clear animals, more or less barrel shaped. The third class the Larvacea are minute animals with long tails.

Lancelets have an elongated fish like blade form and the notocord extended into the head. There is no well developed brain, eyes or other sense organs. The adults live in shallow water inshore and are commonly burrowers.

 Lecture Questions

Echinoderm Reading and Questions

7. Echinoderm reading

1  How do sea cucumbers get their name?

2 Which organs may have poison glands on the sea urchin?

3.  What is Aristotles Lantern and how is it used?

4  Why do sea urchins hold up bits of seaweed ?

5.  What part of the sea urchin is eaten?

6  What is he difference between the habitat of sand dollars and heart urchins?

7.  How many classes of Echinoderms are known and how many are around today?

8. In what type of habitats are echinoderms found?

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