Echinoderms living species

The phylum Echinodermata comprises about 7000 living species [177]. Echinoderm means spiny-skinned and these organisms are characterised by the tube feet, which they use to move about. These have suction discs on the ends, which operate by an internal bulb pumping water in and out of the foot, causing expansion and contraction. The phylum is sub-divided into five classes the asteroids (sea stars), the holothurians (sea cucumbers), the crinoids (sea lilies), the echinoids (sea urchins) and the ophiuroids (brittle stars) [178]. As stated in the introduction to this review, sulfated sterols and saponins, which comprise the majority of echinoderm metabolites containing sulfur, are not included here. 

The absence of any osmoregulatory function may explain the fact that echinoderms have not been able to adapt to freshwater. However, some species, such as Asterias rubens, have been found in the Baltic Sea, where the salinity is only 8%o, and holothurians are present in the Black Sea (18%o) the average salinity of oceans is 35%o. The water in which echinoderms live moves relatively freely through their body, suggesting that these invertebrates are in osmotic and ionic balance with their environment. 

There is a lesson to be learned here if a fish species responds in a certain way to a feature of the environment, the range of its response may be limited according to the environment where it spent its early life. The food of cod on the Faroe Bank consists of organisms that live on or near the sea bottom, such as crustaceans, brittle stars and other echinoderms, and small fish. It is not known how the food supply varies through a whole year, but in our limited experience (Love et al., 1974) we found that the ranges of water contents were almost identical in fish caught on 6 June 1966 and on 13 September 1968. The dates of similar voyages in other years lay between these two extremes. 

Mesozoans are parasites of a wide range of marine invertebrates, including flatworms, roundworms, mollusks, and echinoderms. Two main groupings have been recognized the order Qrthonectida, which are free-living within their hosts, and the Dicyemida, which have specialized cells on the body wall that serve to attach them to the walls of the kidneys in species such as octopus and squid. 

The appearance of bilateral symmetry (two halves which are mirror images of each other) was an important evolutionary breakthrough. The most primitive bilaterally symmetrical animals are the flatworms (phylum Platyhelminthes). Platyhelminthes gave rise to the coelo-mates, which have a coelom, or internal body cavity. The coelomates split into two evolutionary lines, the proto-stomes (molluscs, annelids, and arthropods) and the deuterostomes (echinoderms and chordates). A few phyla, such as the phylum Bryozoa, are intermediate between the two lines. Of the nearly 20,000 species of bry-ozoans known, only 3,500 are stUl living. 

Less obvious, but just as numerous, are flatworms and segmented worms. Some species are free living, but many inhabit tubes just under the surface of the sand or mud. Crawling slowly among theses relatively simple animals are the larger arthropods—mollusks and echinoderms. 

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