Question

7. Why are the echinoderms exclusively marine? In other words, what anatomical feature precludes (prevents) them from adapting to freshwater or land? Explain. Phylum Chordata 8. What are the synapomorphies of chordates? What is the difference between a notochord and a nerve cord? 9. What were the advantages and challenges for vertebrates colonizing land? How did early amphibians partially solve them? Why might limbs rather than fins have been an advantage, even if the animal spent all or most of its time in the water? 10. How did early reptiles and mammals improve on the solutions that amphibians started? Now add adaptations of apes and hominins, including humans.

Answer 1

Ans 7) The echinoderms ae exclusively marine organism which have radial symmetry and each of the segment of the body consists of a duplicate set of all the organs. They consist of a water vascular system essentially developed for their locomotion and predation. They can pump sea water through a structure called madreporite and this helps it to move by filling its tube feet with water which helps them to extend forwards. They retract back with help of muscles that are located within the tube feet. Most of them have sexual mode of reproduction but some of them can also reproduce asexually by regenerating the body parts. They cannot survive in freshwater or in land because their system and morphology is developed in a way that it can only survive in marine water on seabed or on benthic layer. The anatomical feature which prevents them from adapting to seawater or land is because of the presence of water vascular system. This helps them with gaseous exchange, feeding, locomotion as well as sensory reception. The system opens to the exterior with help of a sieve-like structure called as madreporite which is linked to stone canal that extends to ring canal. From these canals, short lateral canal branches off that ends in ampulla. A certain part of the ampulla can protrude through pore forming the tube feet. The echinoderms pumps water through the madreporite and fills water in the tube feet which helps in distribution of nutrients as well as its locomotion. The mechanism of the echinoderm through water vascular system can be run only in the marine environment and hence this anatomical feature prevents it from adapting to land or freshwater. The water vascular system is an important oxygen source and they are isotonic with their environment. The concentration of the salt they have in their body is equal to the concentration of salt in sea water. They are extremely sensitive to salt concentration and since the concentration of salt concentration is low in freshwater as compared to marine, the echinoderm tend to absorb the water, swell up and burst.