Taking a Look at Arthropods and Molluscs

A. Arthropods inherited many great synapomorphies and unique functions such as an exoskeleton, molting and respiration. These traits that arthropods inherited have opened up many great opportunities in their evolution but they have also block some possibilities and cause some hardship. Firstly, molting and the exoskeleton of arthropods are like a double edged sword. The rigid exoskeleton that the arthropods possess provides protection. This exoskeleton is also the fundamental building materials for an array of tool like appendages. The arthropod exoskeleton is secreted by epidermal cells. The outré layer of the arthropod is generally waxy, being composed of a firm lipoprotein layer underlain by layer of lipid. The cuticle of the arthropod is water impermeable so that the outer body surface cannot serve for gas exchange. This also enhances arthropods resistant to water loss by dehydration. The outermost layer (the epicticle) is very much thin; it contributes about 3% of the arthropods thickness. The other 97% is made up of procuticle, which is composed largely of the polysaccharide chitin. Now, from this short description one can that the exoskeleton is wonderful. However, it also has its downside or flaw. Since the exoskeleton is very hard and the epicticle is non-living, it cannot grow bigger small increments as the human skeleton does. Instead of this the arthropods must go through a process of shedding the old exoskeleton and expanding to a larger size before the new exoskeleton is harden. While going through this process, larger arthropods like the coconut crab are more susceptible to predation than smaller ones. Once a larger arthropod molt, gravity simply takes over and deforms the giant molted organism. Now, with no means o...

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...distributed on the body, protruding through 5 double rows of pores in the ambulacral plates of the test. The water vascular system of echinoid follows the archetypical pattern closely, with a single madreportite opening abnormally, as in the asteroids. A regular urchin has spherical symmetry and irregular urchins (sand dollars) have bilateral symmetry.

The Holthuroidea class or sea cucumbers are modified tube feet which are often feathery tentacles at the anterior end and each the tentacles could be used to capture food. The holothruian digestive system resembles that of the echinoids, except that it is greatly elongated. The water vascular system follows the typical echinoderm pattern, with the ring canal forming a ring about the esophagus. The water vascular system is not directly connected to the outside and the class of holothuroidea is bilaterally symmetric.