emerita

emerita - Today's dissection explores the intricate design...

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Today's dissection explores the intricate design of a decapod crustacean. The example we are using is the mole or sand crab Emerita analoga . Emerita is an anomuran decapod along with other crabs such as hermit crabs and porcelain crabs and can be found buried in the sand in the surf zone of beaches on the West Coast of the United States. You should at minimum get the following from your experience today in lab: 1. observe and sketch the external anatomy of your crab 2. observe (no sketch necessary) the antennas 3. observe and sketch the structure of maxillipeds 1,2 & 3 4. dissect and sketch the circulatory system (including gills) 5. dissect and sketch the digestive and reproductive system (after removing the heart). The Antennas Emerita has an intricate system of antennae to gather both food and oxygen. The animal has two sets of big antennae, a primary and a secondary pair. The former is located on either side of the central rostral spine. The animal buries in the sand facing seaward and points its primary antennae up and forward together, forming a V-shaped trough. Could this be a means of funneling water down to the gills? How does the water get down to the those gills even if it indeed is given this channel to go through? -- after all, the crab's
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exoskeleton lacks cilia that might beat water along. As you dissect this animal, keep trying to think of what its structures actually do, and how they work. That's part of their beauty: that they work . The second antennae are used for filter-feeding. The crab is capable of straining particles down to 4-5 microns in size from receding waves. These particles are trapped in the criss- crossing bristles (setae) on either side of each antennae, and the antennae are alternately swung in and scraped clean by the maxillipeds. When the crab is not feeding, its long second antennae are folded down under the thoracic appendages along the mid-ventral body wall. Why does Emerita feed only on receding waves? What might it do, instead, during incoming waves? Using fine forceps, remove one of the second antennae and look at it under the dissecting scope's highest magnification. Fiddle with the light to reveal extra details. Then, look at the attachment of the second antenna you have left on the crab. Notice the exquisite joint system that allows the animal not only to rotate the antenna to an upright position but also to form a locking mechanism for holding the antenna erect against the force of the retreating waves. NASA engineers with their big bucks ought to see what this little crab engineers for free?
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The Thoracic Limbs Well, is it a boy of a girl? Females of this species are larger (15-30 mm) than males (10- 20 mm). There may be sequential hermaphroditism, too; that is, juveniles may go through a male phase and only eventually become a female -- in some populations or under some conditions. (Odd sex-ratios suggest this -- they also suggest sex-dependent predation patterns -- or maybe differential recruitment by sex -- or, well, you name it, because most
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This lab report was uploaded on 04/16/2008 for the course BIO 120 taught by Professor Idk during the Winter '04 term at UCSC.

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emerita - Today's dissection explores the intricate design...

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