NerPhrag

NerPhrag - Todays lab dissection burrows into the...

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Unformatted text preview: Todays lab dissection burrows into the fascinating world of errant and tube dwelling polychaetes. As ususal, you will be partnering up and observing your partners work as well as your own. Nereis virens: You should expect to accomplish the following in your dissection of Nereis: 1. observe and sketch the cephalic region, pygidium, and parapods (one anterior, one posterior) 2. observe and sketch a cross section including the coelom, musculature, nerve cord, blood vessels and, if possible, nephridia 3. internal anatomy (logitudinal section) identifying the digestive, circulatory and nervous system Happy Hunting! Introduction Nereis virens is known popularly as the pile worm, clam worm or rag worm. It is an eastern species that is farmed commercially for bait. Small populations found locally began when anglars introduce them, loosing them from their hooks or releasing then after fishing. This errant polychaete is an active hunting predator with a homonomous body type with bilateral symmetry . The trunk segments are uniform and have the appearance of rings or annuli . The annuli are internally separated by intersegmental septa. Within each repeated segmented, you will find a portion of the nervous, circulatory and (usually) excretory system and a pair of (left, right) coelomic compartments. There is a pair of locomotory appendages on every segment except in the cephalic anterior region and on the posterior pygidium. Watch a living worm crawl slowly and quickly and then put it in seawater to watch it swim. Which movement seems more efficient? Tease the animal a little to see if it will evert its proboscis, revealing its chitinous jaws. External Anatomy Cephalic Region The cephalic region comprises the prostomium , where two pairs of ocelli, a pair of palps, and tentacles are located, and the peristomium , which has the mouth and cirri.Only a few animals will be fixed with their pharynx already everted to show the chitinous jaws and paragnaths. Share these with others. Most animals will have to be dissected to see the jaws at all. Pygidium The trunk ends posteriorly at the elongated pygidium . Teloblastic growth adds segments to the worm only here next to the pygidium. The pygidium has a pair of long ventral cerci. Locomotory Structures Each trunk segment bares a pair of parapodia . These function as limbs (especially the ventral neuropodium ) and gills (especially the dorsal notopodium ). Detach an entire parapodium from the trunk for examination. The details of the notopodium and neuropodium, along with the parapodial setae , are used for keying out species.The first and second pairs of parapodia differ from the rest. Internally; the first two have only a single acicula , a modified seta which acts as a skeletal support. All other parapodia have two aciculae and two setal bundles per parapodium. The dorsal and ventral cirri are tactile organs. Actually find and draw these structures, opening parapodia if need be to look inside....
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NerPhrag - Todays lab dissection burrows into the...

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