Mollusks Mollusca, from the Latin root for “soft,” might seem an odd descriptor for these animals if you’re mostly familiar with mollusks in a culinary, shell-collecting, or decorative context. In many of our most familiar mollusks, the hard shell is widely considered either the most interesting and valuable part or the chief barrier between you and your meal. If you’re inclined to agree with the former, you view mollusks in much the same way as many snail and bivalve taxonomists do. If you hold the “barrier” view, you are in sympathy with a wide range of marine wildlife, in addition to billions of human diners. Either way, you have a valid point, but it neglects several important groups of mollusks. In addition to the bivalves and snails that immediately spring to the shell-focused mind, consider the octopus and squid, whose highly reduced “shells” are completely internal. And let’s not forget the slugs. There are several branches within the Gastropoda with reduced, barely noticeable shells, much like the cephalopods, and others whose shells have been entirely lost. Then there are the aplacophorans, which throughout their evolution never possessed a shell. If we are to define the mollusks using traits widely shared among the major groups, we must rely heavily on features of their soft tissue. RounDwOrmS Roundworms al Reference Guide Complementary and Alternative Medicine Guide Condition Roundworms Roundworms Introduction Roundworms, or nematodes, are parasites that can infect people. They usually live in the intestines. There are different kinds of worms that can cause infection, and they can range in length from 1 millimeter to 1 meter. Most often, eggs or larvae live in the soil and get into the body when you get them on your hands and then touch your mouth. Some can also get into the body through the skin. Like other parasitic diseases, roundworm infections happen more often in warm, tropical climates. Ascariasis is the most common roundworm infection, and affects as many as 1 billion people worldwide. Segmented Worms Segmented worms are bilaterally symmetrical. Their body consists of a head region, a tail region and a middle region of numerous repeated segments. Each segment is separate from the others by a structure called a septa. Each segment contains a complete set of organs. Each segment also has a pair of hooks and bristles and in marine species a pair of parapodia (appendages used for movement). The mouth is located on the first segment at the head-end of the animal and the gut runs through all segments to the end where an anus is located in the tail segment. In many species blood circulates within blood vessels. Their body is filled with fluid that gives the animal shape through hydrostatic pressure. Most segmented worms burrow in terrestrial soils or sediments at the bottom of freshwater or marine waters. sa segment worms
Posted on: Sat, 24 Jan 2015 10:27:59 +0000
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