Notice the variety of specialized structures near the anterior end of the clam worm in the photograph below. Molecular data suggests that segmentation evolved independently in the annelids, the arthropods, and the chordates because ancestors to each of these three groups were not segmented.
Annelids are bilateral, coelomate protostomes. The coelom is partitioned by septa crosswalls. The fluid-filled coelom acts as a hydrostatic skeleton. When the circular muscles that surround each segment contract, the segment becomes thinner and longer. When the longitudinal muscles that extend from one end of the segment to the other contract, the segment becomes shorter but thicker. Because muscles can only contract and cannot lengthen, other muscles are used to lengthen them.
In annelids, when circular muscles contract to lengthen the segment, the longitudinal muscles are lengthened. When the longitudinal muscles contract to make the segment shorter and thicker, the circular muscles become lengthened. Setae are bristles on the skin that anchor or help move the animal. Annelids exhibit specialization of the digestive tract. Some of these structures are the pharynx, crop, gizzard, intestine, and accessory glands.
Annelids have a closed circulatory system. A pair of cerebral ganglia function as a simple brain. A ventral nerve cord extends the length of the animal and connects to a pair of fused ganglia mass of nervous tissue in each segment.
The ganglia within each segment function to coordinate muscle contractions. Examine the exterior of an earthworm and find the ventral surface. Place the worm in a dissecting pan with the ventral surface down. Identify the clitellum. This structure produces mucous needed for reproduction. Find rows of setae along either side of the ventral surface. These help provide traction as the animal moves through the soil. Find the dorsal blood vessel. Since they are polychaetes, their bodies consist of multiple segments, each with a pair of parapodia.
Their heads have two pairs of eyes at the sides each with lenses. Each pair of eyes is orientated in a different plane. Females can grow up to 20mm while males grow up to about 12mm. Their nickname comes from the fact that they are bioluminescent when they rise to the surface during the mating period. The Bermuda Fireworm typically lives in protected rocky bottoms and they swim to the surface 2 to 3 days after each full moon to spawn.
These worms are only bioluminescent during mating times. They also follow a lunar periodicity pattern, which is why the swim to the surface to mate around the time of a full moon. Lunar periodicity is typically seen in response to changes in light intensity from the sun to light intensity from the stars. Typically, the female worm appears first, swimming up from the rocky bottom to make circles at the surface. The glowing, though amusing for human spectators, is designed to attract the attention of males at the bottom.
If you look closely, you can see the males rush up to the surface to meet the females. These animals lack a well-developed respiratory system, and gas exchange occurs across the moist body surface. Annelids show well-developed nervous systems with a nerve ring of fused ganglia present around the pharynx. The nerve cord is ventral in position and bears enlarged nodes or ganglia in each segment. Annelids may be either monoecious with permanent gonads as in earthworms and leeches or dioecious with temporary or seasonal gonads that develop as in polychaetes.
However, cross-fertilization is preferred in hermaphroditic animals. These animals may also show simultaneous hermaphroditism and participate in simultaneous sperm exchange when they are aligned for copulation. Phylum Annelida contains the class Polychaeta the polychaetes and the class Oligochaeta the earthworms, leeches and their relatives. The many chetae of polychaetes are also arranged within fleshy, flat, paired appendages that protrude from each segment called parapodia , which may be specialized for different functions in the polychates.
The subclass Hirudinea includes leeches such as Hirudo medicinalis and Hemiclepsis marginata. The class Oligochaeta includes the subclass Hirudinia and the subclass Brachiobdella. A significant difference between leeches and other annelids is the development of suckers at the anterior and posterior ends and a lack of chaetae.
Additionally, the segmentation of the body wall may not correspond to the internal segmentation of the coelomic cavity. This adaptation possibly helps the leeches to elongate when they ingest copious quantities of blood from host vertebrates. The subclass Brachiobdella includes species like Branchiobdella balcanica sketi and Branchiobdella astaci , worms that show similarity with leeches as well as oligochaetes. Figure 3. The a earthworm, b leech, and c featherduster are all annelids.
Phylum Annelida includes vermiform, segmented animals. Segmentation is seen in internal anatomy as well, which is called metamerism. Annelids are protostomes.
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