There are four traditional groups of flatworms, the largely free-living turbellarians, which include polycladid marine worms and tricladid freshwater species, the ectoparasitic monogeneans, and the endoparasitic trematodes and cestodes. The circulatory system is derived from the coelomic cavity of the embryo. Some … The group is characterized by the ciliated, wheel-like corona, located on their head. At rest, the proboscis is inside a long tube that may take up a considerable portion of the worm's length, lying just above the gut. Their circulatory system consists of a closed loop formed by a connected pair of lateral blood vessels. Rhabdites are rodlike structures discharged in the mucus produced by some free-living flatworms; Eucoelmate protostomes are schizocoels, in which mesoderm-producing cells typically migrate into the blastocoel during gastrulation likely serve in both defense and to provide traction for ciliary gliding along the substrate. There are four traditional groups of flatworms, the largely free-living turbellarians, which include polycladid marine worms and tricladid freshwater species, the ectoparasitic monogeneans, and the endoparasitic trematodes and cestodes. Cavities then develop within the cell mass to form the coelom. Eggs and sperm are released into the water, and fertilization occurs externally. Each proglottid contains a sexually mature reproductive system that can cross-fertilize within a single tapeworm. The intestine is present in the form of diverticular pouches and ends in a rectum that opens via an anus. Although the circulatory fluid contains cells, it is often colorless. The mature proglottids detach from the body of the worm and are released into the feces of the organism. Nervous System. The juvenile worm infects the intermediate host and takes up residence, usually in muscle tissue. Some species may also have a dorsal vessel or cross-connecting vessels in addition to lateral ones. A mouth opening that is ventral to the rhynchocoel leads into the foregut, followed by the intestine. Two large cerebral ganglia—concentrations of nerve cell bodies at the anterior end of the worm—are associated with photosensory and chemosensory cells. These ribbon-shaped animals bear a specialized proboscis enclosed within a rhynchocoel. Most animals in phylum Nemertea are dioecious, although freshwater species may be hermaphroditic. The blood vessels are contractile, although there is usually no regular circulatory pathway, and movement of blood is also facilitated by the contraction of muscles in the body wall. In these phyla, we also see the beginning of cephalization, the evolution of a concentration of nervous tissues and sensory organs in the head of the organism—exactly where a mobile bilaterally symmetrical organism first encounters its environment.Lophotrochozoa are also protostomes, in which the blastopore, or the point of invagination of the ectoderm (outer germ layer), becomes the mouth opening into the alimentary canal. These ribbon-shaped animals also bear a specialized proboscis enclosed within a rhynchocoel. The rhynchocoel is a fluid-filled cavity that extends from the head to nearly two-thirds of the length of the gut in these animals (The nemerteans, which are primarily predators of annelids and crustaceans, have a well-developed digestive system. They show bilateral symmetry and remarkable contractile properties. (A unique characteristic of this phylum is the presence of an eversible proboscis enclosed in a pocket called a rhynchocoel (not part of the animal’s actual coelom). There may be from two to three hundred pairs of ducts in a single animal. Nemerteans, like flatworms, have excellent powers of regeneration, and asexual reproduction by fragmentation is seen in some species. They show bilateral symmetry and remarkable contractile properties. By the end of this section, you will be able to do the following:Animals belonging to superphylum Lophotrochozoa are triploblastic (have three germ layers) and unlike the cnidarians, they possess an embryonic mesoderm sandwiched between the ectoderm and endoderm. Most species of phylum Ribbon worms vary in size from 1 cm to several meters. However, the offspring do not fully detach from the parents and the formation resembles a chain in appearance. Because of their contractility, they can change their morphological presentation in response to environmental cues. The system is responsible for the regulation of dissolved salts and the excretion of nitrogenous wastes.
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