are platyhelminthes lophotrochozoa

The monoplacophorans were believed extinct and only known via fossil records until the discovery of Neopilina galathaea in 1952. The overall morphology is laterally flattened, and the head region is poorly developed. It also means the beginning of cephalization, the evolution of a concentration of nervous tissues and sensory organs in the head of the organism, which is where it first encounters its environment.

Early development in some species occurs via two larval stages: trochophore and veliger. The parasite infects various organs in the body and feeds on red blood cells before reproducing. Rotifera (clade) 4. The class Turbellaria includes mainly free-living, marine species, although some species live in freshwater or moist terrestrial environments. The juvenile worm infects the intermediate host and takes up residence, usually in muscle tissue. In fact, it is the parasite’s eggs that produce most of the main ill effects of schistosomiasis.

Stem cells that become gametes aggregate within gonads placed along the digestive tract.

Figure 11. Flatworms have three embryonic tissue layers that give rise to epidermal tissues (from ectoderm), the lining of the digestive system (from endoderm), and other internal tissues (from mesoderm). Class Aplacophora (“bearing no plates”) includes worm-like animals primarily found in benthic marine habitats. Economically important species have at times devastated commercial fishing of clams and crabs. Mollusks have a muscular foot, which is used for locomotion and anchorage, and varies in shape and function, depending on the type of mollusk under study. Circular as well as longitudinal muscles are located interior to the epidermis.

A recent possible cladogram is shown which would show that the Lophotrochozoa emerged within Platyzoa as a sister group of the Rouphozoa (the Gastrotricha and Platyhelminthes). Textbook content produced by OpenStax is licensed under a The head is rudimentary and protrudes out of the posterior end of the shell. Members of a species mate, and the female then lays the eggs in a secluded and protected niche.

The Platyhelminthes consist of two monophyletic lineages: the Catenulida and the Rhabditophora.

These worms typically reproduce asexually by budding. The food particles enter the mouth and travel to the mastax (pharynx with jaw-like structures). Most flatworms, such as the planarian shown in Figure 1, have a gastrovascular cavity rather than a complete digestive system.

Because of their contractility, they can change their morphological presentation in response to environmental cues. Members of the clade or class Rhabditophora are now dispersed among multiple orders of Platyhelminthes, the most familiar of these being the Polycladida, which contains the large marine flatworms; the Tricladida (which includes Dugesia [“planaria”] and Planaria and its relatives); and the major parasitic orders: Monogenea (fish ectoparasites), Trematoda (flukes), and Cestoda (tapeworms), which together form a monophyletic clade. The cestodes, or tapeworms, are also internal parasites, mainly of vertebrates (Figure). The name of the phylum is derived from the Latin word annellus, which means a small ring. The proglottid detaches from the body of the worm and is released into the feces of the organism.

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. Most nemerteans are carnivores, feeding on worms, clams, and crustaceans. Hence, they are the most advanced worms. The class Oligochaeta includes the subclass Hirudinia and the subclass Brachiobdella. Proglottids are produced at the scolex and gradually migrate to the end of the tapeworm; at this point, they are “mature” and all structures except fertilized eggs have degenerated.

The trematodes, or flukes, are internal parasites of mollusks and many other groups, including humans. The primary host is almost always a mollusk. Some species also have an anal opening. Digestion is primarily extracellular, with digested materials taken into the cells of the gut lining by phagocytosis. (Figure 28.19). The ventral epidermis of free-living flatworms is ciliated, which facilitates their locomotion. This illustration shows the anatomy of a bdelloid rotifer. The system is responsible for the regulation of dissolved salts and the excretion of nitrogenous wastes.

Nemertea (clade) 6. Phylum Mollusca is a large, marine group of invertebrates.

The lophophore is a feeding structure composed of a set of ciliated tentacles surrounding the mouth.

When the muscle tissue is consumed by the primary host, the cycle is completed. Members of the clade or class Rhabditophora are now dispersed among multiple orders of Platyhelminthes, the most familiar of these being the Polycladida, which contains the large marine flatworms; the Tricladida (which includes Dugesia [“planaria”] and Planaria and its relatives); and the major parasitic orders: Monogenea (fish ectoparasites), Trematoda (flukes), and Cestoda (tapeworms), which together form a monophyletic clade. The other phyla have a pseudocoel, and share characteristics such as the structure of their jaws and pharynx, although these have been secondarily lost in the parasitic Acanthocephala. The proboscis may be extended by hydrostatic pressure generated by contraction of muscle of the rhynchocoel and retracted by a retractor muscle attached to the rear wall of the rhynchocoel. Adopted or used LibreTexts for your course? Paired longitudinal nerve cords emerge from the brain ganglia and extend to the posterior end.

Most of the flatworms are classified in the superphylum Lophotrochozoa, which also includes the mollusks and annelids. The specimen shown here was photographed in the Ross Sea, Antarctica. It is the unsegmented type of worms. Phylum Platyhelminthes is composed of the flatworms: acoelomate organisms that include many free-living and parasitic forms. The mantle cavity develops independently of the coelomic cavity. Animals in this phylum show parasitic and commensal symbioses with other species in their habitat. As a result, the anus of the adult animal is located over the head. The remaining body of the tapeworm is made up of a long series of units called proglottids, each of which may contain an excretory system with flame cells, but contain reproductive structures, both male and female. Infection occurs when the human enters the water and a larva, released from the primary snail host, locates and penetrates the skin. Like most lophotrochozoan protostomes, cleavage is spiral, and development is usually direct, although some species have a trochophore-like larva, in which a young worm is constructed from a series of imaginal discs that begin as invaginations from the body surface of the larva. Many of the eggs are released in feces and find their way into a waterway, where they are able to reinfect the snail host. The head contains a number of eyespots and a bilobed “brain,” with nerves extending into the body.

Some turbellarians are capable of remarkable feats of regeneration: they may regrow the entire body from a small fragment. The flatworms are acoelomate organisms that include many free-living and parasitic forms. A ring of four nerve masses called “ganglia” composes the brain in these animals. Platyhelminthes further classified into three classes; on the other hand, Nematoda further classified into two classes. Figure 2. Have questions or comments? The proboscis worm (Parborlasia corrugatus) is a scavenger that combs the sea floor for food. The excretory system in Platyhelminthes is mainly flame cells that are associated with the Protonephridia. Mollusks display a wide range of morphologies in each class and subclass, but share a few key characteristics, including a muscular foot, a visceral mass containing internal organs, and a mantle that may or may not secrete a shell of calcium carbonate (Figure 8).

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