Lophotrochozoa Although the relationships between Lophotrochozoan phyla are not well resolved a set of 6 somewhat obscure phyla appear to be most closely related to the Platyhelminthes. Although the relationships between Lophotrochozoan phyla are not well resolved a set of 6 somewhat obscure phyla appear to be most closely related to the Platyhelminthes. These are the Mesozoa, Nemertea and four phyla grouped together in a clade called the Gnathifera. These are the Mesozoa, Nemertea and four phyla grouped together in a clade called the Gnathifera.

Mesozoa The Mesozoa are tiny ciliated animals ranging in size from only 0.5 mm to 7mm in length. The Mesozoa are tiny ciliated animals ranging in size from only 0.5 mm to 7mm in length. Mesozoans are very specialized parasites (or in some cases symbionts) of marine invertebrates. About 50 species known. Mesozoans are very specialized parasites (or in some cases symbionts) of marine invertebrates. About 50 species known.

tolweb.org/Mesozoa/2484

Mesozoan characteristics Bilaterally symmetrical. Bilaterally symmetrical. No organs or tissues. No organs or tissues. Body contains no internal cavity. Body contains no internal cavity. Possesses no gut. Possesses no gut. Body only two cell layers in most places. Body only two cell layers in most places. No nervous system. No nervous system.

Mesozoa Mesozoans are made up of only cells arranged in two layers, which are not homologous to the germ layers of other protostomes. Mesozoans are made up of only cells arranged in two layers, which are not homologous to the germ layers of other protostomes. They do not undergo gastrulation unlike other diploblasts and triploblasts. They do not undergo gastrulation unlike other diploblasts and triploblasts. Despite their simplicity molecular evidence suggests that Mesozoans are derived from triploblastic organisms. Despite their simplicity molecular evidence suggests that Mesozoans are derived from triploblastic organisms.

Phylum Nemertea (Rhynchocoela) Ribbonworms The nemerteans (ribbon worms) are long, marine, predatory worms and there are about 1000 species known. The nemerteans (ribbon worms) are long, marine, predatory worms and there are about 1000 species known. Most are less than 20cm in length, but others are many meters in length. Most are less than 20cm in length, but others are many meters in length.

8.19 Baseodiscus mexicanus a nemertean from the Galapagos Islands

Phylum Nemertea (Rhynchocoela) Ribbonworms The general body plan of nemerteans is similar to that of turbellarians. Like turbellarians they have a ciliated epidermis and possess a large number of gland cells. They also have flame cells. The general body plan of nemerteans is similar to that of turbellarians. Like turbellarians they have a ciliated epidermis and possess a large number of gland cells. They also have flame cells. Unlike members of the Platyhelminthes nemerteans have a complete gut with a mouth and anus and a true circulatory system. The flame cells also are associated with the circulatory system and so are used to eliminate metabolic wastes (excretion) rather than osmoregulation as in Platyhelminthes. Unlike members of the Platyhelminthes nemerteans have a complete gut with a mouth and anus and a true circulatory system. The flame cells also are associated with the circulatory system and so are used to eliminate metabolic wastes (excretion) rather than osmoregulation as in Platyhelminthes.

Phylum Nemertea (Rhynchocoela) Ribbonworms Prey is captured using a long muscular proboscis armed with a barb called a stylet. Prey is captured using a long muscular proboscis armed with a barb called a stylet. The proboscis lies in a body cavity called the rhynchocoel and muscular pressure on fluid in the rhynchocoel causes the proboscis to be quickly everted. The rhynchocoel appears to be a modified coelom. The proboscis lies in a body cavity called the rhynchocoel and muscular pressure on fluid in the rhynchocoel causes the proboscis to be quickly everted. The rhynchocoel appears to be a modified coelom. The prey is wrapped in the sticky, slime-covered, proboscis and stabbed repeatedly with the stylet. Neurotoxins in the slime incapacitate the prey. The prey is wrapped in the sticky, slime-covered, proboscis and stabbed repeatedly with the stylet. Neurotoxins in the slime incapacitate the prey.

8.18 Internal structure of female ribbon worm (above). Nemertean with proboscis extended (right)

Clade Gnathifera The clade Gnathifera (“jaw bearing”) groups four phyla together on the basis that they all possess small very similarly structured jaws. The clade Gnathifera (“jaw bearing”) groups four phyla together on the basis that they all possess small very similarly structured jaws. Members of three phyla (Gnathostomulida, Micrognathozoa, and Rotifera) are tiny, free- living aquatic animals and the other phylum (Acanthocephala) are wormlike endoparasites. Members of three phyla (Gnathostomulida, Micrognathozoa, and Rotifera) are tiny, free- living aquatic animals and the other phylum (Acanthocephala) are wormlike endoparasites.

Phylum Gnathostomulida The first Gnathostomulid was not discovered until 1928 and only about 80 species are known. The first Gnathostomulid was not discovered until 1928 and only about 80 species are known. They are tiny (0.5-1mm long) wormlike animals that live in the interstitial spaces of sand and silt. They are tiny (0.5-1mm long) wormlike animals that live in the interstitial spaces of sand and silt. They scrape bacteria and algae from the substratum using a pair of jaws in the pharynx, which is similar in structure to the muscular pharynx (mastax) of rotifers. They scrape bacteria and algae from the substratum using a pair of jaws in the pharynx, which is similar in structure to the muscular pharynx (mastax) of rotifers.

Phylum Gnathostomulida Because they lack a circulatory system, and anus gnathostomulids were first classed as turbellarians. Because they lack a circulatory system, and anus gnathostomulids were first classed as turbellarians. More recently it has been suggested that they are more closely related to the phyla Rotifera and Acanthocephala. More recently it has been suggested that they are more closely related to the phyla Rotifera and Acanthocephala.

8.20 Gnathostomula jenneri

Micrognathozoa The first and only species known was collected in Like gnathostomulids, they are tiny (142uM) interstitial inhabitants and consume bacteria, blue-green algae and similar tiny food items. The first and only species known was collected in Like gnathostomulids, they are tiny (142uM) interstitial inhabitants and consume bacteria, blue-green algae and similar tiny food items. They have a two-part head, thorax and abdomen and a very complex jaw system made up of multiple plates and teeth. They have a two-part head, thorax and abdomen and a very complex jaw system made up of multiple plates and teeth.

Phylum Rotifera Rotifers are named for their characteristic ciliated crown or corona, which when it beats looks like a rotating wheel. Rotifers are named for their characteristic ciliated crown or corona, which when it beats looks like a rotating wheel. Rotifers are tiny animals (most are µm long and the largest only 3mm long) the majority of which live in freshwater and are benthic inhabitants (live on the bottom). Rotifers are tiny animals (most are µm long and the largest only 3mm long) the majority of which live in freshwater and are benthic inhabitants (live on the bottom). About 2000 species have been described. About 2000 species have been described.

9.1

Phylum Rotifera The beating of the cilia in the corona draws in plankton-containing water for food. The beating of the cilia in the corona draws in plankton-containing water for food. The mouth opens to a modified muscular pharynx known as a mastax, which is a structure unique to rotifers. The mouth opens to a modified muscular pharynx known as a mastax, which is a structure unique to rotifers. The mastax has a set of complex jaws, which are used to grasp and chew food. The mastax has a set of complex jaws, which are used to grasp and chew food.

Phylum Rotifera One group of rotifers, the Bdelloid rotifers, are very unusual in that there are no males, hermaphrodites, or evidence of meiosis. One group of rotifers, the Bdelloid rotifers, are very unusual in that there are no males, hermaphrodites, or evidence of meiosis. Molecular evidence suggests that there has been only asexual reproduction in this group for several million years. Molecular evidence suggests that there has been only asexual reproduction in this group for several million years.

Phylum Rotifera Because of the problem of accumulation of deleterious mutations in lineages of exclusively asexually reproducing animals (a process known as Muller’s ratchet) it is unclear how the bdelloids have been able to dispense with sexual reproduction entirely. Because of the problem of accumulation of deleterious mutations in lineages of exclusively asexually reproducing animals (a process known as Muller’s ratchet) it is unclear how the bdelloids have been able to dispense with sexual reproduction entirely. Other rotifers reproduce using a combination of sexual and asexual reproduction. Other rotifers reproduce using a combination of sexual and asexual reproduction.

Phylum Acanthocephala Acanthocephalans are commonly known as spiny-headed worms because of the spiny eversible proboscis they use to attach to the gut of their host. Acanthocephalans are commonly known as spiny-headed worms because of the spiny eversible proboscis they use to attach to the gut of their host. All 1100 species of Acanthocephalan are endoparasitic and most parasitize fish, birds and mammals. All 1100 species of Acanthocephalan are endoparasitic and most parasitize fish, birds and mammals.

9.3

s.jpg Acanthocephalan proboscis

Phylum Acanthocephala The body wall is covered with numerous minute depressions which enormously increase the surface are of the tegument and facilitates (as in cestodes) the absorption of food from the host’s gut. The body wall is covered with numerous minute depressions which enormously increase the surface are of the tegument and facilitates (as in cestodes) the absorption of food from the host’s gut. As is true in cestodes, Acanthocephalans lack a gut. As is true in cestodes, Acanthocephalans lack a gut.

Phylum Acanthocephala Acanthocephalans have a lifecycle in which a vertebrate is the definitive host and an invertebrate the intermediate host. Acanthocephalans have a lifecycle in which a vertebrate is the definitive host and an invertebrate the intermediate host. Acanthocephalans, as other parasites do, modify the behavior of the intermediate host to enhance the chances of its being eaten. Acanthocephalans, as other parasites do, modify the behavior of the intermediate host to enhance the chances of its being eaten.

Phylum Acanthocephala For example, acanthocephalans that parasitize Gammarus, a small freshwater crustacean, cause the Gammarus to alter its behavior in the presence of ducks, a common predator. For example, acanthocephalans that parasitize Gammarus, a small freshwater crustacean, cause the Gammarus to alter its behavior in the presence of ducks, a common predator. Instead of diving to the bottom when a duck appears, the Gammarus swims into the light and grasps tightly onto a piece of vegetation, greatly increasing its chances of being eaten. Instead of diving to the bottom when a duck appears, the Gammarus swims into the light and grasps tightly onto a piece of vegetation, greatly increasing its chances of being eaten.

Phylum Acanthocephala The change in behavior appears to be caused by the Acanthocephalan pumping a serotonin- boosting molecule into the Gammarus’ brain. The change in behavior appears to be caused by the Acanthocephalan pumping a serotonin- boosting molecule into the Gammarus’ brain. This causes the Gammarus to think it’s having sex and cling as it would if mating. Interestingly, the parasite’s manipulation also causes female Gammarus to mimic the males mating behavior. This causes the Gammarus to think it’s having sex and cling as it would if mating. Interestingly, the parasite’s manipulation also causes female Gammarus to mimic the males mating behavior.

Phylum Acanthocephala Another Acanthocephalan that parasitizes pill bugs causes them to reverse their normal behavior and avoid humid, dark areas. Another Acanthocephalan that parasitizes pill bugs causes them to reverse their normal behavior and avoid humid, dark areas. Instead they wander in the open where they are much more vulnerable to birds, the acanthocephalans definitive host. Instead they wander in the open where they are much more vulnerable to birds, the acanthocephalans definitive host. The parasite’s manipulations are very effective. Although fewer than 1% of pill bugs are typically infected with acanthocephalan parasites 30% of pill bugs delivered to nestlings are infected. The parasite’s manipulations are very effective. Although fewer than 1% of pill bugs are typically infected with acanthocephalan parasites 30% of pill bugs delivered to nestlings are infected.