Acoelomorpha, Platyzoa, and Mesozoa Chapter 14

Bilateria Most animals have bilateral symmetry. The vast majority of animal species belong to the clade Bilateria, which consists of animals with bilateral symmetry and triploblastic development.

Bilateral Symmetry Radially symmetrical animals have the world coming at them from all directions. They can catch prey coming from any direction. Animals that begin to move about actively seeking food need a different body organization. Distinct head end with sensory structures. Cephalization

Bilateral Symmetry Animals with bilateral symmetry have a distinct head end and can be divided into right and left halves.

Acoelomate Bilateral Animals Animals that have no space between their gut and body wall are said to be acoelomate. These animals are also triploblastic – they have three embryonic germ layers. Organ-system level of organization – more division of labor among their organs.

Acoelomates Although flatworms undergo triploblastic development, they are acoelomates.

Acoelomates These acoelomate phyla are protostomes and have spiral cleavage. Most have determinate cleavage. These are the simplest animals with an excretory system. Acoelomate phyla belong to the superphylum Lophotrochozoa

Phylum Acoelomorpha Group contains ~350 species. Members were formerly in Class Turbellaria within phylum Platyhelminthes Small flat worms less than 5 mm in length. Typically live in marine sediments; few are pelagic. Some species live in brackish water. Most symbiotic but some parasitic.

Ecdysozoa & Lophotrochozoa Clades within Protostomia Modern molecular phylogenies have grouped acoelomate and coelomate taxa together within the protostomes. Protostomes now divided into two large clades: Ecdysozoa and Lophotrochozoa Ecdysozoa possess a cuticle that is molted as their bodies grow Lophotrochozoa share either an odd horse-shoe shaped feeding structure, the lophophore or have unique larval form called the trochophore

Ecdysozoa & Lophotrochozoa Trochophore larvae Minute, translucent, and roughly top-shaped Have a prominent circlet of cilia and sometimes one or two accessory circlets Occur in the early development of other marine members of Annelida and Mollusca and are assumed to be the ancestors of such groups Trochophore-like larvae also occur in some Platyhelminthes, Nemertean, Echiura and Sipunculida groups.

Clade Platyzoa Clade Platyzoa is a unique group of lophotrochozoan protostomes that contain Platyhelminthes, Gastrotricha and Gnathifera.

Phylum Platyhelminthes Members of phylum Platyhelminthes live in marine, freshwater, and damp terrestrial habitats.

Phylum Platyhelminthes Flatworms are flattened dorsoventrally and have a gastrovascular cavity. Extracellular digestion. Undigested food is egested through the pharynx.

Phylum Platyhelminthes The osmoregulatory system consists of protonephridia (excretory or osmoregulatory organs closed at the inner end) with flame cells. Most metabolic wastes removed by diffusion across the body wall.

Phylum Platyhelminthes The nervous system consists of a ladder- like network of nerves and a bilobed brain. Many have large ocelli – light sensing organs.

Phylum Platyhelminthes Many can reproduce asexually as well as sexually. Asexual reproduction via fission. Sometimes the new individuals remain attached – chains of zooids. Monoecious

Taxonomy Flatworms (phylum Platyhelminthes) are divided into four classes: Class Turbellaria – ex. Planaria Not monophyletic Class Trematoda – parasitic flukes Class Monogenea – parasitic monogenetic flukes Class Cestoda - tapeworms

Phylum Platyhelminthes

Class Turbellaria Turbellarians are nearly all free-living and mostly marine.

Class Turbellaria The best-known turbellarians, commonly called planarians, have light-sensitive eyespots and centralized nerve nets.

Class Trematoda Trematodes live as parasites in or on other animals. They parasitize a wide range of hosts.

Class Trematoda Subclass Digenea, digenetic flukes, have a complex life cycle with a mollusc (snail) as the first host and a vertebrate as the final, or definitive, host.

Class Monogenea All monogeneans are parasites. Often found in the gills or external surfaces of fishes.

Class Cestoda Tapeworms (Class Cestoda) are also parasitic and lack a digestive system. The scolex is equipped with suckers and hooks for attachment to the host. Each proglottid contains a set of reproductive organs.

Class Cestoda Cestodes usually require at least two hosts. Adult cestodes are parasites in the digestive tracts of vertebrates.

Pseudocoelomates Pseudocoelomates have a body cavity (the pseudocoel) between the gut (derived from endoderm) and body wall (derived from mesoderm). Derived from the blastocoel.

Advantages of a Body Cavity A body cavity, pseudocoel or coelom, has several advantages. It provides: Greater freedom of movement. Space for development of organ systems. A simple means for circulation of materials around the body. Storage place for waste products. A hydrostatic organ.

Pseudocoelomates Pseudocoelomates do not form a clade. Some are part of superphylum Lophotrochozoa, others are in superphylum Ecdysozoa. All share the pseudocoelomate body plan.

Lophotrochozoa (10 Phyla) Ancestors possessed complex cuticular jaws – Clade Gnathifera Gnathostomulida Micrognathozoa Rotifera Acanthocephala 6 other lophotrochozoan phyla Gastrotricha Tiny aquatic animals that may be closely related to gnathiferans Molecular characteristics place the following with Lophotrochozoa Cycliophora Entoprocta Ectoprocta Brachiopoda Phoronida

Phylum Gastrotricha Gastrotrichs appear similar to rotifers, but without the ciliated corona and have a bristly looking body. Members of the phylum Gastrotricha are pseudocoelomate and have three embryonic germ layers (triploblastic). Complete digestive system. Hermaphroditic or parthenogenetic.

Clade Gnathifera Possess small cuticular jaws with a homologous microstructure. Numbers of pairs of jaws vary. Gnathostomulida, Micrognathozoa, and Rotifera are tiny, free-living, aquatic animals. Acanthocephalans are worm-like endoparasites living as adults in fish or other vertebrates.

Clade Gnathifera Rotifera and Acanthocephala Presumed sister taxa. Form a clade called Syndermata. Have eutelic syncytial epidermis. Constant number of nuclei. Grouping is controversial.

Phylum Gnathostomulida Phylum Gnathostomulida includes the jaw worms. Very small - <2mm. Live in interstitial spaces of fine coastal sediments. Can endure low O2.

Phylum Micrognathozoa Micrognathozoans are tiny animals that live interstitially (between sand grains). Body consists of a two-part head, a thorax, and abdomen with short tail. Move using cilia and have a unique ventral ciliary adhesive pad that produces glue. Three pairs of complex jaws.

Phylum Rotifera Members of the phylum Rotifera are pseudocoelomate and have three embryonic germ layers (triploblastic). Complete digestive system.

Phylum Rotifera Dioecious (separate sexes) but some species parthenogenetic (females produce diploid eggs). Some are parthenogenetic during part of the year, depending on environmental conditions. Thick shelled eggs that can withstand harsh conditions are sometimes produced.

Phylum Rotifera Rotifers have a ciliated crown, the corona, that is characteristic of the phylum.

Phylum Rotifera Rotifers come in a wide range of colors and shapes. Shapes often correspond to lifestyle (floaters, swimmers, sessile). They may be individual or colonial. Mostly freshwater. Benthic and pelagic forms.

Phylum Acanthocephala All spiny-headed worms (Phylum Acanthocephala) are parasites in the intestines of vertebrates. Over 1100 species known. Occur worldwide and parasitize fish, birds, and mammals. Larvae develop in crustaceans or insects.

Phylum Acanthocephala Proboscis has rows of recurved spines that penetrate and may rupture host intestines. Proboscis with hooks can be inverted into a proboscis receptacle by retractor muscles.

Phylum Acanthocephala No respiratory system. Nutrients are absorbed across the tegument, which bears some enzymes - no digestive tract. Dioecious No species normally parasitizes humans.

Phylum Acanthocephala Acanthocephalans penetrate the intestinal wall with spiny proboscis. Remarkably little inflammation on host wall, but pain of infection is intense. Larval acanthors burrow through beetle intestine. Develop into juvenile cystacanths in the insect hemocoel. Pigs become infected by eating grubs.

Phylum Mesozoa Phylum Mesozoa is considered a “missing link” between protozoa and metazoa. Have a simple level of organization. Minute, ciliated, and wormlike animals. All live as parasites in marine invertebrates.

Phylum Mesozoa Most composed of only 20 to 30 cells arranged in two layers. Layers are not homologous to germ layers of other metazoans. Two classes, Rhombozoa and Orthonectida, are so different that some authorities place them in separate phyla.

Phylum Mesozoa Rhombozoans live in kidneys of benthic cephalopods. Adults called vermiforms and are long and slender. Inner, reproductive cells give rise to vermiform larvae. When overpopulated, reproductive cells develop into gonad-like structures producing male and female gametes. Larvae are shed with host urine into the seawater.

Phylum Mesozoa Orthonectids parasitize a variety of invertebrates. Reproduce sexually and asexually. Asexual reproduction consists of a multinucleated mass called a plasmodium.

Phylogeny of Mesozoans Some consider these organisms primitive flatworms and place them in phylum Platyhelminthes. Mesozoans are identified as lophotrochozoan protostomes based on molecular data, but are not placed in Platyzoa.