Acoelomate Animals

Acoelomate Animals

• These animals have three true embryonic tissue layers (triploblastic): ectoderm, mesoderm, and endoderm.
• A third tissue layer, mesoderm, fills the space between the epidermis and digestive cavity.
• Mesoderm enables the development of muscle layers in the body wall.
• Mesoderm promotes more elaborate organs, specialization, and greater division of labor than in Cnidaria.
• They possess true organs, each a combination of several tissues specialized for a particular function.
• Acoelomate means "without body cavity."
• Organs are embedded in tissue, not in any body space; similar to cnidaria and ctenophora which possesses a single "internal space," the digestive cavity.
• Developmentally, these organisms are protostomes.
• The mouth develops first in the embryo during gastrulation.
• Most exhibit bilateral symmetry, allowing for cephalization (concentration of sense organs and a simple brain in a "front end").
• Cephalization is a major design innovation for more efficient searching for food and mates.

Phylum Platyhelminthes (Flatworms)

• These are the least complex worms, characterized by bilateral symmetry.
• They are acoelomates, having thin, solid bodies.
• They exhibit cephalization, with most sensory structures/nerves located in the anterior region.
• Parasitic tapeworms (class Cestoda) and flukes (class Trematoda/Monogenea) are well-known members.

Feeding and Digestion in Planarians

• Planarians feed on dead or slow-moving organisms.
• They extend a tube-like pharynx out of their mouth.
• Enzymes released by the pharynx initiate digestion outside the body.
• Food is then sucked into the gastrovascular cavity, where particles are broken down.
• Excess water is drawn out by flame cells with cilia.

Nervous Control in Planarians

• Some planarians have a nerve net, while others have the beginnings of a central nervous system (CNS).
• Planarians have two nerve cords that run the length of their body.
• Sensory pits detect chemicals and movement.
• Eye spots detect light and dark.
• A Cerebral ganglion, located on the head, communicates with the body to enable responses to stimuli via muscles.
• They can learn and will normally move from light and will stay still.

Reproduction in Planarians

• Planarians are hermaphrodites.
• Sexual reproduction involves the exchange of sperm, which travels along specialized tubes to reach the eggs.
• Fertilization is internal, and zygotes are released in capsules into the water.
• They hatch into tiny planarians.
• Asexual reproduction can occur through fission (splitting).
• They exhibit regeneration: replacement or regrowth of bodyparts.
• If cut in half, the anterior end can grow a new tail and the posterior end can grow a new head; creating regeneration of new clones of the parent after purposeful splitting or damage. (purposeful=fission, accidental=fragmentation).

Feeding and Digestion in Parasitic Flatworms

• Parasitic flatworms live in or on another organism and depend on that organism for food.
• They have mouthparts with hooks for attachment.
• They do not have complex nervous or muscular tissue

Diversity of Flatworms: Tapeworm Bodies (Class Cestoda)

• Tapeworms can grow to be more than 10 meters long.
• The body consists of a head and repeating sections called proglottids.
• A proglottid is a detachable section of a tapeworm containing muscles, nerves, flame cells, and male and female reproductive organs.
• Each proglottid can contain up to 100,000 eggs, and a worm could have 2000 proglottids.

Classes Trematoda and Monogenea

• This group contains flukes.
• Endoparasites live in the host's blood, intestines, lungs, liver, or other organs.
• Ectoparasites live on the external surface of aquatic hosts.

The Life Cycle of Flukes

• Flukes invade the internal organs of vertebrates and embed in the organ to feed on cells, blood, and other fluids.
• Blood flukes of the genus Schistosoma cause schistosomiasis.
• Schistosomiasis is common in rice-growing countries.
• Snails serve as secondary hosts.

Pseudocoelomate Animals - Phylum Nematoda: Roundworms

• More than 15,000 roundworm species exist, with approximately 50 being parasitic to humans.
• Most are free-living, but many are parasitic.
• They are tapered at both ends and have a thick outer covering (cuticle) to prevent digestion.
• They lack muscles and move in a thrashing motion.
• They have a pseudocoelom and two body openings

Diversity of Roundworms: Ascaris

• Ascaris lives in the intestines of pigs, horses, and humans.
• The parasite mainly infects children who swallow eggs.
• A female Ascaris worm can produce up to 200,000 eggs per day.
• The Ascaris eggs hatch in the intestines and the worms move to the lungs.
• They are then coughed up and swallowed again

Hookworms

• Hookworms are intestinal parasites that use cutting plates to attach to the intestinal wall.
• They feed on the host's blood and can cause anemia.
• Hookworms infect people who walk on contaminated soil with bare feet.

Trichinella

• Trichinella can infect mammals, including humans and pigs.
• Trichina worms live embedded in the wall of the intestine.
• The larvae travel through the blood to the muscles, where they form cysts.
• People become infected with trichinosis by eating undercooked pork with cysts.
• The infection causes muscle pain and stiffness, and can cause death if the heart muscle is invaded.
• Freezing and cooking pork properly prevents infection.

Other Parasitic Worms: Pinworm (Enterobius)

• This is the most common roundworm in the U.S., mostly affecting children.
• Pinworms are small in length (~5-10 mm) and do not cause serious disease.
• Females lay eggs near the anus.
• Reinfestation is common because of itching.

Other Parasitic Worms: Filarial Worms

• Filarial worms are found in tropical countries, infecting ~250 million people.
• Mosquitoes are vectors for these worms.
• Some species of filarial worms reside in the lymphatic system and can grow up to 4 inches long.
• If the worms block lymphatic vessels, it can cause elephantiasis.
• This causes limbs to become swollen, and the skin hardens and thickens.
• Loa loa causes infections in the eye.
• Heartworms infects the heart and large arteries of dogs and cats

Phylum Mollusca

• Animals such as squids, snails, oysters, clams, and slugs belong to the phylum Mollusca.
• Most molluscs are marine, but many live in freshwater or on land.
• They all share a basic body plan.
• The hypothetical ancestral mollusc (HAM) has characteristics that appear among most members of the group.

A Closer Look at HAM

• The foot is a broad, flat muscular organ for locomotion and attachment.
• The visceral mass contains the internal organs.
• The mantle is a fold of tissue that drapes over the visceral mass, and the space between the mantle and visceral mass is called the mantle cavity.

HAM: The Shell

• The mantle is responsible for secreting the shell.
• The shell consists of three layers; the outside of shell is covered by a thin organic layer called the periostracum.
• A middle prismatic layer is characterized by densely packed prisms of calcium carbonate laid down in a protein matrix.
• The inner nacreous layer is composed of calcium carbonate sheets laid down over a thin layer of protein.

HAM: Gills

• HAM gills are bipectinate, meaning they are flattened filaments attached to a longitudinal axis on either side.

HAM: The Radula

• The mouth cavity of HAM possesses a specialized rasping organ called the radula, which sits on a cartilaginous structure called the odontophore.
• Particles of food brought into the mouth are bound in mucus secreted by the salivary glands.

Other Features of HAM

• The nervous system consists of a nerve ring and 2 longitudinal nerve cords.
• The coelom is reduced.
• It has an open circulatory system.
• Excretory organs (metanephridia) open into coelom via ciliated funnel (nephrostome) and waste leaves body via nephridiopore.
• Most molluscs produce a free-swimming ciliated larvae called the trochophore larvae.

Molluscan Larval Stages

• The trochophore larvae develops into the adult, but, in other molluscs there a second larval stage (veliger) which is common in gastropods.

Molluscan Diversity: Class Monoplacophora

• These molluscs are a few centimeters in length.
• Dorsal surface is covered with a shield-shaped shell with a slight anterior peak.
• Ventral surface is broad and flat, with the mantle cavity in the form of 2 grooves located to either side of the foot.
• Mantle groove contains 5 or 6 pairs of monopectinate gills.
• There is serial repetition of certain body parts (not sure if this is pseudosegmentation).

Class Polyplacophora (Chitons)

• Chitons are common on the rocky surfaces of the intertidal zone.
• They have a poorly developed head, while the ventral surface is occupied by a broad, flattened foot.
• The dorsal shell is composed of 8 overlapping plates, arranged linearly along the anterior-posterior axis.
• Lateral margins of the plates are overgrown by the girdle.
• Mantle cavity is limited to two lateral troughs between the foot and the mantle edge, called pallial grooves.
• Within the pallial grooves are many bipectinate gills.

Class Gastropoda

• Three evolutionary innovations occurred among the gastropods; changes in the shell, increased development of the head, the embryonic process of torsion.

Class Gastropoda: Changes in the Shell

• The shell became higher and conical with a reduced aperture.
• The shell also became coiled.
• Shells initially were planospiral.
• Initially a bilaterally symmetrical shell with the whorls lying in the same plane before they evolved.
• Modern day shells are asymmetrical, with each successive coil being a little outside and offset a little above the one below.

Class Gastropoda: Change in Shell Architecture

• The planospiral shell was not as compact as an asymmetrical shell.
• This change creates a shift in the weight to one side of the animal.
• To achieve a better weight distribution the shell needed to shift upward and posterior
• The shell axis then became oblique to the longitudinal axis of the foot (= bilateral asymmetry)
• The heavy bulk of the main body whorl, put weight on the right side of the mantle cavity.
• Organs on the right side became less important during evolution
• The Increased Development of the Head
• The developing head bears 2 pairs of tentacles, with the eyespots at the base of one pair

Class Gastropoda: The Embryonic Process known as Torsion

• During embryonic development, 1 side of the visceral mass grows at a much faster rate than the other causing the mass rotates 180 degrees relative to the head-foot.
• Advantages: head retracted first; gills receive water currents; the osphradium is now directed anteriorly.
• Disadvantage: may cause fouling
• The gastropod shell is typically a single, spiraled shell that can hold the head and the foot into for protection.
• Gastropods can have a hardened plate called the operculum plugs the aperture from the shell when the body is withdrawn.

Class Gastropoda: Nutrition

• Many gastropods are herbivores and use their radula scrap algae from surfaces of rocks.
• Some are active predators with highly modified radulas, such as a drill (oyster drills) or harpoon (venomous gastropods).

Gastropoda: Respiration

• Aquatic gastropods possess gills for respiration.
• Terrestrial gastropods obtain oxygen via a well-vascularized mantle

Class Bivalvia

• Bivalves (clams, mussels, scallops, oysters) are characterized by shells divided into 2 equal halves (valves).
• Mantle tissue is indented in the anterior-posterior margins, with 2 centers of calcification.
• Shells are joined at the dorsal midline by non-calcified protein ligaments called the hinge.
• Pallial muscles insert on the underside of the shell and are attached to the free edge of the mantle and pull the mantle under the shell.
• Ventilating Current:
  • Cilia on the gills are used to trap food particles suspended in the water.
  • These cilia move the trapped food over the surface of gills toward food grooves.
  • Food is then directed anteriorly towards the mouth.
  • On the way to the mouth, cells in the gills secrete copious amounts of mucous.
  • The food particles become entangled
  • Prior to entering the mouth, the mucous food thread is first sorted by the labial palps.

Adaptive Radiation of Bivalves

• Soft Bottom Burrowers:
  • those that live deep in the sand or mud; burrowing is accomplished using the foot that is extended through a specific part of the shell - the pedal gap.
  • These molluscs have long tubular extensions of the mantle called siphons, for inhalent and exhalent openings
• Attached Surface Dwellers:
  • Those that live attached to hard services; some lie in their side with the shell fused or cemented to the substrate; foot is absent
• Common mussels attach to the substrate use byssal threads; secreted by foot glands.
• Reproduction: - Most are dioecious and marine. - They produce free swimming trochophore and veliger larvae. - Freshwater bivalves produce glochidia. - The larvae of some species is also housed in the gill system for reproduction purposes. - Once released, glochidia parasitize fins and gills of fish.

Class Cephalopoda

• Class Cephalopoda (squids, octopuses, nautilus) are fast-moving predators of the marine environment.
• Cephalopods evolved after major readjustments in the HAM body plan. - They have a dorso-ventral axis that is elongated - Anterior-posterior axis became compressed
• Heads migrated to the ventral part of body where it fused to the foot to become a tentacle/arm.
• Prehensile tentacles or arms are used to capture prey. - Arms have studded suckers, around 8-10 tentacles a circle around the head

Class Cephalopoda: Feeding, Locomotion and Shell

• The class is a carnivorous group that has a strong parrot like beak to tear it's meat, it also has a radula to poison.
• They swim using jets and have streamlined bodies.
• They increase it's jet propulsion by the use of tentacles for water flow.

Class Cephalopoda: The Shell

• These organisms had internal shells and coiled external shells.
• There are cephalopods that have lost shells - The primitive had shelled records - Extant = nautilus - Cuttlefish have internal shell.

Other General Features: Reproduction

• Other class features include protection via ink, strong sense/camera eyes with remarkable capacities
• Their closed circulatory system makes the well known class.
• Each sex is split by sperm called spermatophore as a mating call, uses tentacles for fertilization
• males use tentacles to inserts the call into the mantle of the woman where the sperm is inserted.