The Triploblasitc, Acoelomate Body Plan Zoology – Chapter 10

Acoelmates, Pseudocoelomates, and Coelomates

All animals in this chapter are: Triploblastic – have three primary germ layers Acoelomate – without a coelom Classified into three phyla – Phylum Platyhelminthes (flatworms) Phylum Nemertea (unsegmented) Phylum Gastrotricha (bottom dwellers)

Phylum Platyhelminthes: aka – Flatworms Most common = planarian Contains over 34,000 species Adult size from 1 mm or less to 25 m Live in marine, freshwater, and damp terrestrial habitats

Flatworm Characteristics: Bilateral Symmetry Cephalization (have a so called head) Mesoderm tissue includes a loose tissue (parenchyma) that fills spaces between specialized tissues, organs, and body wall. May provide skeletal support, nutrient storage, motility, transport of materials, oxygen storage, etc.

Form and Function in Flatworms Most cells are close to external environment ∴ materials can pass easily into and out of their bodies (via diffusion) Rely on Diffusion for respiration, excretion, and circulation

3 Main Classes of Platyhelminthes: Free-living flatworms: Class Turbellaria Parasitic Species: Class Trematoda Class Cestoidea

Class Turbellaria Mostly free-living bottom dwellers in freshwater and marine environments Crawl over stones, sand, or vegetation Named for the turbulence that their beating cilia create in the water

Class Turbellaria cont. Over 3,000 species Few terrestrial species live in humid tropics and subtropics Less than 1 cm long (rare terrestrial/tropic = up to 60 cm long) First group of bilaterally symmetrical animals to appear

Feeding Carnivores (small invert.), scavengers (dead), some herbivores (algae) Sensory cells (chemoreceptors) on their heads help detect food far away Digestive cavity has a single opening (or mouth) through which food and waste pass

Feeding cont. Pharynx – muscular tube that extends out of the mouth and pumps food into the digestive cavity or gut highly branched gut transports food to all parts of the body (this is lacking in many parasitic species)

Locomotion Cilia on epidermal cells help to glide through the water and over the bottom of a stream or pond A layer of mucus is laid down to aid in adhesion and help cilia gain traction

Locomotion cont. Muscle cells controlled by the nervous system allow them to twist and turn so that they are able to react rapidly to environmental stimuli Dorsoventral muscles essential for maintaining flatness (ie-for diffusion)

Exchanges with Environment: Do not have respiratory organs ∴ done via diffusion Respiratory gases (CO2 and O2) Metabolic wastes (ammonia) Depends on environment Marine = osmotic equilibrium Freshwater = hypertonic solutions

Excretion: Protonephridia is a network of fine tubules for excretion of metabolic wastes. Flame cells are ciliated and induce currents to push fluids through tubules.

Tubules eventually merge and open to the outside of the body wall through a minute opening called a nephridiopore.

Nervous System & Sense Organs: Subepidermal nerve plexus (resembles cnidarians but depends on species) Mechanoreceptor (excited by pressure) at anterior end detects body position(due to gravity) Cerebral ganglia – more centralized nerve net (“brain”) Longitudinal nerve cords – ladderlike appearance (evolutionary advancement to a nervous system)

Auricles – sensory lobes on side of head aid in food location (chemoreceptor) Ocelli – eye spots; orient in direction of light (photoreceptor) See how it works…

Reproduction Asexual reproduction Budding or fission Regeneration

Reproduction cont. Can reproduce either asexually or sexually Asexual = planarian detaches its tail end and each half regrows the lost parts (referred to as zooids). Sexual = each Planaria gives and receives sperm. Benefits to sexual over asexual? Have both testes and ovaries (hermaphroditic) Eggs develop inside the body and are shed in capsules called cocoons Weeks later the eggs hatch and grow into adults. Can also reproduce by regeneration (fragmentation) If it is cut into two halves, both halves may become two new Planaria.

Turbellaria review: What type of symmetry? How do they feed? sensory organs? reproduce?

Parasitic worms: Class Trematoda Class Cestoidea

Class Trematoda 8,000 parasitic species (aka – flukes) Internal parasites (endoparasitic) Complex life cycles specialized in parasitism in animal or human tissues One or more suckers around anterior end (oral sucker) Unsegmented

Subclass Digeneans Life cycle involves two - four hosts and a number of developmental stages, including two types of free-living larvae (most complex in the animal kingdom) The definitive (final) host of adult is always a vertebrate Snails are common intermediate hosts

Eggs reach freshwater and Miracidium (ciliated larva) swims out Finds host (snail) Penetrates snail, loses cilia, develops into a sporocyte (contain embryonic cells) Develops into daughter sporocyte (hundreds can form from one miracidium) Hundreds of next larval stage are produced (cercariae) Cercariae leave the snail and find 2nd or final host Penetrates host and becomes a Metacercaria When the definitive host eats the 2nd intermediate host, it becomes an adult

The species that infect humans can be divided into groups: Schistosomes & non-Schistosomes Dicrocoelium dendriticum and Opisthorchis sp. liver flukes of mammals blood flukes, Schistosoma spp. are among most widespread and serious parasites of humans

Schistosomiasomes (blood flukes)

Class Cestoidea aka – tapeworms Most highly specialized class of flatworms Intestinal parasites No digestive tract ∴ reside in digestive system of vertebrates Absorb nutrients across body wall Adults range from 1mm to 25 m in length

The body consists of an anterior scolex solely for attachment to the host's gut and a string of proglottids, each of which possesses both male and female organs