The Triploblasitc, Acoelomate Body Plan

Zoology Chapter 10

Acoelmates, Pseudocoelomates, and Coelomates

All animals in this chapter are:

1. Triploblastic – have three primary germ layers

2. Acoelomate – without a coelom3. Classified into three phyla:

Phylum Platyhelminthes (flatworms) Phylum Nemertea (unsegmented) Phylum Gastrotricha (bottom dwellers)

Phylum Platyhelminthes: Flatworms Most common =

planarian Contains over 34,000

species Currently, no uniquely

defining characters (synapomorphies) in this phylum

Adult size from 1 mm or less to 25 m

Live in marine, freshwater, and damp terrestrial habitats

Flatworm Characteristics:

Bilateral Symmetry Cephalization 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

Three Classes of Platyhelminthes:

Free-living (not parasites) flatworms: Class Turbellaria

Parasitic Species: Class Trematoda Class Cestoidea

Class Turbellaria Mostly free-living

bottom dwellers in freshwater and marine environments

Crawl on 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 evolve

Feeding Carnivores (small invert.),

scavengers (dead), some herbivores (algae)

Sensory cells on their heads help detect food far away

Digestive cavity has a single opening through which food and waste pass

Feeding cont.

Pharynx – muscular tube pumps food from the mouth 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 are controlled by the nervous system

Allows animal to move & react rapidly to environmental stimuli

Dorsoventral muscles maintain flatness (allows 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

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.

Nephridiopore

An opening on the outside of the body from which tubules eventually merge

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 – ladder-like 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…

Asexual Reproduction

Budding or fission Regeneration

Sexual Reproduction Monoecious: 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. Practice “penis fencing” to

determine which flatworm carries the fertilized eggs

See it here http://www.youtube.com/watch?v=5fx-YgcP8Gg

Turbellaria review:

What type of symmetry?

How do theyfeed?

What type ofsensory organs?

How do theyreproduce?

Parasitic worms:

Class TrematodaClass 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)

The species that infect humans: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

Anterior scolex solely for attachment to the host's gut a string of proglottids, each of which possesses both

male and female organs