Systematic Adaptations of Phylum Platyhelminthes

The Flatworms

Organ level of organization◦ Well developed, specialized anatomical structures

Incomplete gastrulation ◦ Gastrovascular cavity with one opening

Triploblastic◦ True mesodermal tissue filling space between

endoderm and ectoderm (note “parenchyma”) Bilateral Symmetry

◦ 2 symmetrical sides about longitudinal axis Cephalization

◦ Concentration of sensory apparatus at anterior end

Derived Characteristics

Acoelomate Parenchyma derived from mesodermal

tissue Elongated body No appendages

Body Form & Development

Simple integument Ciliated in free living flatworms, but lacking

cilia in parasitic forms Glandular tissue associated with skin

◦ Mucous glands◦ “Dual gland” for adhesion and release

Integumentary

Hydrostatic Skeleton Parenchymal tissue holds a constant (“static”)

volume of water (“hydro”) Compression/contraction in one direction

must necessarily result in expansion/elongation in a different direction

Well developed muscles are present in the body wall◦ Longitudinal◦ Circular◦ Oblique

Skeletal & Muscular

Predict the result from the contraction of:◦ Longitudinal muscle (2)◦ Circular muscle (1)◦ Oblique muscle (3)

Muscular arrangement

https://www.youtube.com/watch?v=zCH37KI_R_E

https://www.youtube.com/watch?v=HekwSO6dBG0

https://www.youtube.com/watch?v=OCOYrHAkjVE

Watch these flatworms move

Centralized nervous system◦ Paired solid, ventral

nerve cords◦ “Ladder” type nervous

organization – Transverse nerves connecting longitudinal nerve cords

Concentration of nerve tissue forming cephalic ganglia (simple brain)

Nervous & Sensory

Cephalization◦ Concentration of

sense organs in anterior body region

Sense organs◦ Photoreceptors (note

“ocelli”)◦ Chemoreceptors

(note “auricles”)

Nervous & Sensory

The development of the digestive cavity is incomplete, forming a gastrovascular cavity with a single opening

Digestion is largely extracellular, although some flatworms complete the process in food vacuoles (intracellular)

The class turbellaria are free living and carnivorous, and have well developed gastrovascular cavities

The other classes of flatworms are parasitic, dependent upon the host for digestion

Digestion

Mouth located ventrally Free living turbellaria have an extensible pharynx The highly branched gastrovascular cavity brings

digested food in close proximity to all body tissues As the gastrovascular cavity has only 1 opening,

wastes are excreted through the pharynx or retained

Digestion

https://www.youtube.com/watch?v=9QJF93WlNew

https://www.youtube.com/watch?v=eUmOJ6fWN9w&feature=endscreen&NR=1

https://www.youtube.com/watch?v=Ich4Y_K6GuM

Watch these flatworms feeding

The gastrovascular cavity in parasitic forms is diminished (as in this photo) or absent (as in the tapeworms)

When present, the gastrovascular cavity is generally Y shaped

Digestion

Osmoregulation is the removal of excess water

Excretion is the removal of metabolic waste (mostly nitrogen compounds like ammonia or urea that accumulate from metabolism of protein) and excess dissolved materials (salts)

The flame cells are concerned with osmoregulation

Metabolic wastes are generally removed by simple diffusion

Osmoregulation vs. Excretion?

Excretion & Osmoregulation

“Protonephridia” – primitive excretory tubules driven by Flame Cells.

The “flame” consists of several flagellae that drive fluids through tubules that lead to collecting tubules

Excretion & Osmoregulation

There is considerable variation in the structure of osmoregulation systems in the flatworm classes, from simple excretory pores to a bladder

Due to the simplicity of the flatworm body form, many are capable of reproducing asexually. In low density populations, asexual reproduction (by budding or fission) may be predominant

Flatworms are monoecious (one reproductive form – AKA hermaphroditic) but generally are unable to self-fertilize

Parasitic forms have complex life cycles, often involving one or more intermediate hosts.

Reproduction

This flatworm is free living. Its muscular and digestive systems, while not shown above, are well developed

Note the presence of both male and female gonads (Monoecious)

Flatworm reproductive organs

This liver fluke is parasitic. It depend on its host for locomotion and digestion, allowing it to devote more of its energy and anatomy to reproduction

Note that is also is monoecious. The testes are more pink, and located posteriorly

The ovary is darker and more centrally located

Fluke reproductive organs

Tapeworms are parasitic. Their digestive system is absent. Almost their entire anatomy is devoted to reproduction

They have a segmented body, with both testes and ovaries located in each segment

Tapeworm reproductive organs

Life cycle of the Liver fluke

https://www.youtube.com/watch?v=r7okJ6wHYLA

https://www.youtube.com/watch?v=EEBbtwGqPEs

https://www.youtube.com/watch?v=9fmx24FbZoM

Trematode Life Cycle

The Phylum platyhelminthes is divided into 4 classes◦ Class Turbellaria – The free living flatworms◦ Class Trematoda – The digenetic flukes

Digenetic = 2 or more hosts◦ Class Monogenea – The monogenetic flukes

Monogenetic = 1 host◦ Class Cestoda – The tapeworms◦ https://www.youtube.com/watch?v=wZBPqITnyME

Classes of Platyhelminthes

The only class of free living flatworms Ciliated epidermis with mucous secreting

rhabdites Acoelomate Extensible pharynx Includes the planaria

Class Turbellaria

Parasitic flukes Syncytial tegument (multinucleate skin) without

cilia Y shaped gastrovascular cavity Oral sucker for obtaining food and ventral sucker

for attachment No hooks for attachment to host Digenetic – 2 or more hosts

◦ Intermediate host – usually an invertebrate, frequently a mollusc

◦ Definitive host – a vertebrate Includes fasciola, schistosoma and clonorchis

Class Trematoda

Parasitic flukes Syncytial tegument (multinucleate skin)

without cilia Free living, ciliated larvae Y shaped gastrovascular cavity A single oral sucker A series of posterior hooks (opisthaptor) for

attachment to the host Monogenetic – parasitic on a single host,

usually a fish, amphibian or reptile

Class Monogenea

Tapeworms, parasitic to the digestive tracts of vertebrates Syncytial tegument without cilia The tegument is covered with fingerlike extensions to

increase the surface area for food absorption Gastrovascular cavity is absent Anterior structure (scolex) with hooks/suckers for

attachment to the host Body is formed into segments called proglottids. Each

proglottid has testes and ovaries. Self-fertilization is possible

Gravid proglottids from feces of infected animals may be consumed by a new host, or may be spread by consuming larvae in the flesh of an infected animal

Class Cestoda

https://www.youtube.com/watch?v=Q5BuXmDIJxs

https://www.youtube.com/watch?v=9qyEfeg-334

Tapeworms