Subkingdom Eumetazoa, Superphylum Bilateria, Phylum Platyhelminthes: The Flatworms

The name of the phylum derives from the Greek platy meaning "flat" and helminth meaning "worm."

Three Classes of Platyhelminthes:

• Class Turbellaria - The Free-Living Flatworms
• Class Trematoda - The Flukes (All parasitic)
• Class Cestoda - The Tapeworms (All parasitic)

In these most primitive bilaterians, we see the origin of:

• true bilateral symmetry and cephalization

• more complex true tissues
  • ectoderm (becomes the epidermis in the adult)
  • endoderm (becomes the gastrodermis in the adult)
  • mesenchyme (celluar, but not quite true mesoderm yet)

The Platyhelminth Body Plan

The following organ systems are present, some seen for the first time in animals:

• integumentary system - YES
  Parasitic forms (flukes and tapeworms) have a tough, protein TEGUMENT in addition to the epidermis. It offers protection against the host, as well as allowing gas exchange.
• digestive system - YES, though with only a mouth and no anus (incomplete digestive system)
  • well-developed incomplete digestive tract in Turbellarians
  • reduced digestive system in Trematoda
  • digestive tract lost in Cestodes; nutrients absorbed through the epidermis
• nervous system - YES
  • well developed in Turbellarians
  • reduced in Trematodes
  • even more reduced in Cestodes (Why do you suppose this is true?)
    In the more primitive turbellarian flatworms, the nervous system consists of a simple nerve net with a minor concentration of neurons in the head.
    More derived forms have a "ladderlike" nervous system reminiscent of the segmentation seen in much more derived taxa, and a well-developed cerebral ganglion.
    • chemoreceptors are often present in flaplike extensions of the head called "auricles"
      

    • Photoreception is made possible with a very simple eye called an ocellus (one on each side of the body). An ocellus is little more than a pigment spot in an pigmented cup. This pigment cup casts a shadow, allowing the animal to determine the direction of incoming light.
    • statocyst (balance cells) are located near the cerebral ganglion
    Note that the nervous system and sense organs are often reduced or absent in parasitic forms.

• muscular system - YES
  • Longitudinal, circular and transverse muscle cells allow contraction in every direction. (Which ones contract the worm in which way?)
  • Muscular system well developed in turbellarians and flukes, vestigial in cestodes.
• reproductive system - YES
  hermaphroditic
• excretory system - YES
  In free-living turbellarians, it is compoesed of non-nucleated FLAME CELLS attached to tiny collecting tubules. The network of flame cells and tubules rings the external margins of the body, and there are external openings called NEPHRIDIOPORES located along the sides of the animal where nitrogenous wastes are excreted.

  The following organ systems are not present:

  • skeletal system - NO (though some forms have a few tiny, calcareous plates)
  • circulatory system - NO
  • respiratory system - NO
  • immune and lymphatic systems - NO
  • endocrine - NO

Symbiosis

The Turbellarians are all free-living, and the trematodes and cestodes are 100% parasitic, with often complex life cycles.

There are as many life cycles as there are parasites, but a a few definitions will serve us well:

• definitive host - organism in which the adult parasite resides
• intermediate host - organism in which various larval/developmental stages of the parasite exists until they is passed to the definitive host and metamorphose into the adult

Most species of parasite are relatively host-specific, but some can inhabit more than one different species of definitive host.

In many life cycles, more than one intermediate host is required for the full life cycle to be completed.

From this point onward in our taxonomic review, are TRIPLOBLASTIC (have three true embryonic tissue layers: ectoderm, endoderm and mesoderm) and bilaterally symmetrical.

However, unlike the acoelomate Platyhelminthes, these animals will have an internal, fluid-filled body cavity, either a PSEUDOCOELOM or a COELOM.

The most recent evolutionary studies suggest that the Bilateria consist of two major clades:

The Ecdysozoa

---

The Lophotrochozoa - A Wealth of "Worms" This clade includes such familiar and unfamiliar phyla as

• Mollusca - The Mollusks
• Annelida - The Segmented Worms
• Phoronida - The Horseshoe Worms
• Brachiopoda - The Lamp Shells
• Bryozoa - The Moss Animals
• Nemertea - The Ribbon Worms
• Sipuncula - The Peanut Worms
• Echiura - The Spoon Worms
• Pogonophora - Deep Sea Tube Worms
  (To your great relief, we'll be studying only the first two, shown in bold font. Both molluscs and annelids are COELOMATE.)

The Coelomate Bauplan:

• true plane of symmetry: bilateral

• three germ layers leading to true tissues
  • ectoderm (becomes the epidermis in the adult)
  • endoderm (becomes the gastrodermis in the adult)
  • mesoderm (derived from the endoderm)
• true organs and organ systems
  • integumentary system - YES
  • digestive system - YES
  • nervous system - YES
  • muscular system - YES
  • reproductive system - YES
  • excretory system - YES
  • circulatory system - YES
  • respiratory system - YES, IN MOST PHYLA

---

PHYLUM ANNELIDA - The Segmented Worms

Annelids all have

• typical protostome coelomate characters
• metamerism reflected in external and internal anatomy
• coelom serving as large, fluid-filled hydrostatic skeleton
• well-developed nervous system with a cephalic ganglion & ventral, ganglionated nerve cord
• a dorsal, closed circulatory system with several hearts
• appendages on each segment (even if simply bristles)
• metanephridia or protonephridia serving as excretory system
• larva a characteristic form called a TROCHOPHORE

Three classes of Annelids

• Polychaeta - (poly = "many"; chaet = "bristle") Marine segmented worms
  • free-swimming or sedentary
  • distinct cephalization with complex sense organs
  • paddle-like appendages on each segment called PARAPODIA

• Oligochaeta - (oligo = "few"; chaet = "bristle") - Earthworms & their allies
  • free-living; freshwater or terrestrial; usually FOSSORIAL
  • reduced cephalization with reduced sense organs
  • small bristles on each segment aid in locomotion through substrate
  • important detritivores in many ecosystems
• Hirudinea - (hirudo = "leech") Leeches
  • free-living predators/ectoparasites
  • coelom very reduced; body wall very muscular
  • salivary secretion "hirudin" is still sometimes used as an anticoagulant
  • many leeches are species specific, and many won't feed on a human

Let's look at some PICTURES.

---

PHYLUM MOLLUSCA - The Mollusks

The Hypothetical Ancestral Mollusk (H.A.M.) had all the major mollusk characters in their most primitive form.

All Mollusks have

• typical protostome coelomate characters
• coelom reduced to a vestigial GONOCOEL (surrounding reproductive organs & a few other structures
• a haemocoel as the primary body cavity
• an open circulatory system
• viscera concentrated in a VISCERAL MASS
• a thick, epidermal-cuticular layer of skin called the MANTLE which secretes the SHELL
• large, well-defined, muscular "foot"
• buccal opening with a rasplike RADULA feeding structure
• large, complex metanephridia serving as excretory system
• primitive forms have a TROCHOPHORE larva; in more derived forms, the trochophore develops into a more complex VELIGER larva before metamorphosis into the adult form

Some major classes of Mollusks

• Polyplacophora - (poly = "many"; plac = "plate"; phor = "to bear") Chitons
  • all marine, benthic
  • of the forms we will consider, this is the closest to H.A.M.

• Gastropoda - (gastr = "stomach"; pod = "foot") - Slugs & snails
  • free-living; marine, freshwater or terrestrial
  • high degree of cephalization with well-developed sense organs
  • torsion
  • single shell, often spiral or conical
• Bivalvia - (bi = "two";valve = "shell"; ) - Clams, oysters, mussels, etc.
  • marine and freshwater
  • gill used for both respiration and gas exchange
  • all are suspension filter-feeders
• Cephalopoda - (cephalon = "head";pod = "foot"; ) - Chambered Nautilus, Squids, Octopus
  • exclusively marine
  • fast-swimming predators
  • camera eye is analogous to the vertebrate eye, and forms complex, color images like our own
  • comparatively intelligent; Octopus are able to solve problems!

Let's look at some PICTURES.

---

Both Annelids and Mollusks have a similar larval stage called a TROCHOPHORE:

...which is further testament to their evolutionary relationship. (Remember what we said about similar embryonic development in related groups!)

In the molluscs, the trochophore goes one step further and becomes a cute little VELIGER:

...before it undergoes further development to become whatever type of mollusk is its fate.