Cnidarians and Ctenophores are beautiful!

The Bilateria The bilaterally symmetrical animals are a vast assemblage of animals who exhibit bilateral symmetry: in longitudinal section, the two halves of the body form mirror images.

The phylogeny of the Bilateria is still being elucidated. Three major, putatively monophyletic groups are:

• Lophotrochozoa
• Ecdysozoa
• Deuterostomia

But the diversity of animals whose evolutionary affinity to those three groups is so immense that it wouldn't be fair to just ignore them. So as a representative of those confusing taxa whose relationships we don't yet know, let's visit the Flatworms. Not only are they economically important, but they also exhibit what is believed to be the most primitive bilaterian body: acoelomate.

Platyhelminthes: The Flatworms

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

There are three major groups:

• Turbellaria - Free-living (non-parasitic) Flatworms
• Trematoda - Flukes (all species parasitic)
• Cestoda - Tapeworms (all species parasitic)

The Platyhelminth Bauplan

In these 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 mesogloea)

The following organ systems are present:

• integumentary system - YES
  Parasitic forms (flukes and tapeworms) have a protective tegument external to the epidermis. It is formed by nonciliated, cytoplasmic extensions of large cells whose bodies lie within the mesenchyme. The tegument affords protection against the host's defenses, but allows gas exchange for the parasite.
• digestive system - YES
  • well developed incomplete digestive tract in Turbellarians
    Note: An incomplete digestive tract has a mouth, but no anus. Waste exits the same way the food came in. A complete digestive tract has both a mouth and an anus.

  • reduced digestive tract in Trematoda
  • digestive tract secondarily lost in Cestodes
• 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 more derived animals, and a well-developed cerebral ganglion (almost a tiny brain).
    • 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 a pigmented cup. The pigment on the cup, present on only one side, casts a shadow, allowing the animal to determine the direction of incoming light.
    • statocysts (balance cells) are located near the cerebral ganglion, and give the animal information on its body position relative to gravity.

    The nervous system and sense organs are usually reduced or absent in parasitic forms.

• muscular system - YES
  • Longitudinal, circular and transverse muscle cells allow contraction in every direction.
    • longitudinal muscles - contraction makes the body shorter and fatter
    • circular muscles (encircle the body) - contraction makes the body long and thin
    • transverse muscles - arranged at angles, these allow movement in many directions.
    Remember: a muscle works only by contracting, never by "pushing" or expanding.

  • Muscular system well developed in turbellarians and flukes, vestigial in cestodes.
• reproductive system - YES
  all forms are hermaphroditic
• excretory system - YES
  In free-living turbellarians, the excretory system consists of a tubular protonephridial system. It is composed of anucleate flame cells attached to collecting tubules. The network of flame cells and tubules rings the external margins of the body, and there are nephridiopores located along the sides of the animal where nitrogenous wastes are excreted. (Flame cells are so called because the motion of the flagella inside, that set up fluid current to draw body liquid through the tubules, appears reminiscent of a candle flame when viewed under a microscope.)

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

Let's look at some pretty pictures.

Flatworm Symbioses

The Turbellarians are all free-living. 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.

Transmission of parasites (or any pathogen, for that matter) may be

• horizontal - from one individual to another via contact
• vertical - from parent to child, at or before birth

And now for some fun parasite stories.