Sponges lack true tissues or a true plane of
body symmetry, though some forms are superficially radially symmetrical.
Their tissues are colorless and translucent, with the same refractive index as water. This makes them nearly invisible in their natural habitat, and well concealed from predators.
Cnidarians and Ctenophores are beautiful!
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.
