Ernst Haeckel (1834-1919) proposed his Gastrea Theory in 1872 in an inspired attempt to synthesize a wide swath of zoological phenomena into a coherent darwinian whole.
Haeckel´s theory has long been the text-book model for talking about the origin of animals.
Haeckel´s genius was to take the descriptive model of the gastrula formation in amphibian development and comprehensively apply it, as an organizing metaphor, to phylogeny.
Haeckel´s creative but liberal use of metaphor was often insightful but also controversial and sometimes misleading.
Nonetheless, we will find an examination of his hypothetical gastrea organism to be useful for organizing our thoughts on the origin event and how it relates to the functional organization of animals as a whole.
In Haeckel´s view, the origins of diploblastic animals was the result of a feeding adaptation for consuming large food items via extracellular digestion and uptake of nutrients within a pouch lined with secretory cells - from here, the nutrients would then be distributed to the cells of the animal as a whole.
In the hypothetical gastrea organism the extracellular digestion of food takes place within a pouch (archentron) formed by the invagination of a spherical ball of cells (blastula) into a gastrula with a blastopore opening - the pouch is now lined with two primary tissues or germ layers, the digestive/reproductive endoderm and an somatic ectoderm.
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Elie Metchnikoff pointed out that extracellular digestion of food must have been preceeded by intracellular digestion and suggested that certain varieties of sponges with parenchemella embryos were a better developmental model for the first metazoans.
Metchnikoff would further challenge Haeckel´s conception of the origin of animals on the basis of discontinuities in the physiological viability of his proposed organism in the transition from unicellularity to multicellularity.
The Gastrea Theory is actually a description of diploblast origins, and perhaps the unambiguous end of the the transition from unicellularity and the emergence of true metazoan.
The embryological concept of germ layers was further articulated by E. Ray Lancaster in an attempt to theoretically encompass both phylogeny and development of the animal bodyplan..
Traditionally, Cnidarians and Ctenophores - collectively referred to as Diploblasts - have been considered to be radially symmetrical animals with possessing two primary embryological tissue types or germ layers:
- Endoderm - forms digestive tract
- Ectoderm - forms epidermis
The Cnidarians are classified based upon skeletal features, life-cycle, and habitat - The three main classes of cnidarian are:
- Hydrozoa - no calcified skeleton, acellular mesoglea, nematocyst restricted to epidermis, epidermal gametes, polyp and medusae forms, marine and freshwater habitats
- Scyphozoa - calcified skeleton, mesoglea with amoeboid mesechyme cells, epidermal and gastrodermal cnidocytes, gastrodermal gametes, reduced polyp with free-swimming medusae, marine habitats
- Anthozoa - calcified skeleton, mesoglea with amoeboid mesenchyme cells, cnidocytes without cnidocils, cnidocyte and gonad mesentries section the gastrovascular cavity, polyp form but no medusae marine habitats,
The defining feature of cnidarian is their possession of stinging cells - the cnidocytes.
Recently, evidence has been presented that suggests that at certain points in their development cnidarians express genes in their striated muscle that is usually associated with triploblasts - raising questions about their status as true Diploblasts ( ).
Cnidarians also contain a gelatinous acellular substance, mesoglea, that is secreted into the space between the germ layers - hence their common name: jellyfish.
Cnidarians are the first organisms to develop a nervous system - an uncentralized nerve net.
Cnidarians posses no centralized brain - nor do they possess a complete digestive tract.
Some cnidarians are predatory and possess nematocyst stingers which are used to stun prey.
The synaptic secretory molecules referred to as "neurotransmitters" predate the emergence of the neuron.
Many of the neurotransmitter substances can be shown to regulate growth and metabolism in a variety of unicellular populations ( ).
An array of formerly externally secreted cell-cell signalling molecules, like serotonin and acetylcholine, begin the evolutionary transformation to neurotransmitters with the arrival of localized synaptic specializations on the cell membrane - a feature that is a definitive characteristic of the neuronal morphology.
The diploblastic Cnidarians are the most primitive grade of animal to possess neuronal cells and an organized nervous system.
Cnidarians possess several distinct neuronal types including sensory-motoneurons, ganglia neurons and mechanoreceptor cells (nematocytes/cnidocytes).
With the emergence of the voltage-gated potassium channel gene families, axonal nerve conduction made possible cell-cell communication at a distance.
These potassium channels play a key part in the physiology of nerve conduction and are distinct from those potassium channels involved in maintaining the normal cell-potential that were inherited from their protozoan ancestors ( ).
Four gene subfamilies of voltage-gated K+ channel were inherited by all triploblasts:
Also of interest is the emergence of neuronal sodium and calcium channels as well.
The emergence of neurons and the neural net enabled cnidarians to engage in complex active behaviors.
The hydra nervous system is formed of:
- sensory-motoneurons
- ganglia neurons
- nematocytes - mechanoreceptor cells
Cnidarians possess a relatively simple nerve net with a nerve ring that is usually positioned at the base of the tentacles.
Over the lifetime of an individual animal the bodyplan may proceed thru several quite distinct stages of organization.
All cnidarians have a sexual stage, producing sperm and eggs and a zygote that develops into a planula larva
This indicates that the genomes of animals have the potential to direct the development of multiple bodyplans over the lifespan of the organsim.