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Basic anatomy of metazoa
Peter Shaw
Overview
We have already reviewed the standard taxonomic approach to
invertebrate phyla - today we will examine the theoretical
underpinning of the higher level classifications, with particular
reference to body cavities. We will also examine other features of
invertebrate body design.
Remember:
Kingdom
Phylum The question today is
Class
how we organise this
Order
hierarchy
Family
Genus
Species
Within the metazoa, there are
several different ways of
classifying animals. Some are
interesting but purely
descriptive, such as a
classification based on
symmetry, or on skeletal
systems.
Classification based on
symmetry:
Radial – cnidaria (more apparent than real: really a 4-radial symmetry
as in scyphozoa, or bilateral as in the internal anatomy of anthozoa)
Pentagonal – echinodermata
Bilateral – almost everything else (?Why?)
Classification based on skeletal
systems:
No differentiated skeletal system: platyhelminths
Mesogloeal tissue: many Cnidaria
Hydrostatic skeleton: nematoda, and annelida (echinodermata, in
addition to an endoskeleton).
Complete exoskeleton: arthropods
Partial exoskeleton: most molluscs, some cnidaria
Endoskeleton: chordates (most), echinodermata.
Classification based on
metamerism (segmentation)
Unsegmented – many phyla
True segmentation (with all body characteristics repeated in each
unit, at least in the primitive state)
Annelids, arthropods, chordates
Pseudo-metamerism
Repetition of some parts of the body
Tapeworm bodies, stalked larval forms of scyphozoa (jellyfish)
Embryology
However, the most useful classification of body forms is that which
is believed to best reflect evolutionary history. Often this has relied
on embryology. Why?
This is because early embryonic stages differ far less between phyla
than do adult forms. The early embryonic development seems to
give us a glimpse into the development of long-lost ancestors.
In a few cases, the adult form of animals is
so degenerate as to be unidentifiable, and
before the advent of DNA-based
techniques the only way top classify these
oddities was by their early embryonic
form.
The classic example of this is the crab
parasite called Sacculina. This exists as a
fungus / cancer-like mass of
undifferentiated cells permeating the
whole of an infected crab’s body,
emerging as a yellow sac at its genital
opening. The larvae turn out to be
identical to larval barnacles – they settle
on crabs just like normal barnacles, then
inject a mass of cells into the crab and
cease to resemble any recognizable
Sacculina
Haeckel’s dictum
It is an old observation that embryonic development seems to retrace evolution – embryonic humans resemble embryonic fish. In
1866 the German biologist Ernst Haeckel published a book titled
Generelle Morphologie der Organismen, claiming that embryonic
development retraced evolutionary history – giving rise to
Haeckel’s dictum:
Embryology recapitulates phylogeny
This is not taken too seriously nowadays, but is still a nice quote.
2 or 3 cell layers?
The basics of embryonic development give us one fundamental division
within the Metazoa. Some (presumably simpler) forms develop from 2
layers of cells, while the more complex forms develop from a 3-layered
embryo.
This gives us diploblastic and triploblastic life forms.
Diploblastic animals have an endoderm (interior => guts) and an
ectoderm (exterior => “skin”), but nothing else. These are the cnidaria
and ctenophora – jellyfish and allies.
Triploblastic forms have a third layer of cells, the mesoderm, which
usually develops into muscles etc. (Oddly, in chordates the central
nervous system develops from the ectoderm). All metazoan animals
apart from cnidaria/ ctenophora are triploblastic.
Blastula – ball of cells
Invaginates to make a gastrula, with2
or 3 cell layers
Blastopore
(becomes mouth in
protostomes, anus in
deuterostomes)
Ectoderm
mesoderm
endoderm
Ectoderm
endoderm
Diploblastic (cnidaria)
Triploblastic (others)
Body cavities
A next set of fundamental division is based on the development of
body cavities during embryonic development.
Most higher animals have fluid-filled cavities within the body. These
allow space for organ development, allow for fluid circulation etc.
The simplest way to produce a body cavity is to retain the space
between the ectodermal and endodermal layers of the embryo. This
cavity is called the blastocoel, and is retained in most metazoa, giving a
fluid-filled cavity variously called the haemocoel, pseudocoel or bloodvascular system. As the names imply, this cavity is often used to
contain blood. In insects, molluscs, and many other invertebrates this
is the only significant body cavity.
Coelom (pron. See - lom)
Additionally, a second cavity can develop during embryonic
development, arising de novo as a space between mesodermal
cells. This is known as the coelom (or true body cavity), and is
lined with a specialised layer of cells, the peritoneum. In
mammals the coelom is the space occupied by guts, liver, heart
etc.
Metazoa with a true coelom are known as coelomate. These
include chordates, annelids, molluscs and echinodermata.
This gives us 3 divisions of animals, based on their body cavities:
Acoelomate – no body cavity:
Cnidaria, ctenophora, mesozoa, platyhelminths, nemerteans
Pseudocoelomates – only with remnants of the blastocoel:
Nematodes, rotifers, and various minor phyla (nematomorpha,
gastrotrichs, entoprocts, acanthocephala + others)
Coelomates: Fully developed coelom (though may be secondarily
reduced):
Molluscs, arthropods, annelids, chordates, echinodermata, + others
A final division within the coelomates is again based on embryology.
Chordates and echinoderms have some common patterns of early
development that differ from other coelomates, notably in the pattern
of cell division and the formation of mesoderm + coelom. This leads
to echinoderms, chordates (and a very minor group the hemichordates)
to be classed together as deuterostomes, while the other coelomates
are classed as protostomes.
Protostomes
Deuterostomes
Platyhelminths
Nematodes
Arthropods
Molluscs
Annelids
Lophophorate phyla
Chordates
Echinoderms
Hemichordates
protostomes and the deuterostomes have different
embryology.
Protostomes
Cleavage of
early egg:
Spiral
Deuterostomes
radial
c
Division
Determinate
(hence we can have identical twins)
Indeterminate
Blastopore
becomes anus
becomes mouth
Coelom
from within mesoderm
pouch from gut wall
Chitin
often present
absent
No tissues:
differentiated tissues:
parazoa
metazoa
Diploblastic
Triploblastic phyla
Cnidaria
ctenophora
Acoelomate Pseudocoelomates Coelomates
rotifers, other minor
Platyhelminths phyla
Nematoda
nemerteans
Protostomes deuterostomes
Arthropods
Molluscs
Annelids
others
Chordates
echinoderms
DNA-derived phylogenies
Genome
Junk DNA – no selection
pressure, varies quasirandomly between
individuals
Useful genes – can’t
vary greatly within 1
species
Active site of crucial
enzyme – changes
hardly ever happen
An example of a crucial
sequence that changes very
slowly and may be used to
derive high-level
taxonomic relationships:
the ribosome has to bid
exactly to mRNA and to all
the tRNAs or the organism
will die before its first cell
division. rRNA
homologies are used to
establish relationships
between phyla.
DNA-derived taxonomy
We can now use these slowly-changing DNA
sequences, notably 16srRNA, to derive an objective
hierarchy for animal classification.
Generally it agrees well with the classical tree based
on embryology, though there are a few changes,
notably in that arthropods are joined with nematodes
in the ecdysoza, while nemerteans and most
platyhelminths join molluscs and annelids in a new
group the lophotrochozoa (all having a prototroch
larva).
DNA-derived
classification of
animal phyla
differentiated tissues:
metazoa
Diploblastic
Cnidaria
ctenophora
Lophotrochozoa
Molluscs
Annelids
nemertines
Platyhelminths
? polyphyletic
Triploblastic phyla
Ecdysozoa
Nematoda Arthropods
deuterostomes
Chordates
echinoderms
Myxozoa – once
protozoan parasites
of fish, now shown
to be degenerate
anemones!