What is different between metazoa and protozoa

1. ANATOMY OF METAZOA
BRINESH, R
WELCOME

2. 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
Class
Order
Family
Genus
Species
The question today is
how we organise this
hierarchy
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.

3. 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?)

4. 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.

5. 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)

6. 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.

7. Sacculina
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

8. Haeckel’s dictum
It is an old observation that embryonic development seems to re-
trace 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.

9. 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.

10. Blastula – ball of cells Invaginates to make a gastrula, with2
or 3 cell layers
Ectoderm
endoderm
Ectoderm
mesoderm
endoderm
Diploblastic (cnidaria)
Blastopore
(becomes mouth in
protostomes, anus in
deuterostomes)
Triploblastic (others)

11. 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 blood-
vascular 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.

12. 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.

13. 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

14. 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
Platyhelminths
Nematodes
Arthropods
Molluscs
Annelids
Lophophorate phyla
Deuterostomes
Chordates
Echinoderms
Hemichordates

15. protostomes and the deuterostomes have different
embryology.
Protostomes Deuterostomes
Cleavage of
early egg: Spiral radial
Division Determinate Indeterminate
(hence we can have identical twins)
Blastopore becomes mouth becomes anus
Coelom from within mesoderm pouch from gut wall
Chitin often present absent
c

16. No tissues:
parazoa
Diploblastic
Cnidaria
ctenophora
Acoelomate
Platyhelminths
nemerteans
Pseudocoelomates
rotifers, other minor
phyla
Nematoda
Coelomates
Protostomes deuterostomes
Arthropods
Molluscs
Annelids
others
Chordates
echinoderms
Triploblastic phyla
differentiated tissues:
metazoa