Phylum Onychophora : Characteristics of the Onychophora Annelid ...

Phylum Onychophora :
• 80 or so species of rare animals
• 1.5 to 15 cm in length
• Found in moist places, tropical and So. temperate regions
• Evolved by the Cambrian
Characteristics
of the
Onychophora
No external segmentation
Hallucigenia
Annelid-Like
Extensible Walking legs
Arthropod-like
-Thin flexible outer cuticle - Growth by ecdysis
- Continuous muscles - Chitinous claws on leg tips
- Fluid filled body cavity - Chitinous jaws
- Appendages unsegmented - Open circulatory system
- Segmental excretory - Trachea, spiracular system
tubes like metanephridia
Is Onychophora the “missing link”
between Arthropods and Annelids?
Is segmentation homologous in arthropods and annelids?
PanArthropoda
Phylum Arthropoda
Chelicerata
(75,000)
Insecta
~ 1 million
Crustacea
45,000
Myriapoda
(13,000)
I. Extant Arthropod Diversity
Sub Phylum Mandibulata
Crustacea
Insecta
Myriapoda
Sub Phylum Chelicerata
Phylum probably contains 5 million species
of which 1 million have been described
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Class Crustacea
Sub Class Malacostraca
Class Insecta
Class Myriapoda
Sub Class Chilopoda
Sub Class
Copepoda
Sub Class Cirripidea
Sub Class Branchiopoda
Sub Class Diplopoda
Sub Phylum Chelicerata
Most Significant Character: Exoskeleton
Epicuticle is of
lipids and
lipoproteins
Class Merostomata
Class Arachnida
endocuticle
Chitin is high MW
Nitrogenous
mucopolysaccheride
Class Pycnogonida
Fig 14.1
Hardness from
scleratized
proteins or from
calcium carbonate
in Crustacea
Advantages and constraints of
the exoskeleton
Advantages:
But there are some constraints
Limitations on movement?
• Protection from injury and physiological stress
• Barrier against osmotic and ionic gradients
• Support
• Attachment for muscles
• Elaborated as a variety of structures:
wings, bristles, antennas, eyes, auditory organs,
outer shields, jaws, beaks, pincers, paddles
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But there are some constraints
Hydrostatic skeleton is of no use
Reduction of coelom: main body cavity is hemocoel
But there are some constraints
How to increase body size?
Arthropod
Growth pattern
Tissue
growth
Growth is by molting
(ecdysis) with tissue
size gains occurring
during the intermolt
period
Blood flow:
tissue sinuses venous blood gills
aortas heart
III. Structural/neurobiochemical aspects of molting
Review of the molting process in insects
all arthropods
have “molt lines”
I.e. a line of
weakness
on the shell
Control of Molting: Crustacea (Insects Later)
X-organ produces the
molting inhibition hormone
MIH stored in sinus gland
Control of Molting in Crustacea
Appropriate Stimulus
CNS inhibits MIH
Production by X-organ
Sinus
gland
X organ
Y- organ in the head of the
animal produces ecdysone
Ecdysone initiates molting
Drop in
Blood MIH
Y-organ produces ecdysone
Molting Initiated
X organ exercises a negative control on molting
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IV. Tendencies in Evolution of
Arthropod Body Plan
• Segmentation- when pronounced it is generally
considered an ancestral condition
Other Tendencies in Evolution of
Arthropod Body Plan
• Segmentation- when pronounced it is generally
considered an ancestral condition
• Tagmosis - tendency to organize segments into regions
having similar structure, function and appendages
Remipidea: blind Crustacea that live in sea caves head
followed by trunk with about 32 identical segments
Other Tendencies in Evolution of
Arthropod Body Plan
• Segmentation- when pronounced it is generally
considered an ancestral condition
• Tagmosis - tendency to organize segments into regions
having similar structure, function and appendages
• Cephalization - in early arthropods the head was scarcely
distinguishable; the development of a strong sensory,
feeding, cephalic region is a common theme in the various
groups.
Model
Ancestor
was
trilobite
like
The elaboration of the
head has taken
different courses in the
major groups but are
still considered to show
“serial homology”
A=antenna
a=acron
C=chelicera
Ci= Chilarum
L = leg
Mnd =Mandible
Mx = maxilla
P= pedipalp
0 =nephridia
Serial homology is the concept that initially existing structures
were gradually modified via discrete intermediary steps until such
time as an evolutionary novelty (e.g., jaws) appeared.
Many examples of serial homology, e.g. the body segments of many
animals (vertebrates, arthropods etc), are examples of gene
duplication on regulatory genes such as homeobox genes, followed
by evolution differentiating the duplicated genes.
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