1. Annelids ? Arthropods? Phylum Onychophora
Internal and external segmentation
Paired, segmental appendages
Teloblastic growth
General arrangement of the nervous system
(ventral, ganglionated nerve cord, ladder like N.S.)
Phylum Onychophora

2. 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
Distribution suggests they evolved before the Cambrian
Hallucigenia

3. Biogeography of Onychophora
Considered a relic Gondwanan lineage
Gondwana supercontinent
mya

4. Annelid-Like Arthropod-like
Characteristics
of the
Onychophora
No external segmentation
Extensible Walking legs
Annelid-Like 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

5. According to this
cladogram are the Onychophora the missing link between arthropods and annelids??
PanArthropoda

6. Phylum Arthropoda Crustacea Chelicerata 45,000 (75,000) Myriapoda
(13,000)
Insecta
~ 1 million
Phylum probably contains 5 million species
of which 1 million have been described

7. I. Extant Arthropod Diversity
Sub Phylum Mandibulata
Crustacea
Insecta
Myriapoda
Sub Phylum Chelicerata

8. Class Crustacea Sub Class Malacostraca Sub Class Copepoda
Sub Class Branchiopoda
Sub Class Cirripidea

9. Class Myriapoda
Class Insecta
Sub Class Chilopoda
Sub Class Diplopoda

10. Sub Phylum Chelicerata
Class Merostomata
Class Arachnida
Class Pycnogonida

11. What might be advantages of the arthropod exoskeleton?
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

12. Most Significant Character: Exoskeleton
Epicuticle is of lipids and lipoproteins
Chitin is high MW Nitrogenous
mucopolysaccheride
endocuticle
Fig 14.1
Hardness from scleratized
proteins or from
calcium carbonate
in Crustacea

13. But there are some constraints
Limitations on movement? Arthro - Poda
Joints of un-scleratized chitin on the exoskeleton
Flexors and extensors from within the skeleton

14. tissue sinuses venous blood gills
But there are some constraints
Hydrostatic skeleton is of no use
Reduction of coelom: main body cavity is hemocoel which is part of an OPEN circulatory system
Blood flow:
tissue sinuses venous blood gills
aortas heart

15. But there are some constraints
How to increase body size?
Arthropod
Growth pattern
Growth is by molting (ecdysis) with tissue size gains occurring during the intermolt period
Tissue
growth

16. III. Structural/neurobiochemical aspects of molting
all arthropods
have “molt lines”
I.e. a line of
weakness
on the shell

17. Review of the molting process

18. Control of Molting: Crustacea (Insects Later)
X-organ produces the molting inhibition hormone
MIH stored in sinus gland
Y- organ in the head of the animal produces ecdysone
Ecdysone initiates molting
Sinus
gland
X organ

19. Control of Molting in Crustacea
Appropriate Stimulus
CNS inhibits MIH
Production by X-organ
Drop in
Blood MIH
Y-organ produces ecdysone
Molting Initiated
X organ exercises a negative control on molting

20. 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.

21. The elaboration of the head has taken different courses in the major groups but are still considered to show serial homology
lost
Cheli
Pedi L
Ant1
Ant2
Mnd
Mx 1
Mx 2
Ant1
Mnd
Mx 1
Mx 2
A=antenna
a=acron
C=chelicera
Ci= Chilarum
L = leg
Mnd =Mandible
Mx = maxilla
P= pedipalp
0 = lost segments
Chelicerates Crustacea Insects
Ant
Leg
Ant
Leg
Ancestor
Trilobites

22. -Protocerebrum and deutocerebrum also in Onychophora
-But tritocerebrum is a unique character of Arthropoda
-Position of cerebrums helps establish homology of segments and segmental appendages