Annelids Arthropods Internal and external segmentation Paired segmental

Annelids ? Arthropods? • 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

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

Biogeography of Onychophora Considered a relic Gondwanan lineage Gondwana supercontinent 500 -200 mya

Characteristics of the Onychophora Annelid-Like No external segmentation Extensible Walking legs -Thin flexible outer cuticle - Continuous muscles - Fluid filled body cavity - Appendages unsegmented - Segmental excretory tubes like metanephridia Arthropod-like - Growth by ecdysis - Chitinous claws on leg tips - Chitinous jaws - Open circulatory system - Trachea, spiracular system

According to this cladogram are the Onychophora the missing link between arthropods and annelids? ? Pan. Arthropoda

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

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

Class Crustacea Sub Class Malacostraca Sub Class Copepoda Sub Class Cirripidea Sub Class Branchiopoda

Class Insecta Class Myriapoda Sub Class Chilopoda Sub Class Diplopoda

Sub Phylum Chelicerata Class Merostomata Class Arachnida Class Pycnogonida

What might be advantages of the arthropod exoskeleton? Advantages: • 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

Most Significant Character: Exoskeleton Epicuticle is of lipids and lipoproteins endocuticle Fig 14. 1 Chitin is high MW Nitrogenous mucopolysaccheride Hardness from scleratized proteins or from calcium carbonate in Crustacea

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

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 aortas venous blood heart gills

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

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

Review of the molting process

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

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

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.

lost Cheli Pedi Ant 1 Ant 2 Mnd L L Mx 1 Mx 2 Chelicerates Crustacea Ant Leg Leg Leg Ancestor Ant 1 Mnd Mx 1 Mx 2 Insects Ant Leg Leg Leg Trilobites 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 = lost segments

-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