Protostome Evolution and the Origin of Arthropods

The first arthropods arose in ancient PreCambrian Seas over 600 million years ago. Since that time they have evolved to become the most abundant and diverse group of animals on Earth. They have successfully invaded virtually every habitat and exploited every imaginable lifestyle and developmental strategy.

This lecture is to demonstration to you the steps in the evolution of arthropods from an annelid like ancestor.

The Annelids - A review of their basic body plan

The annelids are the segmented worms (earthworms, leeches, and polychaetes).
In annelids all of the body, except the central nerve tracts, main blood vessel, and digrestive tract are segmented.

Segments allow more efficient locomotion and permits specialization of body parts.

More efficient locomotion: Each segment has its own separate set of muscles (run circular and longitudinal in each segment; no cross-striations) that can bend or stretch each segment independently. Allows burrowing.

Specialization of body segments: anterior for sensation (simple eyes, chemosensation, food capture); posterior end for excretion; other segments for reproduction or locomortion.

In Annelids, the nervous system consists of a "brain" or dorsal ganglion; two circumpharyngeal (remember that -it is an important apomorphy they share with Arthropods!).

The body is covered in thin, flexible, permeable chitinous cuticle that stretches as the animal grows.

Evolution of Arthropods and their Allies

Arthropod Characteristics

Like Annelids they have:
1. Bilateral, triploblastic protostomes.

2. Body segmented, both internally and externally.

3. Nervous system annelid-like with dorsal brain (=cerebral ganglion) and circumpharyngeal nerves and paired ventral nerve cords.

Unique features include:
4. The phenomenal success of these invertebrates can be attributed in part to their external skeleton.

The exoskeleton is composed of a thin, outer protein layer, the epicuticle, and a thick, inner, chitin-protein layer, the procuticle.

In most terrestrial arthropods, such as insects and spiders, the epicuticle contains waxes that aid in reducing evaporative water loss.

The hardness of various parts of the exoskeleton in different arthropods is related to the thickness and degree of hardness of the cuticle. In crustaceans, additional rigidity is achieved by having the exoskeleton impregnated with varying amounts of calcium carbonate.

Annelids have a thin chitinous cuticle covering their bodies but it is not as well developed as in arthropods.

This exoskeleton clearly endowed arthropods with great selective advantages, as evidenced by their enormous success.---

i. protection - arthropods are armored against ----------
a. physical injury----------
b. physiological stress - effective barrier against osmotic and ionic gradients and is a major means of homeostatic control. ---
ii. Strength - for a given quantity of skeletal material, a hollow tubular structure is stronger than a solid rod of material, as is found in our own skeletons.

5. There is a basic problem with locomotion - the animal is going to do more than sit around in a hard shell. This problem was solved by the evolution of jointed appendages. In fact, the name arthropod means jointed (=arthro) foot (=pod).

flexible joints (thin intersegmental places) and regionalized muscles in the body and appendage. These muscles are striated, not smooth as in annelids, and are organized as intersegmental bands associated with the segments - circular muscles wrapping around the body are lost entirely.

With the loss of body flexibility, the coelom became useless as a hydrostatic skeleton SO:

6. Coelom reduced to providing space for portions of the reproduction and excretory system.

7. The complex process of ecdysis (molting) evolved in which the skeleton is periodically shed to allow for an increase in real body size.

The imposition of a rigid exoskeleton prevents growth by gradual increase in external size.

Instead, the body size increases in staggered increments associated with the loss of the old exoskeleton and the secretion of a new, larger one.

The arthropod exoskeleton is protective, but once molted and before the new skeleton has hardened, the animal is quite vulnerable to injury, predation and osmotic stress.

Many arthropods become reclusive, hiding in protective nooks and crannies and not even feeding when in this "soft shelled" condition.

8. Most groups have a strong tendency toward regional body specialization or tagmosis.

The primitive arthropod had a fairly high number of segments, each with a pair of jointed appendages. The diversity seen today has resulted largely from differentiation and specialization of various segments and appendages.

Tagmosis - regional specialization to produce groups of segments specialized for a host of different functions. These regions are called tagmata (e.g., head, thorax, and abdomen).

9. Tracheae-spiracular system - Gas exchange in many (not all some aquatic forms have gills) is by tracheae that open to the outside through many small spiracles located down the body side.

Steps leading to the Arthropods from the Annelids

Phylum Onychophora

Onychophorans resemble soft-bodied, unsegmented centipedes or caterpillars. They range in length from 1.5 to 15 cm. Living onychophorans are confined to humid habitats and most species occur in the tropics. Most onychophorans are distinctly colored blue, green or orange and the papillae and minute scales that cover their body surface give it a velvety sheen (therefore called velvet worms).

The members of this group have features that are intermediate between those of annelids and arthropods. For these reasons onychophorans are sometimes referred to as "missing links" between the annelids and arthropods.

Annelid-like Features of the Onychophorans:

1. Muscles in the body wall - do not form regional groups.

2. The body is covered in thin, flexible, permeable chitinous cuticle that is not divided into plates. But see arthropod characters below.

3. Unjointed appendages. - They have 14-43 pairs of simple conical walking legs. These are not jointed or segmented but are instead saclike and contain coelomic fluid and external muscle insertions.

Walking is accomplished by similar to the way polychaete annelids move

4. The eyes are not compound eyes, but are simple direct eyes with a chitinous lens and a relatively well developed retinal layer.

5. Little tagmentation - body segments are not fused.

Arthropod-like Features of the Onychophorans:

6. Growth by molting

7. Tracheae-spiracular system Gas exchange is by tracheae that open to the outside through many small spiracles located down the body side.

8. Striated Muscles although they show oblique striations rather than cross striations.------------

Living onychophorans are similar to several Cambrian marine fossils. They are unique in that they have slime palps around their mouth - most prey on small invertebrates such as snails, worms, termites and other insects which they pursue into small cracks and crevices. The slime glands at the base of the slime palps produce and discharge a stream of adhesive (sometimes up to a distance of 30cm). The adhesive hardens quickly, entangling the prey (or would-be predators) in a web-like net for later leisurely dining.

Phylum Tardigrada

The tardigrades make up a small phylum that appears to be closely tied to the annelid-arthropod line.

Over 400 species have been described, Most live in semiaquatic habitats such as the water film on mosses, lichens, liverworts, soil, etc. Others live in various freshwater &;marine benthic habitats. All are small usually 0.1-0.5 mm in length, although some giant 1.7 mm forms have been found.

Under the microscope, tardagrade resemble miniature, 8 legged, bears. They even move with a lumbering bear-like gait and so are commonly called "water bears."


1. The body is covered by a thin, uncalcified cuticle as in annelids. However it is periodically molted as in arthropods and onychophorans.

2. The legs are short hollow extensions of the body wall and the muscle is comes in from the main part of the body so that they resemble those of onychophorans and annelids.

4. There is striated muscle. The striations in the striated muscle are of the cross-striated type as in arthropods rather than the oblique type in onychophorans.

5. Nervous system built on the arthropod-annelid plan.

6. Sensory setae cover body


Tardigrades lack a circulatory system with discrete blood vessels or gas exchange structures.

The first Arthropods: The Trilobites

Of all the fossil invertebrates, trilobites are perhaps the most symbolic as ancient and extinct faunas in the minds of most people. The group includes nearly 4,000 species of arthropods known only from the fossil record. They were very abundant in early Paleozoic seas (Cambrian and Ordovician, 500-600 million years ago) and persisted into the Permian (280 million years ago).

Although trilobites were exclusively marine, they exploited a variety of habitats and lifestyles. Most were benthic - they lived crawling about over the bottom or plowing through the top layer of sediment.

Most of these benthic trilobites were probably scavengers or deposit feeders, although some may have been predators that lay partially hidden in the soft sediments and grabbed passing prey.

General body form
The trilobite body was broadly oval and somewhat flattened dorsoventrally. The exoskeleton is divided into three parts:

1. The chephalon, or head, which has a pair of eyes (the first high definition eyes to appear in the fossil record) and a single pair of antennae.

The mouth is underneath the head and faces backwards. The earliest trilobites were deposit feeders - Apparently as the legs moved over the mud, they stirred up the silty organic material which was then sucked into the mouth.

The large bulbous structure on the top of the head, called the glabella, looks as if it is the animals brain but it is not, it is the stomach.

2. The thorax composed of the body segments. Each body segment had a pair of jointed appendages.
3. Abdomen - reproductive and excretory structures.

The trilobites were extremely successful in the paleozoic, but had gone extinct by the end of the Paleozoic. Only the horseshoe crab, a distant relative is alive today.

Arthropods and the Colonization of Land

Fossil Record
The oldest undisputed terrestrial animal fossils date from the Silurian. These fossils include wingless insects, centipedes, spiders, and scorpions.

The arthropods were the first animals on land. The environmental stimulus may have been predation pressure - too many predators in the aquatic habitats would have made land a safe haven.

Physiological Considerations for Terrestrial Living

A. Water
On land, water loss due to evaporation from the body is a particular problem. Arthropods have been able to solve this problem by:

1. the use of a waterproof waxy exoskeleton covering all exposed parts.

On land, the waxy cuticle not only protects against water loss but may also have conferred some protection against harmful UV radiation and from attack by fungi.

2. Internalizing respiratory surfaces.
a. Gills - in some terrestrial crustaceans (pill bugs, land crabs), gills are kept in an internal brachial chamber.
b. Book Lungs - The chelicerata (scorpions and spiders) have book lungs - internal laminate structures in the abdomen that open to the outside via a pore.
c.. Tracheae - insects, millipedes, centipedes, and onychophorans all respire via a tracheae-spiracle system. These organisms have a series of tubes through which air simply diffuses, the oxygen and carbon dioxide exchanged with the tissues through which the tubes run.

3. Water loss during excretion of ammonia waste is solved by arthropods by the converting ammonia to a solid waste, uric acid.

B. Temperature
Temperature is far more variable and unpredictable on land than in water. In part this is because the rate of heat transfer is much greater in air than in water. A few arthropods have adapted to cold temperatures by developing a sort of antifreeze in their body. A few moths are covered with a fur-like covering of small scales that help them stay warm. But the vast majority of arthropods die or become dormant during cold temperatures.

C. Support and Locomotion
Having lost the buoyancy of water, terrestrial organisms must cope with the consequences of gravity. Arthropods that colonized land developed a hang from their exoskeleton for greater stability and have some form of ankle joint and "foot" to allow them to walk more efficiently.

The exoskeleton limits the size of the arthropod. To grow bigger, the animal must discard its old exoskeleton and grow a new and bigger one to replace it. During this period of molting, the animal lacks support and is very vulnerable to predators. In water, support is less of a problem because water is dense and buoys the flabby animal up. On land, arthropods have to inflate themselves with air to keep their body shape while a new exoskeleton is hardening. The larger the animal, the more difficult the problem becomes. This explains why terrestrial arthropods are smaller than aquatic ones.

D. Reproduction and Dispersal of Young
Some terrestrial arthropods (such as the land crab) return to the water to mate. Their young develop in the water and only move onto land after they have developed the adult characteristics that protect them from drying out on land.

Others, such as insects, have internal fertilization of the eggs, the eggs have a protective covering to prevent them from drying, and the young develop in the egg until they have acquired the features that protect them from drying out.

Insect Evolution

Insects are the most successful group of organisms on the planet. Over the course of evolution, insects have acquired several features that have contributed to their success.
The earliest insects lacked wings:

The evolution of wings in insects was a momentous event -- the first creatures to take to the skies, they were the undisputed lords of the skies for 100 million years. The benefits of flight for these early insects must have been pressing enough. They evolved during the age of amphibians and small reptiles. The arthropods were abundant and the were probably the main prey item for these vertebrates.

The wings of insects are not, as in bird or bat wings, a modified limb. They are a totally new structure. How did it evolve? This is a question to which there is no clear answer. Presumably 1/2 of a wing would be of no use, so how could wings gradually evolve? Preadaptation once again may be the answer. One suggestion is that wings were originally struts that held respiratory structures, by fanning the structures greater exchange of gases was possible. Another suggestion is that they were protective shields over the vulnerable leg joints. Whatever their first function, as wings they are highly successful.

Although useful, wings in their primitive form were also something of an encumbrance. Like modern-day dragonflies and mayflies, the primitive insects were unable to tuck their wings away when at rest. These insects would not have been able to crawl through vegetation, much less burrow in the ground.

Not surprisingly, the next major evolutionary burst followed the evolution of foldable wings, an event clearly marked in the fossil record. Modern equivalents are the locusts and stoneflies.

The next major evolutionary advance in insect history was the development of metamorphosis, which appeared around 270 million years ago. In insects without metamorphosis, the young that hatch from eggs closely resemble miniature adults (e.g., cockroaches) but without wings or reproductive structures. Each stage, or instar, molts to give rise to the next until a mature adult is produced.

In insects with metamorphosis, the egg hatches into a larvae (or caterpillar, maggot, grub), which develops into a dormant pupae, which in turn transforms into an adult.

Insect larvae are very different from the adults. It is a tremendous advantage for the young to live and feed in a totally different environment from the adults. This immediately expands the range and conditions that can be tolerated. It prevents competition between parent and offspring.

The first insects with metamorphosis are the flies and bees followed very closely be beetles.

Beetles evolved about 230 million years ago and had a new adaptation - the first pair of wings is modified into a protective covering that makes the beetles relatively free of predators or parasites. Beetles were so successful they quickly rose to become 40% of the insect life - a percentage they maintain to today.

Conclusion - each major adaptation showed a population explosion of the type that had it and a reduction of the more primitive forms:

1. wingless insects
2. winged insects
3. folded winged insects
4. metamorphosis
5. hardened first pair of wings.