Protostome Evolution

The Protostomes are divided into two groups:

1. The Lophotrochozoa - which includes the segmented worms, molluscs, lophophorates and several more obscure phyla.
2. The Ecdysozoa - which includes the arthropods and several other phyla that periodically molt.


Evolution of Arthropods and their Allies

Arthropod Characteristics

Like the Lophotrochozoa, Arthropods are triploblastic protostomes. They also have a nervous system with a dorsal "brain", circumpharyngeal nerves and paired ventral nerve cords.

Like the annelids, their body is segmented, both internally and externally. This was thought to be evidence that the annelid worms and arthropods were closely related. We now believe that the segmentation is a convergent feature.

Unique features include:

1. The phenomenal success of arthropods 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.

This exoskeleton clearly endowed arthropods with great selective advantages:

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.

2. There is a basic problem with locomotion if the animal is surrounded by 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 the lophotrochozoa, and are 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:

3. Coelom reduced to portions of the reproduction and excretory system.

4. Circulatory system open and uses the coelom as a chamber in which internal organs are bathed directly in body fluids.

5. Usually with a pair of compound eyes.  
As the name suggests, compound eyes are composed of many similar, closely-packed facets (called ommatidia ) which are the structural and functional units of vision.   The number of ommatidia varies considerably from species to species:   some worker ants have fewer than six while some dragonflies may have more than 25,000. Externally,each ommatidium has a lens formed by the convex thickening of transparent cuticle.   Beneath the lens, there is often a crystalline cone which refracts incoming light down into a receptor region containing light-senstive pigments. These pigments absorb wavelengths of light and generate nerve impulses through a photochemical process similar to that of vertebrates.

Insects cannot form a true ( i.e. focused) image of the environment, so that their focused vision is relatively poor compared to that of vertebrates.   On the other hand, their ability to sense movement, by tracking objects from ommatidium to ommatidium, is superior to most other animals. 

Many can distinguish 200 images/second (in humans, still images blur into constant motion at about 30 images/second).   They can detect polarization patterns in sunlight, and discriminate wavelengths in a range from ultraviolet to yellow (but not red).

In addition, surface sense organs, called setae, appear.

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

The 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 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.

7. 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).

8. Trachae-spiracle 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.

9. Extreme diversity - 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.

Most specialists believe that there are millions yet to be discovered. No other group of animals or plants approaches this magnitude of species richness.

Lets investigate the steps in the evolution of arthropods from a lophotrochozoan-like ancestor.

Steps leading to the Arthropods: The Ecdysozoans (molting animals)

Chaetognatha (arrow worms)
These small worms have stiff bodies supported by fins. They are active predators and live in the plankton (close to the surface at night and in deeper waters during the day).

Covered with a flexible cuticle that can be molted. BUT they have a pseudocoelom instead of a true coelom.

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 lophotrochozoans like the 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. Circular muscles in the body wall.

2. The body is covered in thin, flexible, permeable chitinous cuticle that is not divided into plates and does not contain wax . 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. These short, claw-tipped, hollow legs are kept rigid and are extended by hydrostatic pressure.

Walking is 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. Trachae-spiracle 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 (see picture to right).
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 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.

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

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

5. 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 subphylum Trilobitomorpha 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).

Because of their hard exoskeleton, great abundance and wide distribution, the trilobites have left a rich fossil record, and more is known about them than about many other extinct taxa.

The Ute indians called them timpe khanitza pachavee - which roughly translates into "little water bug that lives in a stone house."

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. These were in two parts (biramous), with an outer gill-bearing branch and an inner branch.
3. Abdomen - reproductive and excretory structures.

The trilobites were extremely successful in the paleozoic, but had gone extinct but mya. Only the horseshoe crab, a distant relative is alive today (tell them about the trilobite larvae).

What does the Trilobite tell us about early arthropods?

1. Tagmatintization occurs later

2. 1 pair of legs per segment

Arthropods and the Colonization of Land

One can scarcely imagine the continents as totally unoccupied: not simply empty of life, but untouched by it. There was nothing to halt the scouring erosion; nothing to soften and transform the sands and the clays into organic-rich soils; no sounds beyond those of the physical elements in desolate isolation.

Fossil Record
The oldest undisputed terrestrial animal fossils date from 400 mya (found in Aberdeen Scotland - similar fossils dated from 390 mya from Germany). These fossils include wingless insects, centipedes, spiders, extinct spider relatives called trigonotarbids, and scorpions. The fully terrestrialized features of these organisms cause us to speculate that the first terrestrial arthropods appeared in the Silurian.

Although the arthropods were not the first organisms on land - that honor, as we shall see, belongs to the nonvascular plants - 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. Moving onto land would not have been possible until the Silurian because it was not until then that the amount of ozone (O3) in the atmosphere reached a high enough concentration to block the harsh UV light striking the earth. As the atmosphere evolved, mats of bacteria may have first colonized the land and thus produced habitats for animals and plants.

Physiological Considerations for Terrestrial Living

A. Water
Water is essential for life: virtually all biochemical reactions work in an aqueous medium. 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.

In early aquatic arthropods, a waxy cuticle covering the exoskeleton may have been an adaptation for help in maintaining internal osmotic pressure when that of the animal differs from that of the surrounding environment.
The presence of waxes on the exoskeleton preadapted arthropods for living on land. (Preadaptation is not an anticipation of future needs, but the fortuitous ability to use a structure for functions different than that which it evolved for).
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 (isopods, 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.. Trachae - insects, millipedes, centipedes, and onychophorans all respire via a trachae-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.

Such a system is clearly effective - otherwise insect species would not outnumber all other species three to one. And yet it has placed a severe size constraint on these highly successful organisms. The system of tubes, or trachea, works well over short distances, but becomes less and less efficient over large areas. Some insects have developed various methods for stretching the efficiency of their tracheae, such as making rhythmic muscular movements that assist in pumping air through tubes. But such strategies offer minor improvements, and most insects today and in the past have been small, as opposed to animals equipped with lungs. Of course there have been exceptions - during the Jurassic (when O2 levels were higher and CO2 levels were lower than today) dragon-flies had wing-spans of 30 inches, but these must still be considered small compared to the size of some animals with lungs during the same time.

3. Water loss during excretion of ammonia waste is solved in 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, whether they are mobile or sedentary animals. 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.

Although the exoskeleton is highly adaptable, it has one great drawback: it 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 sever the problem becomes. which 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. One out of every three species of organisms is an insect. There are more species of beetles than there are of all the vertebrates. 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.