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Introduction to Arthropods
Have you ever heard the saying that one of the only things that will be left after a nuclear war will be roaches? Well, that may be because they're arthropods—probably the most successful animals in existence; it appears that they can adapt to anything that nature throws at them. While roundworms may have the greatest number of individual organisms, they cannot compare with the number of species that make up the huge phylum Arthropoda. Biologists estimate that there are about 1 million living species of arthropods—more than all of the other phyla of animals combined! They literally are everywhere—in every habitat, in every part of the world.
Arthropods belong to the phylum Arthropoda. As you might imagine, there is an enormous amount of diversity among arthropods, but all of them fit into three major classifications:
♥Chelicerata: Spiders, scorpions, ticks, mites, and horseshoe crabs
♥Crustacea: Lobsters, shrimp, crayfish, crabs, and barnacles
♥Uniramia: Insects, millipedes, and centipedes
Because there are so many organisms, in so many classifications, we'll begin with some general characteristics that fit most or all arthropods; then, we'll examine some of the adaptations that have enabled them to be the ultimate "survivors."
General Characteristics of Arthropods
♥Arthropods have bilaterally symmetrical, segmented bodies.
♥Arthropods are protostomes.
♥Arthropods have exoskeletons, made primarily of chitin.
♥Arthropods have some form of jointed appendages; most are specialized by function.
♥Arthropods' bodies consist of three major tagmata—head, thorax, and abdomen.
♥Most arthropods have specialized sensitive eyes, often with both compound eyes and simple eyes or ocelli.
♥Arthropods have a true coelom, but it is reduced in size, so that most of the body cavity is a hemocoel.
♥Arthropods have a relatively complex muscular system--consisting of both smooth and striated muscle--which they use to move their appendages.
♥Arthropods have an open circulatory system.
♥Arthropods have a complete digestive system.
♥Arthropods have a brain consisting of cerebral ganglia that is attached to a nerve ring that connects to two nerve cords.
♥Most arthropods are dioecious with a pair of reproductive organs, and fertilization takes place internally.
♥Arthropod development often includes some form of metamorphosis.
♥Arthropods grow through a process, called ecdysis, in which they shed their exoskeleton.
Adaptive Advantages
Now we'll look at some of the reasons that arthropods have been able to handle so many changes over their history. (You probably will notice that some of the items on this list were also on the previous characteristics list.)
♥Exoskeleton: The exoskeleton serves several important functions for arthropods. Protection is its most obvious function, but it also assists some species with locomotion and inhibits drying out in terrestrial species.
♥Jointed Appendages: Jointed appendages allow more freedom of movement and adaptation for arthropods. They act as key elements in locomotion--walking, crawling, and swimming--and they play an important role in catching and manipulating food sources in some species.
♥Eyes: Many arthropods have compound eyes. These consist of multiple complete visual organs that can sense light individually. Because each eye component is complete in itself, the organism gets multiple complete focused images. Simple eyes, like those in humans, gather only a portion of an image, so the pieces must be put together to form a complete image.
♥Complex Brain: Arthropods have a relatively complex brain, which is connected to a solid nerve cord and multiple sense organs.
♥Adaptable Respiratory System: Arthropods' respiratory systems are a direct reflection of their living environment. Arthropods that live in water use gill-like structures. Some terrestrial forms, such as spiders, use book lungs, which are small enclosures that allow gas exchange between air and the organism's blood. Other arthropods, namely insects, have tracheae, air tubes that take oxygen directly to the organism's cells.
♥Metamorphosis: The process of metamorphosis allows the larval stage of the organism to develop without having to compete with the adult because the two life forms live and eat differently. Both stages benefit because metamorphosis provides the opportunity for more of these organisms to exist simultaneously.
Chelicerata
Crustacea
Uniramia
General Characteristics of Arthropods
protostomes
tagmata
compound eyes
striated
metamorphosis
ecdysis
Adaptive Advantages
Simple eyes
book lungs
tracheae
Chelicerata (Spiders, Scorpions, Ticks, Mites, and Horseshoe Crabs)
Chelicerates are members of the subphylum Chelicerata. Spiders, scorpions, ticks, mites, and horseshoe crabs are some of the more common chelicerates. There are approximately 70,000 species. In chelicerates, the head and thorax are fused into a structure known as the cephalothorax, so their body has two segments--the cephalothorax and the abdomen. A chelicerate's head doesn't have antennae, mandibles, or maxillae.
Chelicerates have six pairs of unbranched appendages, all of which are attached to the cephalothorax:
1. Chelicerae: The first set of appendages are called chelicerae. They are used for feeding.
2. Pedipalps: The second pair of appendages are called pedipalps. They are used for feeding and/or as sensory organs.
3. Walking legs: The final four pairs of chelicerates' appendages are legs, used for locomotion. Their eight legs set this group apart from other arthropods.
As you'll see, a significant number of chelicerates are predators, and their appendages play significant roles in their ability to capture prey.
Merostomata (Horseshoe Crabs)
Most of the original members of this classification are extinct. Although they are ancient organisms, there are still five species of horseshoe crabs still in existence. Like many chelicerates, horseshoe crabs are predators who feed on small invertebrates.
The horseshoe crab's most distinctive feature is its telson, a spike that sticks out of the back of its body. Their pedipalps resemble their walking legs, which is another unusual characteristic of these organisms.
Like many other arthropods, horseshoe crabs have compound eyes, but they are smaller than those found in several other varieties in this phylum.
Horseshoe crabs have book gills, which consist of layers of thin tissue through which air passes. Because horseshoe crabs are part of such an ancient classification, some scientists believe that book gills may have been the transition organs that bridged the respiratory gap between gills and book lungs.
Pycnogonida (Sea spiders)
There are approximately 1,000 species of pycnogonids, and all of them are marine organisms. They may look like spiders, thus giving them their name of sea spiders, because they have relatively small bodies in comparison with their long, hinged legs.
Sea spider
Unlike most of the arthropods, sea spiders don't appear to have clearly segmented bodies. The front end contains a long proboscis that the organism uses to suck out the liquid portions of soft-bodied invertebrates--sponges and cnidarians--for food. The mouth is not a part of the proboscis itself; it actually lies between the proboscis and the esophagus.
The first segment also has three or four pairs of appendages, including the first set of walking legs, and sets of specialized appendages, called ovigers, which play a role in reproductive development. There are several segments behind the first division, each of which contains a pair of walking legs. (Unlike most of the other chelicerates, sea spiders can have more than eight legs.) The back segment has a tube that leads to the anus.
Pycnogonids do not have respiratory or excretory organs. Reproductive gonopores are found on the second segment of some of the walking legs. All members of this classification have separate sexes, and some are hermaphrodites. Fertilization takes place externally.
Arachnida (Spiders, scorpions, ticks, and mites)
The most well-known chelicerates are the arachnids—consisting of spiders, scorpions, ticks, and mites. There are approximately 60,000 species, over half of which are types of spiders. Almost all arachnids live on land.
An arachnid's cephalothorax can be either partially or completely encased in a protective exoskeleton. Some, but not all arachnids, have segmented abdomens. In some arachnids, the appendages normally associated with the abdomen are missing or have been modified, as we'll see later with spiders and scorpions. Their respiration is accomplished through tracheae or book lungs.
In spiders, neither of the tagmata is segmented; additionally, there tends to be a narrow division between the two tagmata. All of the appendages arise from the
cephalothorax. Spiders' respiration can be accomplished using book lungs, tracheae, or both. The heart is a dorsal tube, located in the abdomen segment. Their nervous system is highly cephalized. It consists of a nerve ring located in the cephalothorax with nerve extensions. Spiders have multiple simple eyes, but they don't have compound eyes.
All spiders are carnivores that capture live prey. Their appendages play key roles in this process. Special glands, known as spinnerets, which are actually modified appendages, spin silk to create webs that serve as traps for prey. As is the case with most arthropods, the front appendages are used for feeding; in the case of spiders, they are fangs that inject toxin into the prey. Digestion takes place externally using enzymes secreted from the gut.
Several surveys have indicated that among adults, only snakes are more feared than spiders.
Scorpions
Based on fossil evidence, it is believed that the oldest existing terrestrial arthropods are scorpions. Because they are totally nocturnal, they spend most of the day hidden under rocks or other structures, away from the light. Scorpions' bodies consist of six cephalothorax segments and twelve abdominal segments. The most obvious scorpion feature is the long telson stinger that extends from the back of the abdomen segment. Scorpions inject a powerful poison from glands in the telson into their prey.
Scorpions have a circulatory vascular system with a hemocoel. Respiration takes place through book lungs, and scorpions have the same respiratory pigment, hemocyanin, as is found in mollusks. Their nervous system is not as complex as the spiders' nervous system, with much less cephalization. Like spiders, however, their heart is simply a tube in their abdomen.
There are several types of sense organs in scorpions. Their pedipalps are modified pincer appendages, called pectines, which they use as sensory devices.
The mating ritual in scorpions is quite unusual and complex. Fertilization takes place internally with a transfer of spermatophores. The scorpion young are born alive, and the female often cares for the young.
Ticks and mites
Ticks and certain mites are parasitic. Ticks suck blood from living organisms, and often transmit diseases as a result of carrying the blood of one host to another host. The larvae, as well as the adult forms, of some mites can be parasites.
Crustacea (Lobsters, shrimp, crabs, crayfish, and barnacles)
There are approximately 30,000 species in the subphylum Crustacea. Most crustaceans are marine organisms. Crustaceans' bodies, like the majority of arthropods, consist of three tagmata--head, thorax, and abdomen. In addition to these standard features, crustaceans have several important distinguishing features:
♥All crustaceans, like many water-based organisms, use gills for respiration.
♥All crustaceans' heads contain two pairs of antennae, one pair of mandibles, a pair of compound eyes, and two pair of maxillae.
♥All crustaceans have a pair of appendages on each of their body segments of the thorax and abdomen.
♥All crustaceans have a covering over the head and thorax segments.
Most crustaceans are free-living, but there are a few parasites in this classification. Several species are filter-feeders, living on plankton and bacteria in the water. Others are scavengers or active predators. They use their mandibles and maxillae to obtain food. Some capture prey in specialized appendages.
Almost all crustaceans have separate sexes, but their reproductive methods are quite diverse. Some species give birth to live young, while others go through a nauplius larva stage.
There are numerous classes within this subphylum, but we aren't going to cover all of them--just enough to give you a feel for these important creatures.
Branchiopoda (Brine shrimp)
Most of the tiny branchiopods live in freshwater. Despite their small size, usually under 2 centimeters, they have a significant number of segments with less fusing of tagmata. Brine shrimp do not have a true exoskeleton because there is little or no calcification (presence of calcium carbonate).
Branchiopods use gills for respiration, but their gills are located on the appendages that are not close to the head. Their thoracic appendages act like cilia, moving water toward their mouth, which then filters out small particles. Like many of the other arthropods, the branchiopod heart consists of a body-long tube, and the cavity is a hemocoel. Branchiopods show no signs of cephalization in a fairly simple nervous system. They contain paired gonads, and their young pass through a nauplius larval stage.
Brine Shrimp
Maxillopoda (Barnacles)
The maxillopods, like the branchiopods, are quite small. Most of them use the maxillae as their major means of obtaining food because most do not filter feed as the branchiopods discussed previously.
Maxillopods exhibit a basic body plan of 5 head and 10 trunk segments (six thoracic and usually four abdominal). The abdominal segments usually do not have appendages, but like some of the organisms we've already discussed, maxillopods have a telson that extends from the final abdominal segment.
The most commonly known maxillopods are the barnacles. Barnacles are best known for the way that they attach themselves and encrust ships, docks, and other marine structures. Barnacles generally have a pair of antennae, which they use as attachment organs. This means that, once a barnacle finds a suitable site, it attaches itself using a powerful glue and spends the rest of its life "standing on its head." Feeding is accomplished by six pairs of modified thoracic legs, featherlike feeding appendages that are jointed and covered with sensory hairs which brush through the water collecting food for the barnacle to eat. The legs also have gills for gas exchange.
Malacostraca (Lobsters, Shrimp, Crabs, and Crayfish)
Malacostraca is by far the largest class of crustaceans. Most of its 20,000 members are marine organisms, but other than that, there is an enormous amount of diversity. However, because of their importance, we are going to limit the discussion to the largest group in this class, the Decapoda, which includes shrimp, crabs, and crayfish.
Malacostracans have eight thoracic and six abdominal segments, each of which contains a pair of branched appendages.
Most members of this classification are filter feeders, like the branchiopods, by using their thoracic appendages to move water along a groove to their mouth.
Crab
The predators in this group capture their prey and tear it into bite-sized pieces with their appendages. There are also a few scavengers that scrape the substrate for food particles and parasites that live off of other marine life.
Malacostracans are well-known for what they contribute to both ends of the food chain. Lobsters, shrimp, crabs, and crayfish are especially popular food sources for humans. However, lesser known members of this classification are equally important in providing the base for the marine food chain or in recycling nutrients from dead plants and animals.
Uniramia (Centipedes, Millipedes, and Insects)
Uniramia is the third and largest subphylum of arthropods. It contains centipedes, millipedes, and insects. The following physical characteristics are common among uniramians:
♥Unbranched appendages, generally consisting of several parts
♥A body consisting of two or three tagmata and a multisegmented abdomen
♥Head appendages that include a pair of antennae, a pair of mandibles, and two pairs of maxillae. One of the pair of maxillae may be fused.
In addition to these physical attributes, uniramians breathe through tracheae or spiracles, openings in a trachealike structure.
With these features in mind, let's look at the three major classes:
1. Chilopoda: Centipedes
2. Diplopoda: Millipedes
3. Insecta: Insects
Chilopoda (Centipedes)
There are more than 2,500 species of centipedes, and all of them are terrestrial. Centipedes generally have small bodies, consisting of numerous flattened segments. All of these segments, except the one directly behind the head and the last two of the abdomen, contain a pair of appendages. The name "centipede," meaning "100 legs," actually is a misnomer because these organisms can have up to 177 segments, meaning that they could have as many as 348 legs.
All centipedes are predators. The foremost appendages are modified to allow the organism to capture its prey. Each appendage contains sharp fanglike "teeth" that inject poison into the prey, which consists of smaller invertebrates and insects that live in the soil.
Fertilization is internal through a transfer of spermatophores, but like the arachnids, the male and female do not join or copulate. The female lays eggs and cares for them until they hatch into juvenile versions of adult chilopods.
Diplopoda (Millipedes)
There are considerably more species of millipedes than centipedes--approximately 8,000 species of diplopods. Most diplopods are scavengers or herbivores that live off of decayed organic material in the soil.
Like the centipedes, millipedes' bodies are made up of numerous segments. The first four thoracic segments contain one pair of appendages; all of the abdominal segments, often called diplosegments because they are fused pairs, contain two pairs of appendages--thus, the name "millipede," meaning "one thousand legs" (in this case, as opposed to the centipedes, probably a slight exaggeration).
Because they don't need to capture prey, millipedes don't have poisonous fangs. Instead, they possess the means for protecting themselves from predators. Millipedes use two common techniques for protection:
♥They roll themselves into a ball in order to make themselves less visible prey.
♥They emit foul-smelling substances that discourage predators from "tasting" them.
Millipedes also have two pairs of spiracles, ganglia, and arteries on each of the diplosegments, through which they carry out their gas exchange. The diplopod nervous system consists of a brain that contains three parts and a double nerve cord.
Millipedes reproduce in much the same means as centipedes. Their gonopores lie between the second and third segment. Diplopod hatchlings have an interesting and unique feature of development. The young are born with three pairs of legs; as they grow, they add segments with corresponding pairs of legs each time they molt.
Insecta (Insects)
With approximately one million named species (and estimations that there may be at least that many more unnamed), Insecta is by far the largest classification on earth. Insects live, eat, and reproduce everywhere from the driest deserts to the oceans (though only a few exist in the ocean). With so much environmental variety and such large numbers, it isn't surprising that this classification is also one of the most diverse. So, let's look at some of the features that at least most of them share:
♥Insects' bodies consist of three tagmata.
♥Insects have a single pair of antennae.
♥Insect respiration is accomplished through tracheae.
♥Insects produce uric acid as the waste product from their metabolism.
♥Insects have a complete digestive system, beginning with complex adaptive mouthparts.
♥Insects have a relatively complex nervous system, consisting of numerous ganglia and a double nerve cord.
♥Insects have one pair of compound eyes and three simple eyes or ocelli. Some insects have other complex sense organs for hearing and chemoreceptors.
♥Insects have three pairs of walking legs, and many have modified appendages that enable them to fly.
♥Most insects go through metamorphosis.
Three of the above named features deserve more attention--adaptive mouthparts, appendages for flight, and metamorphosis.
While most insects have some common mouthparts, the dominant structures in their mouths seem to be adapted to their eating behaviors. For example:
♥Grasshoppers have mouthparts that enable them to tear and chew fibrous plant material.
♥Dragonflies have mouthparts that allow them to bite and chew other animals (particularly other insects) because they are carnivorous.
♥Mosquitoes have mouthparts that specialize in piercing skin and sucking blood.
♥Butterflies have mouthparts that resemble tubes so that they can extract nectar from flowering plants.
♥Houseflies have mouthparts that allow them to sponge or "mop up" surfaces to gather food from wherever they find it.
Insects are the only invertebrates that can fly; bats and birds are the other "flying animals." Insects are the only animal whose wings are modified appendages that are outgrowths of the body wall. Their ability to fly seems to have been a major factor in their survival because it places them out of the reach of many potential predators.
While insects are not the only organisms that pass through metamorphosis, it is another important factor in their survival. Despite a relatively short lifespan, insects are able to develop without contributing to their own competition for food and habitat resources. Insect hatchlings generally appear in a totally different form than that of the adult. They eat different foods and thrive in different living conditions, and their appearance is very different from that of the adult. When this initial larval stage is complete, the growing hatchling (larva) builds a cocoon that encases its body. While in the cocoon, the chrysalis does not eat, but it goes through a complete transformation, and it emerges from the cocoon as an adult.
Butterflies use a different set of chromosomes than they do when they are caterpillars.
Reproductive behavior varies in insects, but all utilize internal fertilization. In most insects, the female releases pheromones to attract the male. The female mates only once because she stores the sperm for the remainder of her life, regardless of the number of times that she lays eggs. Some insect females lay eggs only once, but they may produce a million eggs.
Insects play both a positive and negative role in life's processes. There is no doubt of the destruction caused by insects. They are the leading carriers of diseases, both in animals and plants. Insects are also responsible for destroying plants themselves, as in the case of grasshoppers and aphids. However, they are essential to the ecosystem. As we learned earlier, pollination of some plants would not be possible without insect pollinators. In addition, insects produce valuable products, such as honey and silk, and recycle nutrients in the soil.
cephalothorax
mandibles
maxillae
Merostomata (Horseshoe Crabs)
telson
book gills
Pycnogonida (Sea spiders)
ovigers
gonopores
Arachnida (Spiders, scorpions, ticks, and mites)
arachnids
spinnerets
nocturnal
pectines
Crustacea (Lobsters, shrimp, crabs, crayfish, and barnacles)
nauplius
Branchiopoda (Brine shrimp)
Maxillopoda (Barnacles)
Malacostraca (Lobsters, Shrimp, Crabs, and Crayfish)
Uniramia (Centipedes, Millipedes, and Insects)
Chilopoda
Diplopoda
Insecta
Chilopoda (Centipedes)
diplosegments
molt
Insecta (Insects)
Introduction to Echinoderms
(Starfish, Sea Urchins, Sand Dollars, and Sea Cucumbers)
There are approximately 6,000 species in the phylum Echinodermata, and all echinoderms are marine organisms. The most recognizable members of this phylum are the starfish, sea urchins, sand dollars, and sea cucumbers.
Let's begin by looking at some characteristics of these most unusual members of the ocean's tide pools:
♥Echinoderms have an internal skeleton, known as an endoskeleton, which consist of several individual plates, called ossicles.
♥Echinoderms are radially symmetrical as adults but bilaterally symmetrical during the larval stage.
♥Echinoderms have no back nor front--only aboral (top) and oral (bottom) sides.
♥Echinoderms, like chordates (to be covered in the next unit), are deuterostomes.
♥Echinoderms have a water vascular system rather than a blood-based circulatory system.
♥Echinoderms do not have a brain or centralized nervous system.
♥Echinoderms can reproduce sexually or asexually and directly or indirectly.
Before moving on to the specific classes of echinoderms, we'll look a little more in depth at three very important features in echinoderms--the endoskeleton, water vascular system, and embryonic development pattern.
Echinoderms' endoskeletons take on quite different appearances, but as their name implies, most consist of hard internal structures with sharp spines that protrude through the skin. ("Echino" means spiny, and "derm" means skin.) Starfish are relatively flexible, but they can make themselves very rigid. Conversely, a less common echinoderm, the sea lily, is so delicate that its "spines" are like petals on a flower--thus the name, "sea lily."
Whereas many animals, including humans, use blood as their major body fluid, echinoderms rely entirely on a water-based vascular system. Their bodies are filled with tubes and canals for moving water. In echinoderms, water's functions aren't limited to carrying nutrients, oxygen, and waste products either; it's an all-purpose tool for moving, feeding, and nearly everything else in their lives.
There is a third very important characteristic of echinoderms. During the earliest stage of development, the embryo contains a single opening. In deuterostomes--only echinoderms and chordates--that opening is the anus. Later a second opening, the mouth, forms at the opposite end. The opposite is true for the invertebrates that we've covered so far--mollusks, annelids, and arthropods. These organisms are called protostomes; they form their mouth first.
Now, we're ready to look at some of the classifications of echinoderms, showing some of the diversity in appearance and function of this phylum.
Crinoidea (Sea lilies)
There are only about 600 species of crinoids. The sea lilies and feather stars appear to be as delicate as their names imply. They look like marine flowers. In reality, their "petals" are branched arms that they use for filter-feeding.
Echinoidea (Sea urchins and sand dollars)
Echinoidea is a slightly larger classification, with about 950 species. The most well-known members of this class are the sea urchins and sand dollars. Both of these organisms have spines that radiate from their central core, but the projections are quite different. Sea urchins have very long stalk-like spines; sand dollars are nearly flat, and their spines poke through pores that make up a five-part flower pattern on the top of their bodies. Most of the members of this class use their spines for locomotion, burrowing, or defense.
Sea urchins, like several members of this class, are relatively sedentary and feed on particles in the water and algae.
Holothuroidea (Sea cucumbers)
There are approximately 1,500 holothuroids, including sea cucumbers. As their name implies, sea cucumbers are long and somewhat soft, but with a leathery skin. Like some of the earlier radially symmetrical organisms that we discussed, sea cucumbers feed by using tentacles that surround their mouth.
Sea cucumbers are bottom feeders, eating whatever comes to them as they lie on the ocean floor. It appears that their radial symmetry assists them in feeding because they can seek food equally well in all directions.
Asteroidea (Starfish, or sea stars)
About 1,500 species make up the class Asteroidea. One of its members, the starfish, also known as sea star, is by far the most famous echinoderm. Starfish are one of the most popular images of ocean life.
The following are some easily recognizable features of starfish:
♥flat, radially symmetrical body,
♥circular-shaped central region, and
♥five (or a multiple of five) relatively stiff, textured arms radiating from the central region.
Although they appear to be very rigid, starfish can change their shape quite easily, by changing the position of their arms, in order to move into narrow spaces--even making themselves asymmetrical in the process. Spikes from their endoskeleton project through their outer surface, creating the appearance of their textured skin.
Starfish are carnivores, feeding predominantly on bivalves, which they open with their tube feet that line each arm. Their water vascular system produces extremely powerful suction, and they place the tube feet of two of their arms on the opposite sides of the upper shell of the bivalve with their mouth close to the area where the two shells meet. Then they alternate suction between the tube feet of the opposing arms until they are able to create an opening between the bivalve's two shells.
Once there is even a small crack between the shells, the starfish pushes part of its stomach out through its mouth and between the bivalve's shells. As the bivalve attempts to close the shell, the starfish excretes digestive enzymes that begin to break down the bivalve's soft tissue. The partially digested food then is moved to the remaining portion of the stomach to complete the digestive process. After the process is complete, waste is removed through the anus, which lies on the aboral side.
In addition to the tube feet, each arm contains its own coelom with a pair of digestive glands, the canal system for the water vascular system, either male or female gonads, and a light-sensitive eyespot.
Like most echinoderms, starfish do not have a complete respiratory, excretory, or circulatory system; they rely instead on their water vascular system. Water enters on the aboral side through an intake plate, known as a madreporite. As stated earlier, the water passes through a series of canals, including those in each arm.
Starfish are relatively sedentary unless they are working to acquire food. Their movement is derived from the water vascular system's pushing water into specialized sacs, ampulla, at the ends of the tube feet. The ampulla expands as it fills with water. When the ampulla touches something hard, the center pulls back to create "suction-cup;" then, the body moves over the arm. The starfish repeats this process to "walk" along any surface.
Gas exchange takes place both through skin gills and the tube feet. Starfish reproduce both sexually and asexually. Like some of the worms, starfish are capable of regeneration, so a whole new organism can form from certain portions of a fragmented starfish.
The main food source of starfish is clams and oysters. As a result, starfish have always been a problem for fishermen who harvest bivalves from the ocean. At one point in history, fishermen used to take the starfish they caught in their nets, cut them into pieces, and throw them back into the ocean. What they didn't realize is that starfish have the remarkable ability to regenerate. Each piece that was cast back into the ocean became a new starfish.
Sexual reproduction occurs when the starfish release their gametes. Fertilization occurs in the open water where the bilateral larvae swim around with other plankton until they reach the ocean bottom and mature into adults that have radial symmetry.
protostomes
Crinoidea (Sea lilies)
Echinoidea (Sea urchins and sand dollars)
madreporite
ampulla
Focus on Silk Worms
Most people perceive silk to be one of the most elegant and luxurious materials, so it's hard to believe that silk is actually the cocoon material of a light-colored gray or off-white worm, the larva of the Bombyz Mori moth. Adult Bombyz Mori moths are relatively large. No known members of this species exist outside of captivity.
The larva stage of the Bombyz Mori is the silk worm. After hatching, silk worms go through four moltings--each resulting in a larger silk worm. During their almost constant growth, the silk worms eat mulberry leaves to the point where they weigh 10,000 times their hatchling weight.
Each of the silk worm's three body segments behind the head contains three pair of jointed legs with a single claw. Five modified appendages with hooks are located on the abdomen. There are two silk glands that run along the sides of the body. The two glands have a common opening on the underside of the silk worm.
After the fifth growth cycle (about 25 days), the silk worm is fully mature and ready to spin its cocoon. Both glands secrete a substance that hardens upon contact with the air to become a single, unbroken thread that is about 1/1,200 of an inch thick and up to 1,200 yards long. The silk worm moves its head in a figure-eight pattern continuously for three days, forming the "silk" cocoon, which usually is approximately an inch long. The chrysalis stage lasts from 10 days to three weeks, after which the adult moth emerges, mates, and dies--having left over 500 eggs to begin the cycle anew.
In order to recover the silk for production, the metamorphosis process has to be interrupted so that the insect does not break through the continuous thread. In order to kill the chrysalis without damaging the cocoons, workers boil the cocoons. Then, they must find the natural end of the thread. Because the threads are so fine, more than one cocoon's thread is wound together. The individual threads naturally adhere to one another to form a thicker thread that is wound onto spools for producing silk fabric.
Bombyz Mori
