Guide to organismal biology

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Class Hirudinea - leeches; 500 sp. (fr. L. hirudo = leech)

Some leeches are predators or scavengers, feeding on worms, snails, and insect larvae. But many species (about 75% of them) suck the blood of mammals and even some crustaceans. They have an anterior and posterior sucker for attaching to the skin of their hosts. They are excellent swimmers, and their suckers also helps attach them to the bottom as they crawl along. Leeches are common in freshwater habitats, but only a few species are marine or terrestrial. One Illinois stream contained 10,000 leeches per square meter!

The coelom is greatly reduced, and not divided into compartments. Because leeches move by swimming or crawling, they have lost these coelomic adaptations for burrowing. The blood meal is stored in special pouches in the digestive tract, so leeches don't need to feed very often. And a good thing, too, because a feeding leech will suck up to five to ten times its own weight in blood! When they attach, leeches secrete a special anticoagulant to keep the host's blood flowing. Because medieval physicians believed that "bad blood" caused diseases, patients were bled with leeches until they often died of anemia. The medicinal leech, Hirudo medicinalis, is enjoying a modern day revival, because its bite is antiseptic, and the anticoagulant that it secretes will dissolve blood clots. Leeches are also used to drain postoperative swelling. Lancing the swelling in order to drain it often leads to infection. The best way to remove a feeding leech is by using the tip of a lit cigarette (or cigarette-like object) or by pouring salt over the leech.


Phylum Annelida

Class Oligochaeta -earthworms (Lumbricus)

Class Polychaeta - tubeworms, paddleworms (Nereis)

Class Hirudinea - leeches

Economic, Ecological, and Evolutionary Importance

Earthworms are critical in soil aeration and soil fertility.

Leeches have been used as a medical anti-coagulant for hundreds of years.

Worm ranching is a major industry, with sales to both gardeners and fishermen.

Consider This

How does the body of the leech reflect its parasitic strategy?

Why do we believe that mollusks and annelids are closely related?

How/why is segmentation a useful adaptation for a burrowing animal?

How does segmentation open up a new pathway for evolutionary success?

5 – Ecdysozoans – Nematodes and Arthropods

Ecdysozoans are animals that need to molt in order to grow. They have an exoskeleton, a tough cuticle containing chitin, that is periodically shed as they develop. Ecdysozoans share numerous other features, including radial cleavage in their embryos (unlike most other protostomes), and movement without the aid of cilia. They also lack the trochophore larva common to the annelids and mollusks. Because of their pattern of radial cleavage, we distinguish ecdysozoans from spiralians as part of the larger clade of Protostomia. Both ecdysozoans and spiralians share a common ancestor.

Phylum Nematoda - roundworms (Ascaris, Tubatrix [vinegar eels]; fr Gr. nema = thread, oda = resembling); 12,000 sp.

Nematodes are incredibly diverse, with over 12,000 named species, living in all aquatic habitats, including the water film that surrounds particles of soil or grains of sand in aquatic habitats (interstitial habitat). They are critically important ecologically, as major recyclers of organic matter in the soil, and in aeration of the soil. Nematodes feed mainly on the abundant soil bacteria and fungi, as well as other small animals, including other nematodes. They are economically important, because the parasitic forms are major pests of agricultural crops, causing an estimated $5 billion in damage each year!

They are round, bilaterally symmetric and pseudocoelomate, with a toughened cuticle, an outer layer that protects the parasitic forms against digestive enzymes. They usually molt four times during the course of their development. The basic body shape seems to be an adaptation for living in interstitial habitats. Nematode worms lack circular muscles. They only have longitudinal muscles, and thus appear to thrash about aimlessly. This type of motion appears rudderless when we see them free floating in water or vinegar (Tubatrix), but works very well in their usual interstitial habitat, where there are plenty of packed grains of soil to push against and wriggle through.

They are mainly aquatic. Even the terrestrial forms are basically aquatic, living in the thin film of water that usually coats grains of soil. Males are often smaller than females, and have a copulatory hook at the posterior end with which they can hold open the genital pore of the female. Nematodes excrete ammonia by diffusion, sometimes in conjunction with special excretory cells that are peculiar to this phylum. They have a rudimentary nervous system, with a nerve ring serving as a brain, nerve cords that run the length of the body, and numerous bristles and other structures for mechanical and chemical senses.

Some of the nastier parasitic forms deserve special mention. Nematodes of the genus Trichinella form cysts in pork, which can lead to a deadly case of trichinosis. When the larval worms begin to tunnel through the body, as many as 500 million at a time, the resulting physical trauma can be fatal, and survivors are often left with permanent muscle damage. Hookworms and pinworms are common nematode parasites found in small children, or in anyone walking barefoot over infected soil. Filarial worms are a serious pest in many tropical countries, and cause the grotesque swelling called elephantiasis. The common canine heartworm, Dirofilaria, is also a filarial worm. Ascaris, the intestinal roundworm, is a common parasite of humans and pigs. A single female holds up to 30 million eggs, and can lay up to 200,000 eggs a day. The eggs are spread from dried feces contaminating the soil. One out of six people worldwide are infected with intestinal roundworms (yuck!).

Because nematodes are not segmented worms, they used to be classified much “farther down the trunk” of the tree of life. Segmentation was thought to link the arthropods with the annelid worms, through some unknown common ancestor. We now believe that segmentation evolved independently in these two groups. Given the complexity of molting, however, we elevated the importance of that trait to define a new group, the Ecdysozoa, containing both the nematodes and arthropods.


Phylum Nematoda - roundworms (Ascaris, Tubatrix [vinegar eels])

Economic, Ecological, and Evolutionary Importance

Nematodes cause billions of dollars in crop damage every year.

Nematodes are important in soil aeration, as global recyclers of bacteria and fungi, and as food for other animals.

Many nematode parasites are medically important, such as Ascaris, the intestinal roundworm, Trichinella, hookworms, pinworms, and filarial worms.

Consider This

How does the nematode's lack of circular muscles help it move through the grains of soil and other particles in its natural habitat?

Introduction to Arthropods

This is not, as the Victorians called it, the Age of Mammals. The planet today is almost completely dominated by a single phylum of animal life. On land, in the sea, even in the air itself, they are the true masters of the Earth. They are the arthropods. Arthropods are coelomate protostomes, dominating the protostome branch of the animal tree, just as vertebrates dominate the deuterostome branch. Arthropods share a common ancestor with polychaete worms, and may even be a direct descendant of polychaetes. But unlike other coelomate invertebrates, the arthropod coelom is greatly reduced in the adult animal.

There are nearly 1.2 million named species in the Phylum Arthropoda, named from the Greek arthros (= jointed) and poda (= foot), including the familiar arachnids, crustaceans, and insects, together with a host of less familiar critters, like centipedes, millipedes and sea spiders. All arthropods have jointed appendages. This evolutionary innovation is probably the key to the stunning success of this diverse group. There are about 1018 (10 billion billion) arthropods alive at any one time. There are over three times as many species of arthropods as there are of all other animals on Earth, and there may be millions more that we haven't even discovered. Arthropods do everything with legs or modified legs. They walk, they swim, they creep and crawl, they use legs to sense with (the antennae), to bite and sting with, and even to chew with. That's one reason arthropods look so alien when we see them up close. They chew sideways, and it's all done with legs.

Their bodies are protected by an tough cuticle made of proteins and chitin, a polysaccharide with added nitrogen groups. A cuticle is a tough outer layer of non living organic material. The cuticle of arthropods acts as an exoskeleton. Most are very small, though a few lobsters reach up to a meter, and one giant crab grows to 3.5 meters long.

Fossil insects were also very large. Ancient dragonflies had wingspans of 17” (430 mm) or more. But living insects are uniformly small. Perhaps smaller insects were better at hiding or escaping from their many predators. Terrestrial arthropods remain small primarily because of the limitation imposed by their exoskeleton. A large insect would need such a thick exoskeleton to withstand its strong muscles that the weight of the cuticle would be too great for the animal to carry around. For a small animal, having your skeleton on the outside is as logical as having it on the inside. But it poses a fundamental problem for arthropods. They must shed their exoskeleton, or molt, in order to grow. The exoskeleton splits open. the animal emerges and swells to a larger size until the newer, larger exoskeleton is hardened. While the animal molts, it is especially vulnerable - just ask a plate of soft-shelled crabs!

Arthropods have segmented bodies, like the annelid worms. These segments have become specialized, however, with one pair of jointed appendages added to each segment. Among living arthropods, the millipedes most closely suggest what the ancestral arthropod might have looked like. Arthropod segments have also fused together into functional units called tagma. This process of segment fusion, or tagmosis, usually results in an arthropod body that consists of three major sections, a head, thorax, and abdomen. Sometimes the head and thorax are fused together into a cephalothorax. Each of these body sections still bear the appendages that went with it, though these appendages are often highly modified. Arthropods are very highly cephalized, often with intricate mouthparts and elaborate sensory organs, including statocysts, antennae, simple eyes and compound eyes. Sensitive hairs on the surface of the body can detect touch, water currents, or chemicals. Their nervous systems are highly developed, with chains of ganglia serving various parts of the body, and three fused pairs of cerebral ganglia forming a brain.

Aquatic arthropods respire with gills. Terrestrial forms rely on diffusion through tiny tubes called trachea, or layers of tissue called book lungs. Trachea are cuticle-lined air ducts that branch throughout the body, and open in tiny holes called spiracles, located along the abdomen. Insects can open and close these spiracles, to conserve water that would otherwise be lost to evaporation from the open tubes. Book lungs are made of sheets of tissue that resemble the pages of a book, providing a lagre surface area for diffusion. One of the reasons that insects are small is that they rely on diffusion for respiration.

Arthropods excrete by means of malphigian tubules, projections of the digestive tract that help conserve water. Terrestrial forms excrete nitrogen as uric acid, as do birds. Their waste is nearly dry, a superb adaptation to life on land. Arthropods have an open circulatory system, and separate sexes. Fertilization is usually internal, another adaptation for terrestrial life. Males and females often show pronounced sexual dimorphism.


Subkingdom Eumetazoa





Phylum Nematoda – roundworms, Ascaris, Tubatrix (vinegar eels)

Phylum Arthropoda

Subphylum Chelicerata

Class Merostomata - horseshoe crabs,

Class Arachnida - spiders, scorpions, ticks, mites

Subphylum Crustacea - crustaceans

Subphylum Myriapoda

Class Chilopoda - centipedes

Class Diplopoda - millipedes

Subphylum Hexapoda - insects

Order Hymenoptera - ants, bees, wasps

Order Coleoptera - beetles

Order Lepidoptera - butterflies, moths

Order Diptera - flies, mosquitoes

Order Orthoptera - grasshoppers, crickets, roaches

Order Odonata - dragonflies

Order Isoptera - termites

Characteristics of Subphyla, Classes, and Orders

Arthropod systematic is currently very fluid, with molecular evidence now contradicting the traditional views of how the major clades are related to each other. Some authorities use the traditional grouping of insects, centipedes and millipedes into the clade Uniramia or Tracheata, others place insects with crustaceans in the clade Pancrustacea.

Subphylum Chelicerata:

In chelicerates, the first pair of appendages are called chelicerae, and are modified to manipulate food. They are often modified as fangs or pincers. Chelicerates lack antennae.

Class Merostomata - horseshoe crabs (Limulus)

Horseshoe crabs have larvae that are very similar to trilobites, and they may be descendants of this long vanished group. Horseshoe crabs are nocturnal, feeding on annelids and molluscs. They swim on their backs, or walk upright on five pairs of walking legs. They live in the deep ocean, migrating inshore in large numbers in the spring to mate on the beaches during moonlight and high tide - much like undergraduates on Spring Break.

Class Arachnida - spiders, scorpions, ticks, mites, and daddy longlegs

This very successful group of arthropods have four pair of walking legs (8 legs). The first pair of appendages are the chelicerae, and the second pair are pedipalps, appendages modified for sensory functions or for manipulating prey. They are mostly carnivorous (many mites are herbivores). Most secrete powerful digestive enzymes which are injected into the prey to liquify it. Once dissolved in its own epidermis, the prey is sipped like a root beer float.

Order Scorpiones (1,340 sp.) - Scorpions have pedipalps modified as pincers, along with a venomous sting in their tail. Scorpions date back to the Silurian, about 425 mya, and may be the first terrestrial arthropods.

Order Araneae (38,000 sp.) - Spiders have special modified posterior appendages called spinnerets, which they use to spin their webs. Not all spiders spin webs. Wolf spiders are the tigers of the leaf litter, and the common jumping spider leaps several times its body length to catch its prey. Spiders use pedipalps as copulatory organs. Spiders breathe by book lungs

Order Acari - (50,000 sp.) - Ticks and mites are the largest and most diverse group of arachnids. Most are very tiny, less than 1 mm long. The thorax and head are fused into a single unit (cephalothorax). Ticks are bloodsucking parasites, and can carry diseases like Rocky Mountain Spotted Fever and Lyme Disease.

Order Opiliones (5,000 sp.) - Daddy Longlegs is a familiar arachnid. It has an oval body with extremely long legs, which they frequently lose in various accidents and brushes with predators. They are predators, herbivores, and scavengers. Look at them closely next time you see one. They carry their eyes atop a little tower on their back (weird!).

Subphylum Crustacea - (38,000 sp.), crabs, shrimp, lobsters, crayfish, isopods, barnacles, brine shrimp

Crustaceans are mostly marine, and dominate the ocean to the same degree that insects dominate the land and air. Despite their aquatic diversity, there are very few terrestrial crustaceans, just as there are very few truly aquatic insects. Crustaceans have biramous appendages. Each leg has an additional process, like a little miniature leg branching off from the main leg. Many groups of crustaceans have lost this extra appendage during subsequent evolution. The Order Decapoda have five pair of walking legs, and include the familiar crabs, lobsters, and crayfish. The first and second pair of appendages are usually modified as antennae. Crustaceans have two pair of antennae. Another set of anterior appendages are modified as mandibles, which function in grasping, biting, and chewing food. Behind the mandibles are five pairs of accessory feeding appendages (two pairs of maxillae and three pairs of maxillipeds). These also assist in the creating a flow of water over the gills. Male crayfish also use one pair of legs as a copulatory organ. All crustaceans share a common type of larva called a nauplius larva.

Order Isopoda, Isopods have many common names, such as Pill bugs, Roly-Polys, Woodlice, Bibble Bugs, Cheesybugs, Cud-worms, Coffin-cutters, Monkey Peas, Penny Pigs, Sink-lice, Slaters, Sowbugs, Tiggyhogs, and (in New Orleans) Doodlebugs. They are one of the few successful terrestrial crustaceans. They feed on decaying vegetation in the leaf litter. Their deep sea cousins can reach an enormous size.

Subphylum Myriapoda - centipedes, millipedes

Uniramians have a single pair of antennae, and uniramous appendages. They probably share a common ancestor with the velvet worms (Onycophora).

Class Chilopoda - (2,800 sp.) Centipedes dwell in damp places under old logs and stones. They are carnivorous, eating mostly insects. They are highly segmented, and have one pair of legs per segment. Despite the name, the number of legs comes out to considerably less than one hundred (centi = 100). The first trunk segment bears poison fangs. Centipedes are very dangerous, and their bite is extremely painful.

Class Diplopoda - (1.1,000 sp.) Millipedes share the same habitat as centipedes, but they are mostly herbivorous, feeding on decaying vegetation in the leaf litter. Animals that feed on detritus are called detritivores. They have two pair of legs per segment, (less than a thousand [= milli], but lots more than a centipede). Each segment of the millipede is actually two segments fused together (hence the double set of legs). They can secrete a defensive fluid that smells bad, and a few species actually secrete tiny amounts of cyanide gas to protect themselves!

Subphylum Hexapoda - insects

Class Insecta - (925,000 sp.) If we knew all the different insects on Earth, there could be as many as 30 million species. Insects evolved about 200 mya, with cockroaches and dragonflies among the first to appear. Insects have a head, thorax, and abdomen, with three pair of legs (6 legs) on the thorax. (Crustaceans have legs on the abdomen as well as on the thorax). Most insects have one or two pairs of wings. They are the only invertebrates that fly. Most have compound eyes, and can communicate by sound and scent, using powerful chemical hormones called pheromones.

Insects have extremely elaborate mouthparts, consisting of an upper lip (labrum), mandibles, a pair of appendages called maxillae which aid in chewing, and a second pair of maxillae which are fused together to form a lower lip (labium). These mouthparts are highly modified in various groups for chewing, sucking, and piercing. Insects undergo metamorphosis as they develop, changing from one form to another as they mature. Some (about 10%) show simple metamorphosis, in which there is no resting stage. The juvenile stages look like tiny versions of the adults. Most (90%) show complete metamorphosis, in which one stage is an inactive pupa, like the cocoon of the moth or the chrysalis of the butterfly. Their larvae are often radically different from the mature adult (like the butterfly and the caterpillar). They not only look different, they live in different places and eat different food.

Economic, Ecological, and Evolutionary Importance

The many ways that arthropods help us and hurt us are almost too numerous to mention.

They provide seafood, and pollinate fruit crops.

They also cause billions of dollars a year in crop damage.

They cause or carry a host of diseases, such as malaria and the plague.

Ecologically, they are critically important herbivores. Arthropods are the primary converters of plant tissue to animal tissue on the planet!

Consider This

How do segmentation and tagmosis account for the success of arthropods?

Why aren't bugs the size of Buicks?

Trilobites were among the most successful arthropods on Earth, once numbering over 10,000 species. Why are they all gone?

How does the smooth flow of muscle contractions in the moving millipede relate to the evolution of segmentation in annelids and arthropods? (Hint: Why is a segmented body plan useful for a burrowing animal?)

6 - Echinoderms and Chordates

Introduction to Echinoderms

Our closest cousin among the invertebrates is a most unlikely taxon, the echinoderms (Phylum Echinodermata, = spiny skin; 6,000 sp ) Echinoderms are coelomate deuterostomes. They show a superficial five part (pentamerous) radial symmetry. The larvae are bilaterally symmetric, cephalized, and motile, but they develop into sessile or sedentary radially symmetric adults.

All echinoderms are marine. They have a calcareous endoskeleton, consisting of numerous small plates covered by a thin epidermis. They are probably the first animals to have evolved an endoskeleton derived from mesodermal tissue. Numerous small spines project from the surface of the body. Echinoderms have an open circulatory system, and respiration and excretion occur by means of dermal gills, small finger-like projections of the skin that stick out near the base of the spines on the surface. The large coelom also functions in circulation and in respiration. Mixed in with the spines and dermal gills on the surface of the animal are numerous small pincers on tiny stalks, structures called pedicillaria. These can snap shut on tiny prey, and help keep the animal's skin clear of any small settlers (they repel boarders).

Echinoderms move by odd little hydraulic structures called tube feet. Each tube foot has a small bulb called an ampulla. The ampulla squeezes water into the tube foot to stretch it out, with a one way valve keeping it from returning to the radial canals until the ampulla relaxes. Longitudinal muscles in the feet contract to shorten them, pulling the animal along. Water enters the animal through a madreporite, a tiny sieve plate that keeps out pieces of debris. Water passes into a ring canal, out into a series of radial canals, and finally into the tube feet. Tube feet can function in both locomotion and in feeding.

Echinoderms have no brain, or central nervous system, consistent with their return to a sedentary life with a radially symmetric body plan. The nervous system consists of a simple nerve ring, with five branches to innervate the arms. Their senses are rudimentary, including light sensitive eyespots and sensory tentacles (modified tube feet) at the tips of the arms, and small patches of cells sensitive to chemicals or touch.

They have an unusual type of connective tissue, mutable or catch connective tissue, which can change consistency at will, from very hard to very soft. This is what allows starfish to flex their arms, or drop an arm if attacked by predators. Catch connective tissue also solidifies to lock the spines of urchins into their defensive position. Asexual reproduction occurs by splitting or fragmentation. Sexes are separate, with external fertilization. They have great regenerative powers; one arm can regenerate an entire starfish!

There are five living classes, but over 20 extinct classes of echinoderms. The ancestral echinoderm was probably an animal like the sea lily, which resembles an upside-down starfish on a stalk. The tube feet and water-vascular system originally functioned in filter feeding. Some echinoderms returned to an "active" existence, detached and flipped over (mouth side now down), with the tube feet now functioning in locomotion.


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