Plant and Animal Diversity

This is Unit Ten of Biology, Part Three: protostomes, and the branch that leads away from them. Part Two established the fork: in a protostome embryo the first opening becomes the mouth, in a deuterostome it becomes the anus. Everything here hangs off that single developmental difference, and the protostome side splits again into two clades. The first, Lophotrochozoa, is held together by molecular evidence rather than by any shared visible feature — flatworms, rotifers, molluscs and annelids look nothing alike, and one of them does not even share the body-cavity type of the others. The book uses it as a worked example of why animal phylogeny keeps revising. The second, Ecdysozoa, is the opposite: defined by one shared constraint. Its members grow inside a rigid cuticle that cannot stretch, so they must shed it to get bigger. Nematodes, tardigrades and every arthropod share that. Then arthropods, the most species-rich plan on Earth, and the pair of entries that fixes what everyday speech blurs — an insect and a spider are not the same kind of thing. By the end you will be able to do five things. One: say what unites Lophotrochozoa. Two: name the mollusc body plan. Three: say why ecdysozoans molt. Four: tell insect from arachnid. Five: say why echinoderm symmetry is secondary.

Four entries. A clade whose members share no defining structure, and three of its phyla. The grouping is worth attention as a case study: it is what a molecule-supported clade looks like when morphology gives you nothing.

Held by molecules.

Bio 2249 Lophotrochozoa Defined

Lophotrochozoa groups protostome phyla united by molecular evidence rather than one shared visible structure — flatworms, molluscs, annelids and rotifers, which look structurally distinct. The grouping rests on molecular support more than morphology, which is exactly why animal phylogeny revises as evidence arrives.

Flat for a reason.

Bio 2251 Phylum Platyhelminthes: Flatworms

Flatworms are acoelomate, triploblastic, bilaterally symmetrical worms with no circulatory or respiratory system, relying on a flattened shape to keep every internal cell close enough to the surface for oxygen to diffuse in. The phylum spans both parasitic lineages like tapeworms and free-living ones like planarians.

A spinning crown.

Bio 2253 Phylum Rotifera

Rotifers are microscopic pseudocoelomate animals named for a ciliated feeding structure that appears to rotate as its cilia beat, sweeping algae and bacteria toward the mouth. They are pseudocoelomate where most of their clade-mates are coelomate — the clade is defined by molecular evidence, not by body-cavity type.

A launched proboscis.

Bio 2255 Phylum Nemertea: Ribbon Worms

Nemerteans capture prey with an eversible proboscis that shoots out from a separate fluid-filled internal cavity to grasp or pierce, then retracts. That proboscis is separate from the digestive tract — captured food still enters through a distinct mouth.

Three entries. A three-part body plan, three classes that modify it almost past recognition, and a segmentation that looks like an arthropod's and is not inherited from the same source.

Three parts.

Bio 2257 Mollusk Body Plan: Foot, Mantle, Visceral Mass

A mollusc body organises around three parts: a muscular foot for movement, a shell-secreting mantle, and a visceral mass housing the organs — a snail gliding on its foot with its mantle laying down shell. Not every mollusc keeps all three in obvious form; an octopus's foot became arms, and many lineages lost the shell.

Three classes.

Bio 2259 Three Major Mollusk Classes

Gastropods such as snails move on one broad foot; bivalves such as clams filter feed through hinged shells; cephalopods bear a foot modified into sucker-lined arms — a striking departure within one phylum. Cephalopods still share the foot-mantle-visceral mass organisation despite that modification.

Repeating units.

Bio 2261 Annelid Segmentation

An annelid's body divides into repeating segments, each often with its own muscles, nerves and excretory structures, supporting precise localised movement — an earthworm's segmented muscles contracting in waves as it burrows. This segmentation evolved independently from the similar-looking arthropod version; molecular evidence puts the two phyla in separate clades.

Three entries. A clade defined by one shared constraint rather than by molecular evidence alone, and two phyla before the arthropods — one of them the most numerous animal group on Earth by count.

Shedding to grow.

Bio 2263 Ecdysozoa Defined: the Molting Cuticle

Ecdysozoa groups protostome phyla whose members grow by moulting a rigid outer cuticle — nematodes, tardigrades and arthropods, each periodically shedding an old cuticle and secreting a larger one underneath. A rigid covering cannot stretch, which is what makes moulting necessary; Lophotrochozoa phyla have no such constraint.

Numerous.

Bio 2265 Phylum Nematoda

Nematodes are pseudocoelomate roundworms with a complete digestive tract running mouth to anus, among the most numerous animals on Earth by count — one small free-living species is a standard model organism for development and genetics. Most nematodes are free-living; the parasitic species draw more attention but are a minority.

Dried out.

Bio 2267 Phylum Tardigrada

Tardigrades are microscopic ecdysozoans able to enter a dormant dehydrated state called cryptobiosis, surviving extremes of temperature, pressure and radiation that their active form could not, then reviving on rehydration. Cryptobiosis nearly stops metabolism — it is not evidence they are immune to damage while active.

Four entries. The most species-rich body plan on the planet, its four subphyla sorted by two simple traits, and then a twinned pair correcting a word everyday speech uses for both of the groups people confuse.

Three traits.

Bio 2269 Arthropod Defining Traits

An arthropod has a segmented body, a rigid chitin exoskeleton, and paired jointed appendages — a plan behind more species than any other phylum, letting a crab support its weight and move precisely. That exoskeleton must be moulted for growth; an arthropod cannot enlarge a rigid covering while wearing it.

Four groups.

Bio 2271 Four Arthropod Subphyla

Arthropoda divides into four living subphyla: chelicerates with pincer-like mouthparts and no antennae, myriapods with many leg-bearing segments, aquatic crustaceans, and hexapods — insects and their six-legged kin. Antenna presence and leg count are the defining criteria, not habitat or size.

One word, two groups.

Bio 2273 "An Insect and a Spider Are Both Bugs"

Calling both an insect and a spider a bug collapses a real distinction: the two belong to separate arthropod subphyla with different body plans. An insect has six legs, three body regions and antennae; a spider has eight legs, two body regions and none.

The precise terms.

Bio 2275 Insects versus Arachnids, Structurally Distinguished

An insect, a hexapod, has three body regions, six legs and antennae — a beetle's head, thorax and abdomen. An arachnid has two body regions, eight legs and no antennae — a scorpion's fused head-thorax with pincers. Bug blurs these plans, and the course reserves insect and arachnid as precise terms.

Three entries taking the other fork. The clade, an echinoderm whose adult symmetry is not what it appears to be, and a phylum sharing a chordate trait without being one.

Anus first.

Bio 2277 Deuterostomia Defined

Deuterostomia groups animals in which the embryo's first opening becomes the anus — echinoderms, hemichordates and chordates. A sea star and a human develop the same way despite utterly different adult plans, so shared deuterostome development says nothing about adult appearance; the clade spans some of the most divergent bodies in the kingdom.

Water under pressure.

Bio 2279 Echinoderm Body Plan: Water Vascular System

An echinoderm moves and feeds using a water vascular system, a network of fluid-filled canals powering tube feet — a sea star's feet gripping and prying open a bivalve. Its adult radial symmetry is a secondary trait: echinoderm larvae are bilaterally symmetrical, matching their deuterostome ancestry.

Nearly a chordate.

Bio 2281 Phylum Hemichordata

Hemichordates, including acorn worms, are marine deuterostomes bearing pharyngeal slits — a trait chordates share — and a dorsal nerve cord that, unlike a chordate's, forms no neural tube. Those shared slits reflect deuterostome common ancestry rather than making hemichordates chordates themselves.

Two entries closing the part. A procedure applying this unit's criteria in a fixed order, and a capstone tracing both branches back to one ancestor. Both carry the same warning: the adult body will mislead you.

An unknown specimen.

Bio 2283 Classifying an Unfamiliar Invertebrate by Body-Plan Criteria

A small marine worm: three germ layers, a fully mesoderm-lined coelom, a mouth forming from the first embryonic opening, and periodic cuticle shedding. Three layers rules out the diploblasts. A lined coelom rules out acoelomates and pseudocoelomates. Mouth-first makes it a protostome, ruling out Deuterostomia whatever it resembles. And moulting places it in Ecdysozoa rather than Lophotrochozoa. Classifying by adult form instead of these embryonic criteria is the error.

The capstone. One ancestor, two branches.

Bio 2285 Tracing the Protostome-Deuterostome Branch Point

The capstone starts from the last common bilaterian ancestor — triploblastic and bilaterally symmetrical — and follows the split where the embryo's first opening became either the mouth or the anus. The mouth-first branch produced the protostomes, diversifying into Lophotrochozoa and Ecdysozoa; a clam traces through it. The anus-first branch produced the deuterostomes; a sea star traces through it, its radial adult form evolved on top of a bilateral larva. Both are coelomate. They diverge only at that fork.

Five things to carry out of Part Three. Lophotrochozoa is united by molecular evidence rather than any shared visible structure. Flatworms, rotifers, molluscs and annelids look nothing alike, and rotifers are pseudocoelomate where most of the clade is coelomate. What does not unite it is morphology, which is precisely why the grouping keeps getting revised. A mollusc body has three parts: a muscular foot for movement, a mantle that secretes shell, and a visceral mass holding the organs. All three persist even where they are unrecognisable — an octopus's foot became its arms, and plenty of lineages lost the shell entirely. Ecdysozoans moult because a rigid cuticle cannot stretch. Nematodes, tardigrades and every arthropod grow inside a hard covering, so getting bigger means shedding the old one and secreting a larger one underneath. Lophotrochozoa faces no such constraint. An insect has three body regions, six legs and antennae. An arachnid has two body regions, eight legs and no antennae. They sit in separate arthropod subphyla, and "bug" collapses that distinction, which is why the course uses the precise terms instead. And an echinoderm's radial symmetry is secondary, not ancestral. Its larvae are bilaterally symmetrical, matching its deuterostome ancestry, and the adult radial form evolved on top of that. Judging a sea star by its adult shape would file it with the cnidarians, which is exactly wrong. Part Four closes the unit with the chordates.