An introduction to the diversity of animal life. One cell or many?. We start dividing up animals here. Some animals have just one cell – many others have large numbers of differentiated cells. 1 cell - Protozoa. Many cells – parazoa and metazoa. The Protozoa – the single celled animals.
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We start dividing up animals here.
Some animals have just one cell – many others have large numbers of differentiated cells.
1 cell - Protozoa
Many cells – parazoa and metazoa
In fact many of these are photosynthetic and are claimed as plants by botanists, while some are both photosynthetic and carnivorous! The animal -plant - fungus split does not make sense at this level.
Old system: exclude green species, lump the rest in Phylum protozoa, which has 4 classes:
ciliates (Paramecium caudatum) – many small cilia
flagellates (Euglena, Trypanosoma) – one big cilium (flagellum)
Rhizopoda (Amoeba proteus) – no cilia
+ a less well known class of parasitic species: Sporozoa (Plasmodium vivax)
Ciliates are covered in hundreds of tiny motile hairs = cilia (sing. cilium). Are common in freshwater, also benign gut inhabitants.
flagellates move by a small number of long motile hairs = flagellae (sing. flagellum). Free living, also rumen flora and some gut parasites.
Rhizopoda free living in sediments etc, moving by slow protrusion of pseudopodia. A few are nasty parasites (Entamoeba dysenterica, Naegleria spp.)
Sporozoa (Plasmodium vivax causes malaria, the biggest killer in human history)
Instead of the drastic shoe-horning described above, the current version is to regard all single-celled organisms as belonging to the kingdom Protozoa with many phyla (27 at last count!)
This is probably more realistic, but much harder to remember.
These are essentially colonial protozoa, whose colonies are reinforced with solid spicules of various shapes and composition. Silica SiO2 and Calcite CaCO3 are the commonest.
They are exclusively aquatic, mainly marine, and live by filter feeding. The feeding cells are called choanocytes, which incorporate a central flagellum pumping water through the sponge, and the water passes through a collar of cilia-like filtering projections. The other main cell type is ameoba-like, making the supporting tissues and moving nutrients around.
Typically sponges suck water in from around their bodies and exhale it from a common central siphon. Due to their diffuse form, and often variable colour, identifying them is often difficult / impossible in the field and relies on microscopic examination of spicules.
Metazoa: These are animals with fully differentiated tissues, including muscles and nerves.
1 cell - Protozoa
No clear tissues: parazoa
The next level up in organisation takes us to the group of animals that used to be classed as phylum coelenterata (jellyfish, anemones and sea gooseberries). These are now split into 2 phyla, based on deep differences in design of their their stinging cells:
Cnidaria – jellyfish and anemones
Ctenophora – sea gooseberries.
1 cell - Protozoa
No clear tissues: parazoa
2 cell layers in embrya
Phyla cnidaria and ctenophora
3 cell layers in embryo
Remaining animal Phyla
There are about 10 phyla in which the basic body design involves a body cavity lined with cells (called a coelom), but of these I will only cover 4 today – these are the important common ones. One grouping is probably 3 distantly related phyla.
Phylum annelida – the segmented worms
Phylum mollusca: snails and allies
Phylum echinodermata – starfish and allies
Phylum (superphylum?) arthropoda – insects, spiders and crustaceans.
Phylum chordata – everything with a backbone (including us)
(examples: molluscs, annelids,
(a) Cleavage. In general, protostomedevelopment begins with spiral, determinate cleavage.Deuterostome development is characterized by radial, indeterminate cleavage.
Spiral and determinate
Radial and indeterminateCleavage
(b) Coelom formation. Coelom formation begins in the gastrula stage. In protostome development, the coelom forms from splits in the mesoderm (schizocoelous development). In deuterostome development, the coelom forms from mesodermal outpocketings of the archenteron (enterocoelous development).
folds of archenteron
masses of mesoderm
split and form coelom
Figure 32.9bCoelom Formation
An ectoproct, a lophophorate
Structure of trochophore larva
Figure 32.13a, b
Other bilaterians (including
Mollusca, and Annelida)
PORIFERA (5,500 species)
PLACOZOA (1 species)
KINORHYNCHA (150 species)
A placozoan (LM)
A kinorhynch (LM)
ROTIFERA (1,800 species)
PLATYHELMINTHES (20,000 species)
A marine flatworm
A rotifer (LM)
PHORONIDA (20 species)
ECTOPROCTA (4,500 species)
NEMERTEA (900 species)
A ribbon worm
ACANTHOCEPHALA (1,100 species)
CTENOPHORA (100 species)
A ctenophore, or comb jelly
MOLLUSCA (93,000 species)
ANNELIDA (16,500 species)
A marine annelid
LORICIFERA (10 species)
PRIAPULA (16 species)
A loriciferan (LM)
NEMATODA (25,000 species)
A scorpion (an arachnid)
CYCLIOPHORA (1 species)
TARDIGRADA (800 species)
A cycliophoran (colorized SEM)
Tardigrades (colorized SEM)
HEMICHORDATA (85 species)
ONYCHOPHORA (110 species)
An acorn worm
ECHINODERMATA (7,000 species)
CHORDATA (52,000 species)
A sea urchin
is lined with feeding cells called
choanocytes. By beating flagella,
the choanocytes create a current that
draws water in through the porocytes.
Azure vase sponge (Callyspongia
passing through porocytes
enters a cavity called the
Porocytes. Water enters
the epidermis through
channels formed by
cells that span the body wall.
The movement of the choanocyte
flagella also draws water through its
collar of fingerlike projections. Food
particles are trapped in the mucus
coating the projections, engulfed by
phagocytosis, and either digested or
transferred to amoebocytes.
Epidermis. The outer
layer consists of tightly
packed epidermal cells.
transport nutrients to other cells of
the sponge body and also produce
materials for skeletal fibers (spicules).
Mesohyl. The wall of this
simple sponge consists of
two layers of cells separated
by a gelatinous matrix, the
mesohyl (“middle matter”).
Jellyfish and allies. These alternate 2 phases in their life cycle: the free-living medusoid phase (“jellyfish”), and a sessile hydroid phase. Both feed by capturing planktonic food using tentacles armed with a cnidarian speciality, the class of stinging cell called nematocysts. Some are entangling, some inject barbed points to anchor, some inject toxins. A few a lethal to humans - NEVER EVER swim with box jellies (sea wasps, class Cubomedusae).
The main classes are:
Scyphozoa = jellyfish, Aurelia aurita in the common UK moon jelly (harmless to humans)
Anthozoa: sessile forms: sea anemones, corals, sea fans
Hydrozoa: various medusoid radiations, often with several body forms fused into one animal ie Physalia physalis, the infamous, portugese man o’war (avoid!).
The simplest of these phyla are the flatworms, platyhelminths. These have no body cavity (acoelomate), and a “bottle gut” (ie mouth and anus are the same orifice).
Combined mouth and anus, leading into gut
Many are free living, the planaria, and are active hunters. One recently introduced species from New Zealand is a serious earthworm predator - Arthiopostioa triangulata.
A few are internal parasites, ie liver fluke Fasciola hepatica. Bilharzia is caused by a flatworm Schistosoma that lives inside blood vessels - a serious medical problem.
None of the phyla mentioned so far have any internal fluid-filled body cavities. In fact most animal phyla do – these turn out to be highly important for making sense of phyla.
3 cell layers in embryo
No body cavity
(and the closely related phylum nemertini, bootlace worms.)
Has body cavity
Lined with cells
Not lined with cells
There are quite a few rather obscure phyla here, mainly of tiny (<2mm) and unfamiliar creatures that live in the water between grains of sand, in sediments etc – Phyla rotifera, gastrotricha and others (look up “minor pseudocoelomate phyla”). There is only one of these phyla that is really significant in terms of species richness.
These are the roundworms, phylum nematoda.
Nematodes: Almost all have the same body shape - round, pointy at both ends. (A very few plant parasitic species look like balloons, being immobile and full of eggs).
All have a thick collagen body wall retaining a high internal hydrostatic pressure - they are almost impossible to squash under normal circumstances.
Most of you here will have been infected with nematodes,. Luckily the commonest nematode in humans is tiny and harmless - the pinworm Enterobius vermicularis.
Nematode eggs are very tough (collagen wall again) and stay viable for months or years.
The most familiar of these is the common earthworm, Lumbricus terrestris.
(In fact, ecologically, this is one of the oddest annelids!)
All have true metameric segmentation, with each segment carrying gut, musculature and part of the nerve cord. There is often some differentiation of segments, ie the collar (clitellum) of earthworms.
The classes are:
Class chaetopoda - annelids with chaetae
order Polychaetes - marine worms, often very spiky with chaetae on lateral projections called parapodia (Beware: divers do not touch)
order oligochaeta - freshwater / terrestrial, small chaetae
Class hirudine - leeches; predators / ectoparasites with anterior + posterior suckers.
These have a soft, mucus-covered body with a muscular foot, often with a calcareous shell.
Class gastropoda - limpets, slugs and snails. Originally marine grazers, have emerged to become major terrestrial herbivores.
Class Lamellibranchs (=Bivalves) - aquatic filter feeders, using their gills to capture suspended food particles.
Class Cephalopoda - octopuses, squids, ammonites, nautilus (ie common octopus; Octopus vulgaris). Very different to other molluscs, with the muscular foot becoming 8-10 tentacles for food capture. They have independently evolved an eye almost identical to vertebrates, and seem to be the most highly intelligent invertebrates. They also include the largest invertebrates - a giant squid can be >5m long, with another 10m of tentacles.
All have an unexplained pentagonal symmetry, and a calcite exoskeleton supporting a complex system of tube feet used for slow locomotion. Any fossil – if it is pentagonal, it’s an echinoderm!
Asteroidea - starfish
Echinoidea - sea urchins
Ophiuroidea - brittle stars
Holothuridae - sea cucumbers
Crinoidea - feather stars
Starfish are predators, echinoids are herbivores, holothuridae are detritivores, the remainder filter feeders.
This is the biggest phylum in existence.
All these animals have a hard external skeleton and jointed legs. (‘Arthropod’ means jointed foot or limb). For many years these were treated as one huge phylum with three clear subphyla. More recently various lines of work, notably DNA analyses, suggest that the differences in these 3 subphyla are so great that they probably evolved the ‘armoured’ body form independently, and should be seen as 3 distinct phyla.
Forgive me if I still use the term ‘Arthropod’! It may yet come back, and if it doesn’t it remains a handy abbreviation.
(all have exoskeleton)
Mouthparts are mandibles, 2 pairs antennae.
Crabs, shrimps, lobsters, woodlice etc.
All have calcified cuticle.
Mouthparts are claw-like (chelicera), no antennae.
Spiders, mites, and horseshoe crabs.
Mouthparts are mandibles, 1 pair antennae.
Insects, millepedes, centipedes etc
Insects have 3 pairs of legs
All chordates have a dorsal nerve cord running along the body. There is an anterior swelling (‘brain’), and segmentalised body with segmented blocks of muscle. Unlike the arthropods and molluscs the brain does not encircle the gut – happens to be a good design for large body sizes.
Most chordates have bones along their nerve cord, making them vertebrates. Not all – some of our phylum are invertebrates!
Sea squirts (subphylum urochordates) have a larval form that is built much like a tadpole, barring a lack of bone, and are clearly from the chordate mould. But the adults forsake this for a sedentary life filtering sea water through a mucus net. There are a few other less well known invertebrate chordates.
The bony animals divide neatly into 5 classes, all of which you will recognise:
Amphibia – frogs newts etc (smooth skin)
Reptiles – lizards etc (scales)
Mammals (us, whales and everything else warm and furry)
Inevitably, the harder one looks at the fossil record, the less clear-cut these boundaries become!