1. An introduction to the diversity of animal life

2. Aim for today
To introduce you to several characteristics found in animals and the range of animal life on the planet. In one lecture I can do no more than scrape the surface, but want to give you a basic structure to carry in your head into which any animal may be fitted.
This framework has a hierarchical structure (meaning it can be shown as a dendrogram) founded in taxonomy.
Taxonomy – the study of the classification of life forms.
Dendrogram

3. Taxonomic hierarchies
These are about seeking common features unifying all the organisms in a named group. The deepest split of all is between two ways of organising cells – the eukaryotic cell (with a nucleus and organelles) and prokaryotic cells (with DNA loops floating free in the cytoplasm). These are divided into 5 kingdoms in modern systems:
Eukaryotes:
Animals
Plants
Fungi
Prokaryotes
Eubacteria
Archaebacteria
(Viruses would count as a 6th, if you regard them as alive).

4. Phyla
In this course we will concentrate on just one kingdom, the animals. Luckily there are few hidden catches here – it is usually pretty obvious if a life form is an animal or not, though at the single celled level things can get rather blurred. (Volvox is a single celled green, photosynthetic entity which can ingest particulate food. It has good claims to be both animal and plant).
The next level down from kingdom is the one that REALLY matters for classifying animals. It is called Phylum, plural phyla.
There are about 30 phyla, each with a deep underlying similarity of body form. Once you can place an animal in its phylum you have made an excellent start towards understanding its anatomy.
(NOT FILA, as a student once wrote in a failed exam paper…)

5. The full hierarchy Kingdom - animalia Phylum - mandibulata
Class - Insecta
Order - Collembola
Family - Entomobryidae
Genus Entomobrya
Species Entomobrya nivalis
(I dislike the term “latin name”, since it is not Latin but merely latinised. Others find it acceptable, but I would encourage ‘Scientific name’)
 Species - the basis of taxonomy, dignified by a Latinised binomial = the scientific name: Homo sapiens, Apodemus sylvaticus, Lumbricus terrestris.

6. How to write a scientific name!
Homo sapiens OR Homo sapiens
1st name has a capital letter, 2nd does not
So many students get this wrong that I want to tell you now, at the start of your careers, how to write these names. Remember that getting it wrong is equivalent to saying “I have not been formally trained in biology”.
When writing by hand underline the name.
On a PC make the font italic

7. Several characteristics of animals
Concept 32.1: Animal are multicellular, heterotrophic eukaryotes with tissues that develop from embryonic layers
Several characteristics of animals
Sufficiently define the group

8. Their bodies are held together
By structural proteins such as collagen
Nervous tissue and muscle tissue
Are unique to animals
Animals are multicellular eukaryotes

9. Reproduction and Development
Most animals reproduce sexually
With the diploid stage usually dominating the life cycle
Animals are heterotrophs
That ingest their food

10. After a sperm fertilizes an egg
The zygote undergoes cleavage, leading to the formation of a blastula
The blastula undergoes gastrulation
Resulting in the formation of embryonic tissue layers and a gastrula

11. Cross section of blastula
Early embryonic development in animals
The zygote of an animal undergoes a succession of mitotic cell divisions called cleavage. 1
Only one cleavage
stage–the eight-cell
embryo–is shown here. 2
In most animals, cleavage results in the
formation of a multicellular stage called a blastula.
The blastula of many animals is a hollow ball of cells. 3
Zygote
Cleavage
Eight-cell stage
Blastula
Cross section of blastula
Blastocoel
Gastrula
Gastrulation
Endoderm
Ectoderm
Blastopore
The endoderm of
the archenteron de-
velops into the tissue
lining the animal’s
digestive tract. 6
The blind pouch
formed by gastru-
lation, called
the archenteron,
opens to the outside
via the blastopore. 5
Most animals also undergo gastrulation, a rearrangement of the embryo in which one end of the embryo folds inward, expands, and eventually fills the blastocoel, producing layers of embryonic tissues: the ectoderm (outer layer) and the endoderm (inner layer). 4
Figure 32.2

12. All animals, and only animals
Have Hox genes that regulate the development of body form
Although the Hox family of genes has been highly conserved
It can produce a wide diversity of animal morphology

13. 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

14. The Protozoa – the single celled animals
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)

15. 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)

16. New version – kingdom Protozoa
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.

17. Sponges – Phylum parazoa
 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.

18. Metazoa: These are animals with fully differentiated tissues, including muscles and nerves.
Many cells
1 cell - Protozoa
No clear tissues: parazoa
Tissues: metazoa
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.

19. Phylum Cnidaria (radially symmetric, 2 cell layers in body)
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!).

20. Bilateria: this comprises c
Bilateria: this comprises c. 25 phyla all with bilateral symmetry (at least as larvae) and 3 layers of cells in the embryo.
Many cells
1 cell - Protozoa
No clear tissues: parazoa
Tissues: metazoa
Radial symmetry
2 cell layers in embrya
Phyla cnidaria and ctenophora
Bilateral symmetry
3 cell layers in embryo
Remaining animal Phyla

21. Phylum Platyhelminths
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).
<1mm deep
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.

22. Body cavities
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.
Bilateral symmetry
3 cell layers in embryo
No body cavity
Flatworms
Phylum platyhelminths
(and the closely related phylum nemertini, bootlace worms.)
Has body cavity
Lined with cells
Coelomate phyla
Not lined with cells
Pseudocoelomate phyla

23. Pseudocoelomates, especially phylum nematoda, the roundworms
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.

24. Phylum nematoda – the roundworms
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.

25. The big 5 coelomate 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)

26. Phylum Annelida – the segmented worms.
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.

27. Phylum Mollusca – snails and allies
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.

28. Phylum Echinodermata – starfish and allies
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!
Classes
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.

29. Superphylum Arthropoda – insects, spiders and crustaceans
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.

30. Superphylum Arthropoda (all have exoskeleton)
Phylum Crustacea
Mouthparts are mandibles, 2 pairs antennae.
Crabs, shrimps, lobsters, woodlice etc.
All have calcified cuticle.
Phylum Mandibulata
Mouthparts are mandibles, 1 pair antennae.
Insects, millepedes, centipedes etc
Insects have 3 pairs of legs
Phylum Chelicerata
Mouthparts are claw-like (chelicera), no antennae.
Spiders, mites, and horseshoe crabs.

31. Our phylum – the chordates
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.

32. Vertebrates
The bony animals divide neatly into 5 classes, all of which you will recognise:
Pisces (fishes)
Amphibia – frogs newts etc (smooth skin)
Reptiles – lizards etc (scales)
Birds (feathers)
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!

33. The animal kingdom includes not only great diversity of living species
Concept 32.2: The history of animals may span more than a billion years
The animal kingdom includes not only great diversity of living species
But the even greater diversity of extinct ones as well

34. The common ancestor of living animals
May have lived 1.2 billion–800 million years ago
May have resembled modern choanoflagellates, protists that are the closest living relatives of animals
Single cell
Stalk
Figure 32.3

35. Was probably itself a colonial, flagellated protist
Colonial protist,
an aggregate of
identical cells
Hollow sphere of unspecialized cells (shown in cross section)
Beginning of cell specialization
Infolding
Gastrula-like “protoanimal”
Somatic cells
Digestive
cavity
Reproductive cells
Figure 32.4

36. Concept 32.3: Animals can be characterized by “body plans”
One way in which zoologists categorize the diversity of animals
Is according to general features of morphology and development
A group of animal species
That share the same level of organizational complexity is known as a grade

37. The set of morphological and developmental traits that define a grade
Are generally integrated into a functional whole referred to as a body plan

38. Animals can be categorized
Symmetry
Animals can be categorized
According to the symmetry of their bodies, or lack of it

39. Some animals have radial symmetry
Like in a flower pot
Radial symmetry. The parts of a radial animal, such as a sea anemone (phylum Cnidaria), radiate from the center. Any imaginary slice through the central axis divides the animal into mirror images.
(a)
Figure 32.7a

40. Some animals exhibit bilateral symmetry
Or two-sided symmetry
Bilateral symmetry. A bilateral animal, such as a lobster (phylum Arthropoda), has a left side and a right side. Only one imaginary cut divides the animal into mirror-image halves.
(b)
Figure 32.7b

41. Bilaterally symmetrical animals have
A dorsal (top) side and a ventral (bottom) side
A right and left side
Anterior (head) and posterior (tail) ends
Cephalization, the development of a head

42. Tissues Animal body plans Tissues
Also vary according to the organization of the animal’s tissues
Tissues
Are collections of specialized cells isolated from other tissues by membranous layers

43. Triploblastic animals
Animal embryos
Form germ layers, embryonic tissues, including ectoderm, endoderm, and mesoderm
Diploblastic animals
Have two germ layers
Triploblastic animals
Have three germ layers

44. In triploblastic animals
Body Cavities
In triploblastic animals
A body cavity may be present or absent

45. A true body cavity Is called a coelom and is derived from mesoderm
Figure 32.8a
Coelom
Body covering
(from ectoderm)
Digestive tract
(from endoderm)
Tissue layer
lining coelom
and suspending
internal organs
(from mesoderm)
Coelomate. Coelomates such as annelids have a true coelom, a body cavity completely lined by tissue derived from mesoderm.
(a)

46. A pseudocoelom
Is a body cavity derived from the blastocoel, rather than from mesoderm
Pseudocoelom
Muscle layer
(from
mesoderm)
Body covering
(from ectoderm)
Digestive tract
Pseudocoelomate. Pseudocoelomates such as nematodes have a body cavity only partially lined by tissue derived from mesoderm.
(b)
Figure 32.8b

47. Organisms without body cavities
Are considered acoelomates
Body covering
(from ectoderm)
Tissue-
filled region
(from
mesoderm)
Digestive tract
(from endoderm)
Acoelomate. Acoelomates such as flatworms lack a body cavity between the digestive tract and outer body wall.
(c)
Figure 32.8c

48. Protostome and Deuterostome Development
Based on certain features seen in early development
Many animals can be categorized as having one of two developmental modes: protostome development or deuterostome development

49. In protostome development
Cleavage
In protostome development
Cleavage is spiral and determinate
In deuterostome development
Cleavage is radial and indeterminate
Protostome development
(examples: molluscs, annelids,
arthropods)
Deuterostome development
(examples: echinoderms,
chordates)
Eight-cell stage
Spiral and determinate
Radial and indeterminate
(a) Cleavage. In general, protostome development begins with spiral, determinate cleavage. Deuterostome development is characterized by radial, indeterminate cleavage.
Figure 32.9a

50. Coelom Formation In protostome development In deuterostome development
The splitting of the initially solid masses of mesoderm to form the coelomic cavity is called schizocoelous development
In deuterostome development
Formation of the body cavity is described as enterocoelous development
Figure 32.9b
Archenteron
Blastopore
Mesoderm
Coelom
Schizocoelous: solid
masses of mesoderm
split and form coelom
Enterocoelous:
folds of archenteron
form coelom
(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).

51. In protostome development
Fate of the Blastopore
In protostome development
The blastopore becomes the mouth
In deuterostome development
The blastopore becomes the anus
Figure 32.9c
Anus
Mouth
Mouth develops
from blastopore
Anus develops
Digestive tube

52. Zoologists currently recognize about 35 animal phyla
Concept 32.4: Leading hypotheses agree on major features of the animal phylogenetic tree
Zoologists currently recognize about 35 animal phyla
The current debate in animal systematics
Has led to the development of two phylogenetic hypotheses, but others exist as well

53. One hypothesis of animal phylogeny based mainly on morphological and developmental comparisons
Porifera
Cnidaria
Ctenophora
Phoronida
Ectoprocta
Brachiopoda
Echinodermata
Chordata
Platyhelminthes
Mollusca
Annelida
Arthropoda
Rotifera
Nemertea
Nematoda
“Radiata”
Deuterostomia
Protostomia
Bilateria
Eumetazoa
Metazoa
Ancestral colonial
flagellate
Figure 32.10

54. One hypothesis of animal phylogeny based mainly on molecular data
Calcarea
Silicarea
Ctenophora
Cnidaria
Echinodermata
Chordata
Brachiopoda
Phoronida
Ectoprocta
Platyhelminthes
Nemertea
Mollusca
Annelida
Rotifera
Nematoda
Arthropoda
“Radiata”
“Porifera”
Deuterostomia
Lophotrochozoa
Ecdysozoa
Bilateria
Eumetazoa
Metazoa
Ancestral colonial
flagellate
Figure 32.11

55. Points of Agreement
All animals share a common ancestor
Sponges are basal animals
Eumetazoa is a clade of animals with true tissues

56. Most animal phyla belong to the clade Bilateria
Vertebrates and some other phyla belong to the clade Deuterostomia

57. Disagreement over the Bilaterians
The morphology-based tree
Divides the bilaterians into two clades: deuterostomes and protostomes
In contrast, several recent molecular studies
Generally assign two sister taxa to the protostomes rather than one: the ecdysozoans and the lophotrochozoans

58. Ecdysozoans share a common characteristic
They shed their exoskeletons through a process called ecdysis
Figure 32.12

59. An ectoproct, a lophophorate Structure of trochophore larva
Lophotrochozoans share a common characteristic
Called the lophophore, a feeding structure
Other phyla
Go through a distinct larval stage called a trochophore larva
Figure 32.13a, b
Apical tuft
of cilia
Mouth
Anus
(a)
An ectoproct, a lophophorate
(b)
Structure of trochophore larva

60. Chapter 33 Invertebrates- sponges
Overview: Life Without a Backbone
Invertebrates
Are animals that lack a backbone
Account for 95% of known animal species

61. A review of animal phylogeny
Ancestral colonial
choanoflagellate
Eumetazoa
Bilateria
Deuterostomia
Porifera
Cnidaria
Other bilaterians (including
Nematoda, Arthropoda,
Mollusca, and Annelida)
Echinodermata
Chordata
Figure 33.2

62. Exploring invertebrate diversity
PORIFERA (5,500 species)
A sponge
CNIDARIA (10,000 species)
A jelly
PLACOZOA (1 species)
KINORHYNCHA (150 species)
0.5 mm
A placozoan (LM)
A kinorhynch (LM)
250 µm
PLATYHELMINTHES (20,000 species)
ROTIFERA (1,800 species)
A marine flatworm
A rotifer (LM)
ECTOPROCTA (4,500 species)
PHORONIDA (20 species)
Ectoprocts
Phoronids
Figure 33.3

63. A ctenophore, or comb jelly
Exploring invertebrate diversity
BRACHIOPODA (335 species)
NEMERTEA (900 species)
A brachiopod
A ribbon worm
ACANTHOCEPHALA (1,100 species)
CTENOPHORA (100 species)
An acanthocephalan
A ctenophore, or comb jelly
MOLLUSCA (93,000 species)
ANNELIDA (16,500 species)
An octopus
A marine annelid
LORICIFERA (10 species)
PRIAPULA (16 species)
5 mm
50 µm
A loriciferan (LM)
A priapulan
Figure 33.3

64. A cycliophoran (colorized SEM) Tardigrades (colorized SEM)
Exploring invertebrate diversity
NEMATODA (25,000 species)
ARTHROPODA (1,000,000 + species)
A roundworm
A scorpion (an arachnid)
CYCLIOPHORA (1 species)
TARDIGRADA (800 species)
100 µm
A cycliophoran (colorized SEM)
Tardigrades (colorized SEM)
ONYCHOPHORA (110 species)
HEMICHORDATA (85 species)
An onychophoran
An acorn worm
ECHINODERMATA (7,000 species)
CHORDATA (52,000 species)
A sea urchin
A tunicate
Figure 33.3

65. Sponges are sessile and have a porous body and choanocytes
Sponges, phylum Porifera
Live in both fresh and marine waters
Lack true tissues and organs

66. Sponges are suspension feeders
Capturing food particles suspended in the water that passes through their body
Azure vase sponge (Callyspongia
plicifera)
Osculum
Spicules
Water
flow
Flagellum
Collar
Food particles
in mucus
Choanocyte
Phagocytosis of
food particles
Amoebocyte
Choanocytes. The spongocoel
is lined with feeding cells called
choanocytes. By beating flagella,
the choanocytes create a current that
draws water in through the porocytes.
Spongocoel. Water
passing through porocytes
enters a cavity called the
spongocoel.
Porocytes. Water enters
the epidermis through
channels formed by
porocytes, doughnut-shaped
cells that span the body wall.
Epidermis. The outer
layer consists of tightly
packed epidermal cells.
Mesohyl. The wall of this
simple sponge consists of
two layers of cells separated
by a gelatinous matrix, the
mesohyl (“middle matter”).
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.
Amoebocyte. Amoebocytes
transport nutrients to other cells of
the sponge body and also produce
materials for skeletal fibers (spicules). 5 6 7 4 3 2 1
Figure 33.4

67. Choanocytes, flagellated collar cells
Generate a water current through the sponge and ingest suspended food
Most sponges are hermaphrodites
Meaning that each individual functions as both male and female

68. All animals except sponges
Concept 33.2: Cnidarians have radial symmetry, a gastrovascular cavity, and cnidocytes
All animals except sponges
Belong to the clade Eumetazoa, the animals with true tissues
Phylum Cnidaria
Is one of the oldest groups in this clade