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An introduction to the diversity of animal life
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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.
Dendrogram
Taxonomy – the study of
the classification of life
forms.
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).
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.
The full hierarchy
Kingdom - animalia
Phylum - mandibulata
Class - Insecta
Order - Collembola
Family - Entomobryidae
Genus Entomobrya
Species Entomobrya nivalis
Species - the basis of taxonomy, dignified by a Latinised binomial =
the scientific name: Homo sapiens, Apodemus sylvaticus, Lumbricus
terrestris.
How to write a scientific name!
1st name has a capital letter, 2nd does not
Homo sapiens
OR Homo sapiens
On a PC make the font italic
When writing by hand
underline the name.
• Concept 32.1: Animal are multicellular,
heterotrophic eukaryotes with tissues that
develop from embryonic layers
• Several characteristics of animals
– Sufficiently define the group
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Their bodies are held together
– By structural proteins such as collagen
• Nervous tissue and muscle tissue
– Are unique to animals
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Reproduction and Development
• Most animals reproduce sexually
– With the diploid stage usually dominating the
life cycle
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Early embryonic development in animals
1 The zygote of an animal
undergoes a succession of mitotic
cell divisions called cleavage.
2 Only one cleavage
stage–the eight-cell
embryo–is shown here.
3 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.
Blastocoel
Cleavage
Cleavage
6 The endoderm of
the archenteron develops into the tissue
lining the animal’s
digestive tract.
Zygote
Eight-cell stage
Blastula
Cross section
of blastula
Blastocoel
Endoderm
5 The blind pouch
formed by gastrulation, called
the archenteron,
opens to the outside
via the blastopore.
Ectoderm
Gastrula
Gastrulation
Blastopore
Figure 32.2
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
4 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).
• 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
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)
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.
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.
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.
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!).
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
Radial symmetry
2 cell layers in embrya
Phyla cnidaria and ctenophora
Tissues: metazoa
Bilateral symmetry
3 cell layers in embryo
Remaining animal Phyla
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.
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
Has body cavity
Flatworms
Phylum platyhelminths
(and the closely related
phylum nemertini,
bootlace worms.)
Lined with cells
Not lined with cells
Coelomate phyla
Pseudocoelomate phyla
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.
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.
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)
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.
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.
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.
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.
Superphylum Arthropoda
(all have exoskeleton)
Phylum
Mandibulata
Phylum
Chelicerata
Phylum
Crustacea
Mouthparts are
mandibles, 1 pair
antennae.
Insects, millepedes,
centipedes etc
Insects have 3 pairs
of legs
Mouthparts are
claw-like
(chelicera), no
antennae.
Spiders, mites, and
horseshoe crabs.
Mouthparts are
mandibles, 2 pairs
antennae.
Crabs, shrimps,
lobsters, woodlice
etc.
All have calcified
cuticle.
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.
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!
• 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
• 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
– Was probably itself a colonial, flagellated
protist
Digestive
cavity
Somatic cells
Reproductive cells
Colonial protist,
an aggregate of
identical cells
Figure 32.4
Hollow sphere of
unspecialized
cells (shown in
cross section)
Beginning of cell
specialization
Infolding
Gastrula-like
“protoanimal”
• 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
• The set of morphological and developmental
traits that define a grade
– Are generally integrated into a functional whole
referred to as a body plan
Symmetry
• Animals can be categorized
– According to the symmetry of their bodies, or
lack of it
• Some animals have radial symmetry
– Like in a flower pot
(a) 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.
Figure 32.7a
• Some animals exhibit bilateral symmetry
– Or two-sided symmetry
(b) 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.
Figure 32.7b
• 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
Tissues
• Animal body plans
– Also vary according to the organization of the
animal’s tissues
• Tissues
– Are collections of specialized cells isolated
from other tissues by membranous layers
• Animal embryos
– Form germ layers, embryonic tissues,
including ectoderm, endoderm, and mesoderm
• Diploblastic animals
– Have two germ layers
• Triploblastic animals
– Have three germ layers
Body Cavities
• In triploblastic animals
– A body cavity may be present or absent
• A true body cavity
– Is called a coelom and is derived from
mesoderm
Coelom
(a) Coelomate. Coelomates such as
annelids have a true coelom, a body
cavity completely lined by tissue
derived from mesoderm.
Tissue layer
lining coelom
and suspending
internal organs
(from mesoderm)
Digestive tract
(from endoderm)
Figure 32.8a
Body covering
(from ectoderm)
• A pseudocoelom
– Is a body cavity derived from the blastocoel,
rather than from mesoderm
Body covering
(from ectoderm)
(b) Pseudocoelomate. Pseudocoelomates
such as nematodes have a body cavity only
partially lined by tissue derived from
mesoderm.
Pseudocoelom
Digestive tract
(from ectoderm)
Figure 32.8b
Muscle layer
(from
mesoderm)
• Organisms without body cavities
– Are considered acoelomates
Body covering
(from ectoderm)
(c) Acoelomate. Acoelomates such as
flatworms lack a body cavity between
the digestive tract and outer body wall.
Digestive tract
(from endoderm)
Figure 32.8c
Tissuefilled region
(from
mesoderm)
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
Cleavage
• In protostome development
– Cleavage is spiral and determinate
• In deuterostome development
– Cleavage is radial and indeterminate
Protostome development
(examples: molluscs, annelids,
arthropods)
Eight-cell stage
Spiral and determinate
Figure 32.9a
Deuterostome development
(examples: echinoderms,
chordates)
Eight-cell stage
Radial and indeterminate
(a) Cleavage. In general, protostome
development begins with spiral,
determinate cleavage.
Deuterostome development is
characterized by radial, indeterminate
cleavage.
Coelom Formation
• In protostome 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
Coelom
Archenteron
Coelom
Mesoderm
Blastopore
Mesoderm
Blastopore
Enterocoelous:
Schizocoelous: solid
folds of archenteron
masses of mesoderm
form coelom
split and form coelom
Figure 32.9b
(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).
Fate of the Blastopore
• In protostome development
– The blastopore becomes the mouth
• In deuterostome development
– The blastopore becomes the anus
Mouth
Anus
Digestive tube
Mouth
Figure 32.9c
Mouth develops
from blastopore
Anus
Anus develops
from blastopore
• 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
“Radiata”
Deuterostomia
Metazoa
Figure 32.10
Ancestral colonial
flagellate
Nematoda
Nemertea
Rotifera
Arthropoda
Annelida
Protostomia
Bilateria
Eumetazoa
Mollusca
Platyhelminthes
Chordata
Echinodermata
Brachiopoda
Ectoprocta
Phoronida
Ctenophora
Cnidaria
Porifera
• One hypothesis of animal phylogeny based
mainly on morphological and developmental
comparisons
Arthropoda
Nematoda
Rotifera
Annelida
Mollusca
Nemertea
Platyhelminthes
Ectoprocta
Phoronida
Brachiopoda
Chordata
Echinodermata
Cnidaria
Ctenophora
Silicarea
Calcarea
• One hypothesis of animal phylogeny based
mainly on molecular data
“Radiata”
“Porifera”
Deuterostomia
Lophotrochozoa
Bilateria
Eumetazoa
Metazoa
Figure 32.11
Ancestral colonial
flagellate
Ecdysozoa
Points of Agreement
• All animals share a common ancestor
• Sponges are basal animals
• Eumetazoa is a clade of animals with true
tissues
• Most animal phyla belong to the clade Bilateria
• Vertebrates and some other phyla belong to
the clade Deuterostomia
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
• Ecdysozoans share a common characteristic
– They shed their exoskeletons through a
process called ecdysis
Figure 32.12
• Lophotrochozoans share a common characteristic
– Called the lophophore, a feeding structure
• Other phyla
– Go through a distinct larval stage called a
trochophore larva
Apical tuft
of cilia
Mouth
Figure 32.13a, b
(a) An ectoproct, a lophophorate
Anus
(b) Structure of trochophore larva
Chapter 33 Invertebrates- sponges
• Overview: Life Without a Backbone
• Invertebrates
– Are animals that lack a backbone
– Account for 95% of known animal species
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Chordata
Echinodermata
Other bilaterians (including
Nematoda, Arthropoda,
Mollusca, and Annelida)
Porifera
Cnidaria
• A review of animal phylogeny
Deuterostomia
Bilateria
Eumetazoa
Ancestral colonial
choanoflagellate
Figure 33.2
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Exploring invertebrate diversity
PORIFERA (5,500 species)
A sponge
PLACOZOA (1 species)
CNIDARIA (10,000 species)
A jelly
KINORHYNCHA (150 species)
0.5 mm
250 µm
A placozoan (LM) A kinorhynch (LM)
ROTIFERA (1,800 species)
PLATYHELMINTHES (20,000 species)
A marine flatworm
ECTOPROCTA (4,500 species)
Figure 33.3
A rotifer (LM)
PHORONIDA (20 species)
Ectoprocts Phoronids
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Exploring invertebrate diversity
BRACHIOPODA (335 species)
NEMERTEA (900 species)
A brachiopod
ACANTHOCEPHALA (1,100 species)
A ribbon worm
CTENOPHORA (100 species)
5 mm
An acanthocephalan
A ctenophore, or comb jelly
MOLLUSCA (93,000 species)
ANNELIDA (16,500 species)
An octopus
A marine annelid
PRIAPULA (16 species)
LORICIFERA (10 species)
50 µm
Figure 33.3
A loriciferan (LM)
A priapulan
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Exploring invertebrate diversity
ARTHROPODA (1,000,000 + species)
NEMATODA (25,000 species)
A roundworm
A scorpion (an arachnid)
CYCLIOPHORA (1 species)
TARDIGRADA (800 species)
100 µm
100 µm
A cycliophoran (colorized SEM) Tardigrades (colorized SEM)
HEMICHORDATA (85 species)
ONYCHOPHORA (110 species)
An onychophoran
An acorn worm
ECHINODERMATA (7,000 species)
Figure 33.3
A sea urchin
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
CHORDATA (52,000 species)
A tunicate
• 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Sponges are suspension feeders
– Capturing food particles suspended in the
water that passes through their body
5 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.
Azure vase sponge (Callyspongia
plicifera)
4 Spongocoel. Water
passing through porocytes
enters a cavity called the
spongocoel.
3 Porocytes. Water enters
the epidermis through
channels formed by
porocytes, doughnut-shaped
cells that span the body wall.
2 Epidermis. The outer
layer consists of tightly
packed epidermal cells.
Figure 33.4
Flagellum
Collar
Food particles
in mucus
Choanocyte
Osculum
Phagocytosis of
food particles
Spicules
Water
flow
1 Mesohyl. The wall of this
simple sponge consists of
two layers of cells separated
by a gelatinous matrix, the
mesohyl (“middle matter”).
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Amoebocyte
6 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.
7 Amoebocyte. Amoebocytes
transport nutrients to other cells of
the sponge body and also produce
materials for skeletal fibers (spicules).
• 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 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
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