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Chapter 33. Invertebrates. Overview: Life Without a Backbone. Invertebrates are animals that lack a backbone They account for 95% of known animal species. Fig. 33-2. Calcarea and Silicea. ANCESTRAL PROTIST. Cnidaria. Lophotrochozoa. Common ancestor of all animals. Eumetazoa.

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Chapter 33

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Chapter 33

Chapter 33


Overview life without a backbone

Overview: Life Without a Backbone

  • Invertebrates are animals that lack a backbone

  • They account for 95% of known animal species

Chapter 33

Fig. 33-2








ancestor of

all animals





Chapter 33

A sea


A marine


A jelly

A sponge

A ctenophore, or comb jelly

Chapter 33

A rotifer

Acoel flatworms


1.5 mm

A marine annelid

A brachiopod

An octopus

Chapter 33

A priapulan

A ribbon worm

A scorpion

(an arachnid)

A roundworm

An onychophoran

Phyla calcerea and silcea porifera

Phyla Calcerea and Silcea (Porifera)

  • Sponges are sedentary animals

  • They live in both fresh and marine waters

  • Sponges lack true tissues and organs

  • Most sponges are hermaphrodites: Each individual functions as both male and female

Chapter 33

  • Sponges are suspension feeders, capturing food particles suspended in the water that pass through their body

  • Choanocytes, flagellated collar cells, generate a water current through the sponge and ingest suspended food

  • Water is drawn through pores into a cavity called the spongocoel, and out through an opening called the osculum

  • Sponges consist of a noncellular mesophyl layer between two cell layers

Chapter 33

Fig. 33-4






Phagocytosis of

food particles








Clade eumetazoa true tissues phylum cnidarians

Clade Eumetazoa (true tissues): Phylum Cnidarians

  • Wide range of both sessile and motile forms including jellies, corals, and hydras

  • They exhibit a relatively simple diploblastic, radial body plan

  • The basic body plan of a cnidarian is a sac with a central digestive compartment, the gastrovascular cavity

Chapter 33

Fig. 33-5














•A single opening functions as mouth and anus

•There are two variations on the body plan: the sessile polyp and motile medusa

Chapter 33

  • Cnidarians are carnivores that use tentacles to capture prey

  • The tentacles are armed with cnidocytes, unique cells that function in defense and capture of prey

  • Nematocysts are specialized organelles within cnidocytes that eject a stinging thread

Chapter 33

Fig. 33-6










Chapter 33

Table 33-1

Chapter 33

Fig. 33-7


Sea wasp (class


(d) Sea anemone (class


(b) Jellies (class


(a) Colonial polyps (class


Classes of phylum cnidaria


- Alternate between polyp and medusa forms


- Jellies (medusae) are the prevalent form of the life cycle

Classes of Phylum Cnidaria

Video: Hydra Budding

Video: Jellyfish Medusae separating from polyp

Video: Jellyfish Medusae


Video: Jellyfish tentacles

  • Includes box jellies and sea wasps

  • The medusa is box-shaped and has complex eyes

  • - Cubozoans often have highly toxic cnidocytes


  • Includes the corals and sea anemones

  • Occur only as polyps

Video: Sea anemone

Video: Box Jelly sting

Clade lophotrochozoans have the widest range of animal body forms

Clade Lophotrochozoans have the widest range of animal body forms

  • Includes the flatworms, rotifers, ectoprocts, brachiopods, molluscs, and annelids

Phylum platyhelminthes

Phylum Platyhelminthes

  • Flatworms live in marine, freshwater, and damp terrestrial habitats (mostly parasitic)

  • They are tribloblastic acoelomates

  • They are flattened dorsoventrally and have a gastrovascular cavity (one opening)

  • No organs for gas exchange or circulation

Chapter 33

  • Flatworms are divided into four classes:

    1. Turbellaria (mostly free-living flatworms called planarians; hermaphrodites)

    2. Monogenea (are fish parasites)

    3. Trematoda (trematodes, or flukes are parasites)

    4. Cestoda (tapeworms, parasites in intestine of vertebrates)

Video: Tapeworm

Chapter 33

Table 33-2

Phylum rotifera

Phylum Rotifera

  • Rotifers are tiny animals that inhabit fresh water, the ocean, and damp soil

  • Rotifers are smaller than many protists but are truly multicellular and have specialized organ systems

- Rotifers have an alimentary canal (not a gastrovascular cavity), a digestive tube with a separate mouth and anus that lies within a fluid-filled pseudocoelom

- Parthogenesis: females producing more females with out the help of males

Video: Rotifer

Phylum mollusca

Phylum Mollusca

  • Includes snails and slugs, oysters and clams, and octopuses and squids

  • Most are marine, though some inhabit fresh water and some are terrestrial

  • Soft-bodied animals, but most are protected by a hard shell

Chapter 33

  • All molluscs have a similar body plan with three main parts:

    • Muscular foot

    • Visceral mass

    • Mantle

  • Many molluscs also have a water-filled mantle cavity, and feed using a rasplike radula

  • Most molluscs have separate sexes with gonads located in the visceral mass

  • Many snails are hermaphrodites.

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Table 33-3

Chapter 33



  • Snails and slugs

  • The most distinctive characteristic of gastropods is torsion, which causes the animal’s anus and mantle to end up above its head



  • Chitins

  • Oval-shaped marine animals encased in an armor of eight dorsal plates

Class bivalvia



Class Bivalvia

  • Squids and octopuses

  • Carnivores with

  • beak-like jaws surrounded

  • by tentacles of their modified foot

  • Cephalopods have a closed circulatory system, well-developed sense organs, and a complex brain

- Clams, oysters, mussels,

and scallops

- Shell is divided into two


- The mantle cavity

contains gills that are

used for feeding as well as

gas exchange (suspension feeders)


Phylum annelida

Phylum Annelida

  • Annelids have bodies composed of a series of fused rings

  • Segmented worms and are coelomates

  • Closed circulatory system

  • Sea, frseshwater habitats, and damp soil




  • Paddle-like parapodia that work as gills and aid in locomotion

  • Sparse chaetae, bristles made of chitin

  • Earthworms

  • Earthworms are hermaphrodites but cross-fertilize

  • Alimentary canal


Blood-sucking parasites, such as leeches

Clade ecdysozoans

Clade Ecdysozoans

  • Covered by a tough coat called a cuticle

  • The cuticle is shed or molted through a process called ecdysis

  • The two largest phyla are nematodes and arthropods

Phylum nematoda


  • Roundworms are found in most aquatic habitats, in the soil, in moist tissues of plants, and in body fluids and tissues of animals

  • They have an alimentary canal, but lack a circulatory system

  • pseudocoelomate

  • Non segmented

  • Tough coat = cuticle

Phylum arthropoda

Phylum Arthropoda

  • Coelomates

  • Body plan consists of a segmented body, hard exoskeleton, and jointed appendages

  • The body of an arthropod is completely covered by the cuticle, an exoskeleton made of layers of protein and the polysaccharide chitin

  • When an arthropod grows, it molts its exoskeleton

  • Arthropods have an open circulatory system

  • A variety of organs specialized for gas exchange have evolved in arthropods

  • Aquatic and terrestrial

Chapter 33

Table 33-5

Sub phyla cheliceriforms

Sub Phyla Cheliceriforms

- Clawlike feeding appendages called chelicerae

- Include spiders, scorpions, ticks, and mites

- Have an abdomen and a cephalothorax, which has six pairs of appendages

Sub Phyla Myriapods

- Millipedes and centipedes

- Are terrestrial, and have jaw-like mandibles

- Millipedes (each segment has two pairs of legs)

- Centipedes are carnivores (one pair of legs per segment)

Sub phyla hexapoda

Sub Phyla Hexapoda

  • Insects

  • Complex organ systems

  • Evolution of flight

  • Some insects are beneficial as pollinators, while others are harmful as carriers of diseases, or pests of crops

  • Insects are classified into more than 30 orders

Chapter 33

  • In incomplete metamorphosis, the young, called nymphs, resemble adults but are smaller and go through a series of molts until they reach full size

  • - Insects with complete metamorphosis have larval stages known by such names as maggot, grub, or caterpillar









Sub phyla crustaceans

Sub Phyla Crustaceans

  • Marine and freshwater environments

  • Branched appendages that are extensively specialized for feeding and locomotion

  • Isopods include terrestrial, freshwater, and marine species

    • Pill bugs are a well known group of terrestrial isopods

  • Decapods are all relatively large crustaceans and include lobsters, crabs, crayfish, and shrimp

Ghost crab



Clade deuterostomes

Clade deuterostomes

  • Phylum Echinodermata (sea stars) and phylum Chordata (vertebrates)

  • Shared characteristics define deuterostomes (Chordates and Echinoderms)

    • Radial cleavage

    • Formation of the mouth at the end of the embryo opposite the blastopore

Phylum echinodermata

Phylum Echinodermata

  • Slow-moving or sessile marine animals

  • Have a unique water vascular system, a network of hydraulic canals branching into tube feet that function in locomotion, feeding, and gas exchange

  • Males and females are usually separate, and sexual reproduction is external

  • Central disk has nerve ring and nerve cords

Video: Biology of Echinoderm

Chapter 33

Table 33-6

Chapter 33

Fig. 33-40

(a) A sea star (class Asteroidea)

(b) A brittle star (class Ophiuroidea)

(c) A sea urchin (class Echinoidea)

(d) A feather star (class Crinoidea)

(f) A sea daisy (class Concentricycloidea)

(e) A sea cucumber (class Holothuroidea)

Class asteroidea

Class Ophiuroidea

Class Asteroidea

  • - Brittle stars

  • Have a distinct central disk and long, flexible arms, which they use for movement

  • Tube feet lack suckers

- Sea stars have multiple arms radiating from a central disk

- The undersurfaces of the arms bear tube feet, each of which can act like a suction disk

  • Sea stars can regrow lost arms

  • Turn stomach inside out

Class Crinoidea

Class Echinoidea

  • - Sea lilies live attached to the substrate by a stalk

  • Feather stars can crawl using long, flexible arms

  • Mouth directed upward

- Sea urchins and sand dollars have no arms but have five rows of tube feet

Class Holothuroidea

Class Concentricycloidea

  • Sea cucumbers lack spines, and do not look like other echinoderms

  • 5 rows of tube feet

- Sea daisies; only three species are known

Chapter 33

Fig. 33-UN6

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