Invertebrates

Invertebrates

Life Without a Backbone Invertebrates – –

Are animals that lack a backbone Account for 95% of known animal species

Eumetazoa

Ancestral colonial choanoflagellate Bilateria

Chordata

Echinodermata

Other bilaterians (including Nematoda, Arthropoda, Mollusca, and Annelida)

Cnidaria

Porifera

A review of animal phylogeny

Deuterostomia

Sponges (Porifera) • Sponges are sessile and have a porous body • Sponges

– Live in both fresh and marine waters – Lack true tissues and organs

Sponge Anatomy

31.1

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)

Flagellum Collar

1

Phagocytosis of food particles

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.

Mesohyl. The wall of this simple sponge consists of two layers of cells separated by a gelatinous matrix, the mesohyl (“middle matter”).

Choanocyte

Osculum

4 Spongocoel. Water passing through porocytes enters a cavity called the spongocoel. 3

Food particles in mucus

Amoebocyte

6

Spicules

Water flow

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).

31.1

Cnidarians • Exhibit a relatively simple diploblastic, radial body plan • Have a sac with a central digestive compartment, the gastrovascular cavity

– A single opening in this cavity functions as both mouth and anus

31.2

Examples

(a) These colonial polyps are members of class Hydrozoa.

(b) Many species of jellies (class Scyphozoa), including the species pictured here, are bioluminescent. The largest scyphozoans have tentacles more than 100 m long dangling from a bell-shaped body up to 2 m in diameter.

(c) The sea wasp (Chironex fleckeri) is a member of class Cubozoa. Its poison, which can subdue fish and other large prey, is more potent than cobra venom.

(d) Sea anemones and other members of class Anthozoa exist only as polyps.

Most animals have bilateral symmetry The vast majority of animal species belong to the clade Bilateria – Which consists of animals with bilateral symmetry and triploblastic development

Flatworms • Members of phylum Platyhelminthes – Live in marine, freshwater, and damp terrestrial habitats – Are flattened dorsoventrally and have a gastrovascular cavity – Are bilaterally symmetrical

• Although flatworms undergo triploblastic development – They are acoelomates 31.3

Flatworms are divided into four classes

Molluscs • Phylum Mollusca – Includes snails and slugs, oysters and clams, and octopuses and squids – Most molluscs are marine though some inhabit fresh water and some are terrestrial – Molluscs are soft-bodied animals but most are protected by a hard shell

31.4

• All molluscs have a similar body plan with three main parts – A muscular foot – A visceral mass (guts) – A mantle

31.4

Scallop

Annelids • Phylum Annelida • Annelids – Have bodies composed of a series of fused rings

31.5

Annelids

31.5

Nematodes • Phylum Nematoda • Nematodes are nonsegmented

pseudocoelomates covered by a tough cuticle

– Among the most widespread of all animals, nematodes, or roundworms, are found in most aquatic habitats, in the soil, in moist tissues of plants, and in the body fluids and tissues of animals 31.6

• The cylindrical bodies of nematodes

are covered by a tough coat called a cuticle

25 µm

31.6

• Some species of nematodes – Are important parasites of plants and animals Encysted juveniles

Muscle tissue

Trichinosis is caused by eating uncooked or undercooked pork 31.6

Trichinella

50 µm

Arthropods • Arthropods are segmented coelomates that have an exoskeleton and jointed appendages • Two out of every three known species of animals are arthropods • Members of the phylum Arthropoda – Are found in nearly all habitats of the biosphere 31.7

General Characteristics of Arthropods • The diversity and success of arthropods

– Are largely related to their segmentation, hard exoskeleton, and jointed appendages

31.7

• Early arthropods, such as trilobites – Showed little variation from segment to segment

31.7

• As arthropods evolved – The segments fused, and the appendages became more specialized

• The appendages of some living arthropods

– Are modified for many different functions

31.7

• The body of an arthropod – Is completely covered by the cuticle, an exoskeleton made of chitin

• When an arthropod grows – It molts its exoskeleton in a process called ecdysis

31.7

– Arthropods have an open circulatory system in which fluid called hemolymph is circulated into the spaces surrounding the tissues and organs – A variety of organs specialized for gas exchange have evolved in arthropods

31.7

Insects • Are more species-rich than all other forms

of life combined • Live in almost every terrestrial habitat and in fresh water • Flight is obviously one key to the great success of insects – An animal that can fly can escape predators, find food, and disperse to new habitats much faster than organisms that can only crawl 31.8

Echinoderms • Phylum Echinodermata • Sea stars and most other echinoderms

– Are slow-moving or sessile marine animals

• A thin, bumpy or spiny skin

– Covers an endoskeleton of hard calcareous plates

• The radial anatomy of many 31.9

echinoderms

Example Echinoder ms

31.9