Chapter 33 Invertebrates

Overview: Life Without a Backbone Invertebrates Are animals that lack a backbone Account for 95% of known animal species Figure 33.1

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

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

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

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

Sponges are sedentary Have a porous body and choanocytes Sponges, phylum Calcarea & Silicea Mistaken for plants Among simplest animals Live in both fresh and marine waters Lack true tissues and organs

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

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

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

Cnidarians Have diversified into a wide range of both sessile and floating forms including jellies, corals, and hydras But still 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 A single opening Functions as both mouth and anus

There are two variations on this body plan The sessile polyp and the floating medusa Mouth/anus Tentacle Gastrovascular cavity Gastrodermis Mesoglea Epidermis Body stalk Medusa Polyp Figure 33.5

Cnidarians are carnivores That use tentacles to capture prey The tentacles are armed with cnidocytes Unique cells that function in defense and the capture of prey Tentacle “Trigger” Nematocyst Coiled thread Discharge Of thread Cnidocyte Prey Figure 33.6

The phylum Cnidaria is divided into four major classes Table 33.1

Hydrozoa, Scyphozoa, Cubozoa, and Anthozoa (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. Figure 33.7a–d

Hydrozoans Most hydrozoans Alternate between polyp and medusa forms Other polyps, specialized for reproduction, lack tentacles and produce tiny medusae by asexual budding. 3 Some of the colony’s polyps, equipped with tentacles, are specialized for feeding. 2 The medusae swim off, grow, and reproduce sexually. 4 Feeding polyp Reproductive Medusa bud ASEXUAL REPRODUCTION (BUDDING) Gonad MEIOSIS FERTILIZATION SEXUAL Egg Sperm Developing Portion of a colony of polyps Mature Planula (larva) Key Haploid (n) Diploid (2n) 1 mm Zygote Figure 33.8 A colony of interconnected polyps (inset, LM) results from asexual reproduction by budding. 1 The planula eventually settles and develops into a new polyp. 6 The zygote develops into a solid ciliated larva called a planula. 5

Scyphozoans In the class Scyphozoa Jellies (medusae) are the prevalent form of the life cycle

In the class Cubozoa, which includes box jellies and sea wasps Cubozoans In the class Cubozoa, which includes box jellies and sea wasps The medusa is box-shaped and has complex eyes

Class Anthozoa includes the corals and sea anemones Anthozoans Class Anthozoa includes the corals and sea anemones Which occur only as polyps

Concept 33.3: 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

Members of phylum Platyhelminthes Flatworms Members of phylum Platyhelminthes Live in marine, freshwater, and damp terrestrial habitats Are flattened dorsoventrally and have a gastrovascular cavity Although flatworms undergo triploblastic development They are acoelomates

Flatworms are divided into four classes Table 33.2

Turbellarian Turbellarians Are nearly all free-living and mostly marine Figure 33.9

The best-known turbellarians, commonly called planarians Have light-sensitive eyespots and centralized nerve nets Pharynx. The mouth is at the tip of a muscular pharynx that extends from the animal’s ventral side. Digestive juices are spilled onto prey, and the pharynx sucks small pieces of food into the gastrovascular cavity, where digestion continues. Digestion is completed within the cells lining the gastro- vascular cavity, which has three branches, each with fine subbranches that pro- vide an extensive surface area. Undigested wastes are egested through the mouth. Ganglia. Located at the anterior end of the worm, near the main sources of sensory input, is a pair of ganglia, dense clusters of nerve cells. Ventral nerve cords. From the ganglia, a pair of ventral nerve cords runs the length of the body. Gastrovascular cavity Eyespots Figure 33.10

Monogeneans and Trematode Monogeneans and trematodes Live as parasites in or on other animals Parasitize a wide range of hosts

Trematodes that parasitize humans Spend part of their lives in snail hosts These larvae penetrate the skin and blood vessels of humans working in irrigated fields contaminated with infected human feces. Asexual reproduction within a snail results in another type of motile larva, which escapes from the snail host. Blood flukes reproduce sexually in the human host. The fertilized eggs exit the host in feces. The eggs develop in water into ciliated larvae. These larvae infect snails, the intermediate hosts. Snail host 1 mm Female Male 5 2 3 4 Figure 33.11 Mature flukes live in the blood vessels of the human intestine. A female fluke fits into a groove running the length of the larger male’s body, as shown in the light micrograph at right. 1

Most monogeneans Are parasites of fish

Tapeworm Tapeworms Are also parasitic and lack a digestive system Proglottids with reproductive structures 200 µm Hooks Sucker Scolex Figure 33.12

Rotifers, phylum Rotifera 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 0.1 mm Figure 33.13

Rotifers have an alimentary canal A digestive tube with a separate mouth and anus that lies within a fluid-filled pseudocoelom Rotifers reproduce by parthenogenesis In which females produce more females from unfertilized eggs

Lophophorates: Ectoprocts, Phoronids, and Brachiopods Lophophorates have a lophophore A horseshoe-shaped, suspension-feeding organ bearing ciliated tentacles

Ectoprocts Are colonial animals that superficially resemble plants Lophophore Ectoprocts, such as this sea mat (Membranipora membranacea), are colonial lophophorates. (a) Figure 33.14a

Phoronids Are tube-dwelling marine worms ranging from 1 mm to 50 cm in length Lophophore In phoronids such as Phoronis hippocrepia, the lophophore and mouth are at one end of an elongated trunk. (b) Figure 33.14b

Brachiopods superficially resemble clams and other hinge-shelled molluscs But the two halves of the shell are dorsal and ventral rather than lateral, as in clams Lophophore Brachiopods have a hinged shell. The two parts of the shell are dorsal and ventral. (c) Figure 33.14c

Members of phylum Nemertea Nemerteans Members of phylum Nemertea Are commonly called proboscis worms or ribbon worms Figure 33.15

The nemerteans unique proboscis Is used for defense and prey capture Is extended by a fluid-filled sac Nemerteans also have a closed circulatory system In which the blood is contained in vessels distinct from fluid in the body cavity

Most molluscs are marine Concept 33.4: Molluscs have a muscular foot, a visceral mass, and a mantle 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

All molluscs have a similar body plan with three main parts A muscular foot A visceral mass A mantle

Visceral mass Mantle Foot Coelom Intestine Gonads cavity Anus Gill Nerve cords Esophagus Stomach Shell Radula Mouth Nephridium. Excretory organs called nephridia remove metabolic wastes from the hemolymph. Heart. Most molluscs have an open circulatory system. The dorsally located heart pumps circulatory fluid called hemolymph through arteries into sinuses (body spaces). The organs of the mollusc are thus continually bathed in hemolymph. The long digestive tract is coiled in the visceral mass. Radula. The mouth region in many mollusc species contains a rasp-like feeding organ called a radula. This belt of backward- curved teeth slides back and forth, scraping and scooping like a backhoe. The nervous system consists of a nerve ring around the esophagus, from which nerve cords extend. Figure 33.16

Most molluscs have separate sexes With gonads located in the visceral mass The life cycle of many molluscs Includes a ciliated larval stage called a trochophore

There are four major classes of molluscs Table 33.3

Class Polyplacophora is composed of the chitons Oval-shaped marine animals encased in an armor of eight dorsal plates Figure 33.17

About three-quarters of all living species of molluscs Gastropods About three-quarters of all living species of molluscs Belong to class Gastropoda A land snail (a) A sea slug. Nudibranchs, or sea slugs, lost their shell during their evolution. (b) Figure 33.18a, b

Most gastropods Slugs lack a shell Are marine, but there are also many freshwater and terrestrial species Possess a single, spiraled shell Slugs lack a shell Or have a reduced shell

The most distinctive characteristic of this class Is a developmental process known as torsion, which causes the animal’s anus and mantle to end up above its head Anus Mantle cavity Stomach Intestine Mouth Figure 33.19

Molluscs of class Bivalvia Bivalves Molluscs of class Bivalvia Include many species of clams, oysters, mussels, and scallops Have a shell divided into two halves Figure 33.20

The mantle cavity of a bivalve Contains gills that are used for feeding as well as gas exchange Hinge area Gut Coelom Heart Adductor muscle Anus Excurrent siphon Water flow Incurrent Gill Mantle cavity Foot Palp Mouth Shell Figure 33.21