Invertebrates: Sponges & Cnidarians

• Invertebrates lack a backbone.
• They comprise over 97% of known animal species.

Invertebrate Clades

• Porifera
• Cnidaria
• Lophotrochozoa
• Ecdysozoa
• Deuterostomia

Sponges

• Sponges are basal animals that lack true tissues.
• They filter feeders, capturing food particles suspended in the water passing through their body.
• Water is drawn through pores into the spongocoel cavity, exiting via the osculum opening.
• Sponges lack true tissues and organs.
• Choanocytes, flagellated collar cells, generate water currents and ingest suspended food.
• The mesohyl is a gelatinous noncellular layer between two cell layers in sponges.
• Amoebocytes are within the mesohyl to aid in digestion and structure.
• Most sponges are hermaphrodites.

Cnidarians

• Cnidarians are an ancient phylum of eumetazoans.
• Eumetazoa includes all animals except sponges and a few other groups; these animals have true tissues.
• Phylum Cnidaria is one of the oldest groups in the Eumetazoa clade.
• Cnidarians are diverse and have sessile and motile forms like jellies, corals, and hydras.
• They exhibit a simple diploblastic, radial body plan.
• The basic cnidarian body plan is a sac with a gastrovascular cavity (central digestive compartment).
• A single opening functions as mouth and anus.
• There are two variations of the body plan are the polyp and the medusa.
• Polyps adhere to the substrate by the aboral end.
• Medusa has a bell-shaped body with its mouth on the underside.
• Medusae do not attach to the substrate but move freely.
• Cnidarians are carnivores that use tentacles to capture prey.
• Tentacles are armed with cnidocytes which are unique cells functioning in defense and prey capture.
• Nematocysts are specialized organelles within cnidocytes that eject stinging threads.

Medusozoans

• Scyphozoans (jellies)
• Cubozoans (box jellies)
• Hydrozoans
• Medusozoans include all cnidarians that produce a medusa.

Hydrozoans

• They alternate between polyp and medusa forms.
• Hydra, a freshwater cnidarian, only exists in polyp form and reproduces asexually by budding.

Scyphozoans

• The medusa is the predominant stage in the life cycle of most scyphozoans and cubozoans.
• Costal Scyphozoans have a brief polyp stage, whereas open ocean species lack a polyp stage.

Cubozoans

• Medusa is box-shaped.
• Cubozoans often have highly toxic cnidocytes.

Anthozoans

• Anthozoa clade includes corals and sea anemones.
• Anthozoans occur only as polyps.
• Corals often form symbioses with algae and secrete a hard exoskeleton (external skeleton).

Lophotrochozoans

• Lophotrochozoans, a clade identified by molecular data, contains the widest range of animal body forms.
• Bilaterian animals have bilateral symmetry and triploblastic development.
• Most have a coelom and an alimentary canal.
• Bilateria contains Lophotrochozoa, Ecdysozoa, and Deuterostomia.
• They develop a lophophore for feeding, others a trochophore larval stage, and a few neither feature.
• Lophotrochozoa includes the flatworms, rotifers, ectoprocts, brachiopods, molluscs, and annelids.

Flatworms

• Members of phylum Platyhelminthes inhabit marine, freshwater, and damp terrestrial habitats.
• Although flatworms undergo triploblastic development, flatworms are acoelomates.
• Flatworms have a gastrovascular cavity with one opening.
• Gas exchange occurs across the surface, and protonephridia regulate osmotic balance.
• Dorsoventrally flattened shapes maximize surface area for gas exchange.
• Flatworms are divided into Catenulida or “chain worms,” which reproduce asexually by budding.
• The other classification is Rhabditophora, which are more diverse and include both free-living and parasitic species.

Free-Living Flatworm Species

• The best-known rhabditophorans are planarians.
• Planarians inhabit fresh water and prey on smaller animals.
• Planarians have light-sensitive eyespots and centralized nerve nets.
• The planarian nervous system is more complex and centralized than cnidarian nerve nets.
• Planarians are hermaphrodites and can reproduce sexually, or asexually through fission.

Parasitic Flatworm Species

• Parasitic rhabditophorns live in or on other animals.
• Trematodes (flukes) and cestodes (tapeworms) are two groups of parasitic rhabditophorns.
• Trematodes (flukes) parasitize a wide range of hosts, and most have alternating sexual and asexual stages.
• Schistosoma, trematodes that parasitize humans, spend part of their lives in snail hosts.
• Tapeworms are parasites of vertebrates, and they lack a digestive system.
• Tapeworms absorb nutrients from the host's intestine.
• The scolex contains suckers and hooks for attaching to the host.
• Proglottids are units with sex organs that form a ribbon behind the scolex.
• Fertilized eggs, produced by sexual reproduction, leave the host's body in feces.

Rotifers

• Belonging to phylum Rotifera, are tiny animals inhabiting 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, a digestive tube, with a separate mouth and anus that lies within a fluid-filled pseudocoelom.
• Rotifers reproduce by parthenogenesis, where females produce from unfertilized eggs.
• Some species lack males entirely.

Ectoprocts

• Also called bryozoans, are sessile colonial animals that superficially resemble plants (moss).
• A hard exoskeleton encases the colony, and some species build reefs.
• They reproduce sexually and asexually.

Brachiopods

• They superficially resemble clams and other hinge-shelled molluscs, but the two halves of the shell are dorsal and ventral rather than lateral.
• Brachiopods are marine and attach to the seafloor by a stalk.
• They reproduce sexually, and some perform parthenogenesis.

Molluscs

• They include snails and slugs, oysters and clams, and octopuses and squids.
• While most molluscs are marine, some inhabit fresh water, and some snails and slugs are terrestrial.
• Molluscs are soft-bodied animals, but most are protected by a hard shell.
• All 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, but many snails are hermaphrodites.
• The life cycle of many molluscs includes a ciliated larval stage called a trochophore.
• Four of the major classes of molluscs are Polyplacophora , Gastropoda, Bivalvia, and Cephalopoda.

Chitons

• Chitons are oval-shaped marine animals encased in armor of eight dorsal plates.
• They use their foot like a suction cup to grip rock, and their radula scrapes algae off of the rock surface.

Gastropods

• Gastropods make up about three-quarters of all living species of molluscs.
• They move slowly by a rippling motion of the foot or by cilia.
• Most have a single, spiraled shell that functions in protection from injury, dehydration, and predation.
• Most gastropods are herbivores, but some species use modified radula to feed on prey.

Bivalves

• Bivalves have a shell divided into two halves drawn together by adductor muscles.
• Some bivalves have eyes and sensory tentacles along the edge of their mantle.
• The mantle cavity of a bivalve contains gills for feeding and gas exchange.
• Bivalves have an open circulatory system.
• Most species are sedentary, but some have limited motility.

Cephalopods

• Cephalopods are carnivores with beak-like jaws surrounded by tentacles, a modified foot.
• They can immobilize prey with poison present in their saliva.
• Most octopuses creep along the sea floor in search of prey.
• Squids use their siphon to fire a jet of water, which allows them to swim rapidly.
• One small group of shelled cephalopods, the nautiluses, survives today.
• Cephalopods have a closed circulatory system, well-developed sense organs, and a complex brain.
• Shelled cephalopods called ammonites were common but went extinct at the end of the Cretaceous, 65.5 million years ago.

Protecting Molluscs

• Molluscs are the animal group with the largest number of recent extinctions.
• The most threatened groups are freshwater bivalves, including pearl mussels, and terrestrial gastropods, including Pacific island land snails.
• These molluscs are threatened by habitat loss, pollution, and non-native species

Annelids

• Annelids are coelomates with bodies composed of a series of fused rings.
• The phylum Annelida was traditionally divided into three clades:
  • Polychaeta
  • Oligochaeta
  • Hirundinea

Clade Errantia

• Contains mobile, marine organisms.
• Many have a pair of paddle-like or ridge-like structures called parapodia (“beside feet") on each body segment.
• Each parapodium has numerous chaetae, bristles made of chitin.
• Parapodia are not unique to this clade.

Clade Sedentaria

• Tend to be less mobile than errantians.
• Some species burrow into the substrate, while others live in protective tubes.
• Tube-dwelling sedentarians often have elaborate gills or tentacles for filter feeding.
• This clade also contains leeches and earthworms.

Leeches

• Most species of leeches live in fresh water; some are marine or terrestrial.
• Leeches include predators of invertebrates, and parasites that suck blood.
• Leeches have an open circulatory system.
• Leeches secrete a chemical called hirudin to prevent blood from clotting.

Earthworms

• Earthworms eat through soil, extracting nutrients, and it moves through their alimentary canal.
• Earthworms have a closed circulatory system.
• Earthworms are hermaphrodites but cross-fertilize.
• Some reproduce asexually by fragmentation.

Ecdysozoans

• They are are 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.

Nematodes

• Nematodes, or roundworms, are found in most aquatic habitats, in soil, in moist tissues of plants, and in body fluids and tissues of animals.
• They have an alimentary canal but lack a circulatory system.
• Body wall muscles are all longitudinal, meaning their contraction produces a thrashing motion.
• Caenorhabditis elegans is a model organism in research.
• Some species of nematodes are parasites of plants and animals.
• Trichinella spiralis can be acquired by humans from undercooked pork.

Arthropods

• 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.
• The arthropod body plan consists of a segmented body, hard exoskeleton, and jointed appendages.
• This body plan dates to the Cambrian explosion, 535-525 million years ago.
• Early arthropods show little variation from segment to segment.
• Arthropod evolution is characterized by a decrease in the number of segments and an increase in appendage specialization.
• These changes may have been caused by changes in Hox gene sequence or regulation.
• The appendages of some living arthropods are modified for functions such as walking, feeding, sensory reception, reproduction, and defense.
• These modified appendages are jointed and come in pairs. It is called heteronomous metamerism.
• The body of an arthropod is completely covered by the cuticle, an exoskeleton made of protein layers and the polysaccharide chitin.
• When an arthropod grows, it molts its exoskeleton.
• Arthropods have eyes, olfactory receptors, and antennae that function in touch and smell.
• Arthropods have an open circulatory system in which hemolymph is circulated into the spaces surrounding the tissues and organs.
• They use malphighian tubules to process nitrogenous waste.
• A variety of organs specialized for gas exchange have evolved in arthropods.
• Morphological and molecular evidence suggests that living arthropods consist of three major lineages that diverged early in the phylum's evolution:
  • Chelicerates (sea spiders, horseshoe crabs, scorpions, ticks, mites, and spiders)
  • Myriapods (centipedes and millipedes)
  • Pancrustaceans (lobsters and other crustaceans, as well as insects and their relatives)

Clade Chelicerata

• Named for clawlike feeding appendages called chelicerae.
• They have an anterior cephalothorax and a posterior abdomen.
• The earliest chelicerates were the extinct eurypterids (water scorpions).
• Many marine chelicerates (including eurypterids) are extinct, but some species survive today, including horseshoe crabs.
• Most modern chelicerates are arachnids, including spiders, scorpions, ticks, and mites. -Arachnids have six paired appendages: the chelicerae, the pedipalps, and four pairs of walking legs.
• Gas exchange in spiders occurs in respiratory organs called book lungs.
• Many spiders produce silk, a liquid protein, from specialized abdominal glands.

Clade Myriapoda

• Includes millipedes and centipedes.
• All living myriapods are terrestrial.
• They have a pair of antennae and three pairs of appendages modified as mouthparts.
• Millipedes eat decaying leaves and plant matter.
• They have many legs, with two pairs per trunk segment.
• Centipedes are carnivores.
• Centipedes have are one pair of legs per trunk segment.
• Poison claws (forcipules) are the first modified pair of legs trunk segment paralyze prey and aid in defense.

Clade Pancrustacea

• Terrestrial insects are more closely related to crustaceans than myriapods.
• Some lineages of crustaceans are more closely related to insects than other crustaceans.
• Together, insects and crustaceans form the clade Pancrustacea.
• Crustaceans live in marine and freshwater environments.
• Many crustaceans have highly specialized appendages.
• Small crustaceans exchange gases through the cuticle; larger crustaceans have gills.
• Most crustaceans have separate males and females.
• Isopods include terrestrial, freshwater, and marine species, for example, pill bugs are a well-known group of terrestrial isopods.
• Decapods are all relatively large crustaceans and include lobsters, crabs, crayfish, and shrimp.
• They use green gland (antennal and maxillary glands) to process nitrogenous waste.
• Planktonic crustaceans include many species of copepods, which are among the most numerous of all animals.
• They are nearly as abundant as the shrimplike krill.
• Barnacles are a group of mostly sessile crustaceans.
• They have a cuticle that is hardened into a calcium carbonate shell.
• They attach to submerged surfaces.
• Hexapoda is an enormous clade, including insects and their relatives.
• Insects live in almost every terrestrial habitat and in fresh water.
• Tracheal tubes transport gases to and from cells.
• Malpighian tubules process nitrogenous wastes.
• Flight is 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.
• Insect wings are an extension of the cuticle.
• Many insects undergo metamorphosis during their development.
• In incomplete metamorphosis, the young, called nymphs, resemble adults but are smaller, and they go through a series of molts until they reach adult size.
• Insects with complete metamorphosis have larval stages known by names such as maggot, grub, or caterpillar.
• The larval stage looks entirely different from the adult stage.
• Most insects have separate males and females and reproduce sexually.
• Individuals find and recognize other members of their own species by bright colors, sounds, or odors.
• 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.

Echinoderms and Chordates

• They are deuterostomes.
• Echinoderms (phylum Echinodermata) include sea stars and sea urchins and other echinoderms.
• Chordates (phylum Chordata) include the vertebrates.
• Echinoderms and chordates constitute the clade Deuterostomia.
• Deuterostomes share developmental characteristics:
  • Radial cleavage
  • Mesoderm development
  • Formation of the anus from the blastopore
• Deuterostomes are defined primarily by DNA similarities, not developmental similarities.

Echinoderms

• Echinoderms include sea stars and most other slow-moving or sessile marine animals.
• A thin epidermis overs an endoskeleton of hard calcareous plates.
• They have a unique water vascular system, a network of hydraulic canals branching into tube feet that function in locomotion and feeding.
• Males and females are usually separate, and sexual reproduction is external.
• Most adult echinoderms have radial symmetry with multiples of five.
• Echinoderm larvae have bilateral symmetry.
• Living echinoderms are divided into five clades: Asteroidea, Ophiuroidea, Echinoidea, Crinoidea, Holothuroidea.

Asteroidea

• Sea stars and sea daisies have multiple arms radiating from a central disk.
• Tube feet on the undersurface of each arm grip the substrate with adhesive chemicals.
• They feed on bivalves by prying them open with their tube feet, everting their stomach, and digesting their prey externally with digestive enzymes.
• Sea daisies are a group of armless species in the clade Asteroidea, only three species are known.
• They live on submerged wood and absorb nutrients through a membrane that surrounds their body.

Ophiuroidea

• Brittle stars have a distinct central disk and long, flexible arms used to move.
• Some species are suspension feeders, while others are predators or scavengers.

Echinoidea

• Sea urchins and sand dollars have no arms but have five rows of tube feet.
• Their spines are used for locomotion and protection.
• They feed on seaweed using a jaw-like structure on their underside.

Crinoidea

• Includes sea lilies and feather stars.
• Sea lilies live fixed to the substrate by a stalk.
• Feather stars can crawl with long, flexible arms and have no stalk.
• Both are suspension feeders.
• Crinoidea have changed little throughout evolution.

Holothuroidea

• Sea cucumbers that lack spines, have an extremely reduced endoskeleton and do not look much like other echinoderms.
• They have five rows of tube feet; some of these are developed as feeding tentacles.

Chordates

• Phylum Chordata consists of two basal groups of invertebrates as well as vertebrates.
• Chordates are bilaterally symmetrical coelomates with segmented bodies.
• Chordates did not evolve from echinoderms but have evolved separately from them for at least 500 million years.