Chapter 29 Echinoderms and Invertebrate Chordates Section 1

Chapter 29: Echinoderms and Invertebrate Chordates Section 1: Echinoderms

Echinoderms • Includes starfish, sea urchins, sand dollars, etc. • Belong to the phylum Echinodermata – Stretches back to the beginning of the Cambrian Period, more than 580 million years ago

What Is an Echinoderm? • Echinoderms are spiny skinned animals • In addition to having a spiny skin, echinoderms are characterized by five-part radial symmetry, an internal skeleton, a water vascular system, and suction-cuplike structures called tube feet • The internal skeleton, or endoskeleton, is made up of hardened plates of calcium carbonate, which are often bumpy or spiny • The water vascular system consists of an internal network of fluid-filled canals connected to external appendages called tube feet – Water vascular system is involved in many essential life functions in echinoderms

Form and Function in Echinoderms • Adult echinoderms have a body plan with five parts organized symmetrically around a center • Typically have neither an anterior nor a posterior end and no brain • The side where the mouth is located is called the oral surface, and the opposite side is called the aboral surface

Form and Function in Echinoderms • The water vascular system opens to the outside through a sieve like structure called the madreporite • The entire water vascular system operates like a series of living hydraulic pumps that can propel water in or out of the tube feet • The tube feet act like living suction cups • All echinoderms “walk” with their tube feet, and some use their tube feet for feeding

Feeding • Carnivores – Use their tube feet to pry open the shells of bivalve mollusks such as clams and scallops – Once opened, the carnivores flips its stomach out of its mouth, pours out enzymes, and digests its prey in the prey’s own shell • Herbivores – Scrape algae from rocks by using their five-part jaw • Filter feeders – Use tube feet on flexible arms to capture plankton that float by on ocean currents • Detritus feeders – Move like a bulldozer across the ocean floor, taking in a mixture of sand detritus

Respiration • Echinoderms need to exchange carbon dioxide for oxygen • In most species the thin walled tissue of the tube feet forms the main respiratory surface • In some species small outgrowths called skin gills also function in gas exchange

Internal Transport • The functions of transporting oxygen, food, and wastes are shared by different systems in echinoderms • The distribution of nutrients is performed primarily by the digestive glands and the fluid within the body cavity

Excretion • Solid wastes are released through the anus in the form of feces • Excrete nitrogen-containing wastes primarily in the form of ammonia • Wastes seem to be excreted in many of the same places around the body in which gas exchange takes place – the tube feet and the skin gills

Response • Have primitive nervous systems • Have a nerve ring that surrounds the mouth and radial nerves that connect the ring with the body sections • Also have light sensitive cells to help them tell whether it is night or day • Many echinoderms hide under rocks and in crevices by day, coming out to feed at night, when most predators are asleep

Movement • Most echinoderms use tube feet and thin layers of muscle fibers attached to the plates of the endoskeleton to move • Mobility is determined by the structure of the endoskeleton – Some plates are fused – Some have flexible joints

Reproduction • Most are either male or female • Some are hermaphrodites • In starfish, the sperm and egg are produced in testes or ovaries which fill the arms during the reproductive season • Shed their sperm and eggs into the water • Larvae swim around for some time • Eventually, they will swim to the ocean bottom, where they mature and metamorphose into adults that have radial symmetry

The Echinoderm Classes • Almost 6000 species of living echinoderms • Found in almost every ocean in the world • No echinoderms have ever entered fresh water, and they cannot survive for long on land • Echinoderms are remarkably diverse in appearance

Starfish • Contains the common starfish, which are also known as sea stars • Occur in many colors • Many species have more than 5 arms • Starfish creep slowly along the ocean bottom • Most are carnivores, preying upon the bivalves they encounter as they move • Some species are important predators in rocky areas along the coast

Brittle Stars • Live in tropical seas, especially on coral reefs • Look much like a common starfish, but they have longer, more flexible arms and are able to move much more rapidly • Brittle stars protect themselves by shedding one or more of their arms when attacked • The detached parts keep wriggling violently, distracting predators, while the rest of the animal escapes • Are filter and detritus feeders that hide by day and wander around in search of food at night

The brittle star gets its name from the fact that it can shed its arms when it is threatened. This distracts predators so that the brittle star can escape. In time, it will regrow the missing arm. Some starfish, such as the sun star, have more than five arms.

Sea Urchins and Sand Dollars • Includes disk-shaped sand dollars, oval heart urchins, and round sea urchins • Most are grazers that eat large quantities of algae • Others are detritus feeders • Heart urchins and sand dollars live hidden in burrows • Most sea urchins wedge themselves in crevices in rock during the day and only come out at night • Many sea urchins have long, sharp spines

The slate urchin has thick, strong spines that were once harvested for use as implements for writing on slateboards. Sea urchins have a lanternlike set of bony plates inside their body that power their jaws. The sand dollar gets its name from its flattened, coin-shaped appearance.

Sea Cucumbers • Look like warty moving pickles with a mouth at one end an anus at the other • Most are detritus feeders • Not numerous in shallow water • Herds containing hundreds of thousands of them often cover areas of the sea floor at great depths • A few species expel sticky substances that attach to a predator • The predator is immobilized as it is glued into a helpless ball

Sea Lilies and Feather Stars • These filter feeders, which have 50 or more long, feathery arms, comprise the most ancient class of echinoderms • Not common today • Sea lilies are sessile animals that are attached to the ocean bottom by a long, stem like stalk • Many feather stars live on coral reefs, where they perch on top of rocks at night and use their tube feet to catch floating plankton

How Echinoderms Fit into the World • • • Numerous in most marine habitats Control the populations of other animals Control the distribution of algae Considered delicacies by some people Useful as research subjects and as possible sources of medicine

Chapter 29: Echinoderms and Invertebrate Chordates Section 2: Invertebrate Chordates

Invertebrate Chordates • Phylum Chordata – Fish, frogs, birds, snakes, dogs, cows, and humans • Most of the chordates are vertebrates, which means they have backbones, so they are placed in the subphylum Vertebrata • The invertebrate chordates are divided into two subphyla – tunicates and lancelets

What Is a Chordate? • Belong to phylum Chordata • Chordates are animals that are characterized by a notochord, a hollow dorsal nerve cord, and pharyngeal (throat) slits • All chordates display these three characteristics at come stage in their life

st 1 Characteristic • The notochord, is a long, flexible supporting rod that runs through at least part of the body, usually along the dorsal surface just beneath the nerve cord • In most vertebrates, the notochord is quickly replaced by the backbone

nd 2 Characteristic • The hollow dorsal nerve cord, runs along the dorsal surface just above the notochord • In most chordates, the front end of this nerve cord develops into a large brain • Nerves leave this cord at regular intervals along the length of the animal and connect to internal organs, muscles, and sense organs

rd 3 Characteristic • Pharyngeal slits, are paired structures in the pharyngeal, or throat, region of the body • In aquatic chordates the pharyngeal slits are gill slits that connect the pharyngeal cavity with the outside • Many invertebrates have gills of some sort in various places, but only chordates have pharyngeal gills

rd 3 Characteristic • In terrestrial chordates that use lungs for respiration, pharyngeal slits are present only for a brief time during development • These slits soon close up as the embryo develops • In chordates such as humans, pouches form in the pharyngeal region but never open up to form slits • For this reason, some scientists regard pharyngeal pouches, not slits, as the “true” chordate characteristic

Tunicates • Small marine chordates that eat plankton they filter from the water • Get their name from a special body covering called the tunic • Only the tadpole-shaped larvae of tunicates have a notochord and a dorsal nerve cord • When tunicate larvae mature, they undergo metamorphosis and become sessile adults that grow into colonies attached to a solid surface • Both larval tunicates and adults filter feed and breathe at the same time through a pharyngeal basket pierced by gill slits

Lancelets • Small fishlike creatures that live in the sandy bottom of shallow tropical oceans • Have a definite head • Have a mouth that opens into a long pharyngeal region with up to 100 pairs of gill slits • Feed by passing water through their pharynx, where food particles are caught in a sticky mucus • This mucus is swallowed into a digestive tract that starts at one end of the pharynx and continues straight through the animal to the anus, near the tail

Lancelets • Have a simple, primitive heart that pumps blood through vessels in a closed circulatory system • They show evidence of segmentation in the arrangement of their nerves and muscles • Muscles are organized into V-shaped units that are paired on either side of the body • Each muscle unit receives a branch from the main nerve cord • Lancelets have no jaw • Their mouth is composed entirely of soft tissues • Also lack appendages and can move only by bending their bodies back and forth

How Invertebrate Chordates Fit into the World • It is important to remember that living vertebrates did not evolve from living lancelets or tunicates • Both these subphyla have evolved over time • However, similarities in structure and embryological development indicate that vertebrates and invertebrate chordates evolved from common ancestors many millions years ago

Homework • Page 646 #1 – 3