chapter 29 echinoderms and invertebrate chordates

Chapter 29 Echinoderms and Invertebrate Chordates

Section 29-1Echinoderms

OBJECTIVES:–Relate the structure of the echinoderms to essential life functions.

–Describe the characteristics of the classes of echinoderms.


Phylum Echinodermata- starfish, sea urchins, sand dollars, etc.–echino- means spiny; dermis means skin

–these are spiny-skinned animals

–Cambrian period; 580 million yr.


In addition to having spiny skin, they are characterized by:–5-part radial symmetry–internal skeleton–water vascular system–suction-cuplike structures called tube feet


The internal skeleton (or endoskeleton) is made up of hardened plates of calcium carbonate; often bumpy or spiny.

The water vascular system consists of an internal network of fluid-filled canals connected to the external appendages called tube feet.


The water vascular system is essential for:–feeding; respiration; internal transport; elimination of wastes; and movement.

Echinoderms have an internal skeleton like Chordates, and some similar development.


Thus, some biologists feel that among invertebrates, echinoderms are most closely related to humans!

Echinoderms are very well adapted to life in the sea; some have changed very little in millions of years of evolution.


Adult echinoderms have a body plan with five parts organized symmetrically around a center.–neither anterior nor posterior end; no brain

–but, they are two-sided–mouth side is the oral surface


Side opposite the mouth is the aboral surface.

They have a unique system of internal tubes called a water vascular system.–Opens to the outside through a sieve-like structure called the madreporite.


In starfish, the madreporite connects to a tube called the ring canal that forms a circle around the animal’s digestive system.–Figure 29-3, page 639–From the ring canal, five radial canals extend into each body segment.


Attached to each radial canal are hundreds of movable tube feet.–This entire system acts like a series of living hydraulic pumps that can propel water in or out of the tube feet.

–Can create a partial vacuum to hold on to whatever it is touching


Feeding in carnivores, such as starfish, use their tube feet to pry open the shells of bivalve mollusks.

• Then the starfish flips the stomach out of its mouth, pours out enzymes, and digests its prey in its own shell; then pulls the stomach back, leaving an empty shell.


Herbivores, such as sea urchins, scrape algae from rocks by using their 5-part jaw.

Filter feeders, such as sea lilies, basket stars, and some brittle stars, use tube feet on flexible arms to capture plankton that float by on ocean currents.


Detritus feeders, such as sea cucumbers, move much like a bulldozer, taking in a mixture of sand and detritus.–Similar to earthworms, they digest the organic material and pass the sand grains out in their feces.


Respiration 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 (transporting oxygen, food, and wastes which is normally done by a circulatory system) is shared by different systems in echinoderms.–They don’t really need a system for gases, because of gills and skin.


The distribution of nutrients is performed primarily by the digestive glands and the fluid within the body cavity

Excretion in almost all echinoderms, solid wastes are released through the anus (on the aboral surface) as feces.


The nitrogen-containing cellular wastes are excreted primarily as 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 since they have no head, they have primitive nervous systems.–They do have scattered sensory cells to detect food.

–Starfish also have up to 200 light-sensitive cells clustered in eyespots at the tip of each arm.


However, they can do little more than tell whether it is light or dark.–Also may have statocysts for balance, to tell them whether they’re right side up.

The spiny surface is not really very good protection; good only in a few such as the crown-of-thorns starfish.


Many predators have learned that if they turn these animals over, they can attack them through their unprotected underside.–Thus, many echinoderms hide during the day and are active at night when most predators are asleep.


Movement = use tube feet and thin layers of muscle fibers attached to the plates of the endoskeleton to move.–In sand dollars and sea urchins, the plates are fused together to form a rigid box that encloses the animal’s internal organs.


–In sea cucumbers, the plates are reduced to tiny vestiges inside a soft, muscular body wall. The loss of the plates makes the body of sea cucumbers very flexible.


Reproduction = most echinoderms are either male or female, although some are hermaphrodites.–Egg and sperm are released and fertilization takes place in the water.

–The larvae have bilateral symmetry = very advanced.


When the larvae mature and metamorphose into adults, they develop radial symmetry.

Many starfish have incredible powers of regeneration.–Each piece can grow into a new animal as long as it contains a portion of the central part.


Almost 6,000 species found in almost every ocean (salt water) in the world.–No echinoderms have ever entered fresh water, and they cannot survive for long on land.


Echinoderm Classes –Starfish–Brittle Stars–Sea Urchins and Sand Dollars–Sea Cucumbers–Sea Lilies and Feather Stars


1. Starfish = this class contains the common starfish, which are also known as sea stars.–Some have more than 5 arms–Figure 29-7, page 642–Carnivorous, preying upon the bivalves they encounter


2. Brittle Stars = live in tropical seas, especially on coral reefs–Look like common starfish, but have longer, more flexible arms, thus able to move much more rapidly

–Protection by shedding one or more arms when attacked; are filter and detritus feeders


3. Sea Urchins and Sand Dollars = includes disk-shaped sand dollars, oval heart urchins, and round sea urchins. Fig. 29-8, p. 643–Are grazers that eat large quantities of algae; may burrow into the sand or mud; may protect themselves by long sharp spines.


4. Sea Cucumbers = look like warty moving pickles, with a mouth at one end and an anus at the other.–Figure 29-9, page 644 top–Most are detritus feeders–Some produce a sticky material to “glue” a predator helpless


5. Sea Lilies and Feather Stars = filter feeders, have 50 or more long feathery arms.–The most ancient class of echinoderms; not common today, but once were widely distributed.

–Sea lilies; sessile animals, p.644


How Echinoderms Fit Into the World:–Starfish are important carnivores, controlling other animal populations; a rise or fall in numbers affects other populations.


For example, several years ago the coral-eating crown-of-thorns starfish suddenly appeared in great numbers in the Pacific Ocean.–Within a short period of time, they caused extensive damage to many coral reefs.


In many coastal areas, sea urchins are important because they control distribution of algae.

In various parts of the world, sea urchin eggs and sea cucumbers are considered delicacies by some people.


Several chemicals from starfish and sea cucumbers are currently being studied as potential anti-cancer and anti-viral drugs.

Sea urchins have been helpful in embryology studies, since they produce large eggs; fertilize externally; develop in sea water.

Section 29-2Invertebrate Chordates

OBJECTIVES:–Name and discuss the three distinguishing characteristics of chordates.

–Describe the two subphyla of invertebrate chordates.


The phylum Chordata, to which fishes, frogs, birds, snakes, dogs, cows, and humans belong, will be discussed in future chapters.–Most chordates are vertebrates, which means they have backbones, and are placed in the subphylum Vertebrata.


But, there are also invertebrate chordates; these are divided into two subphyla:–1. The tunicates–2. The lancelets

Due to similar structures, the chordate vertebrates and invertebrates may have evolved from a common ancestor.


Chordates are animals that are characterized by a notochord, a hollow dorsal nerve cord, and pharyngeal (throat) slits.–Some chordates posses these characteristics as adults; others as only embryos; but all have them at some stage of development.


Notochord = a long, flexible supporting rod that runs through at least part of the body, usually along the dorsal surface just beneath the nerve cord.–Most chordates only have this during the early part of embryonic life.


Vertebrates will replace the notochord quickly with the backbone.

The second chordate 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 its internal organs, muscles, and sense organs.


The third chordate characteristic, the pharyngeal slits, are paired structures in the pharyngeal (or throat) region of the body.–In aquatic chordates, such as lancelets and fishes, the pharyngeal slits are gill slits that connect the pharyngeal cavity with the outside.


In terrestrial chordates that use lungs for respiration, pharyngeal slits are present for only a brief time during the development of the embryo.–They soon close up as the embryo develops. Page 283.


In humans, pouches form in the pharyngeal region, but never open up to form slits.–Thus, some scientists consider the pharyngeal pouches, not slits, as the “true” chordate characteristic.


Tunicates = small marine chordates that eat plankton they filter from the water.–Name from a special body covering called the tunic.

–Only the tadpole-shaped larvae have the notochord and dorsal nerve cord.


Examples of tunicates are the sea squirts. Figure 29-11, page 646.–Adults are sessile, living as colonies attached to a solid surface; larvae are free swimming.


Lancelets = small fishlike creatures; live in sandy bottoms of shallow tropical oceans.–Unlike tunicates, the adult lancelets have a definite head; a mouth that opens into a long pharyngeal region with up to 100 pairs of gills.


Figure 29-12, page 646–They feed by passing water through their pharynx, where food particles are caught in a sticky mucus; lack any jaws.

–They have a primitive heart pumping blood through closed circulation.


Lancelet’s muscles are organized into V-shaped units on either side of the body.–Each muscle unit receives a branch from the main nerve cord; lack any appendages.

–Similar system found in all living vertebrates.


How Invertebrate Chordates Fit Into the World:–By studying the invertebrate chordates, it is like using a time machine to study the ancestors of our own subphylum.

chapter 29 echinoderms and invertebrate chordates

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tube feetsection

hundreds of movable

echinodermsadult echinoderms

classes of echinoderms

sea urchins

sea lilies

echinodermsin starfish

sea cucumbers