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Chapter 32:  Echinoderms and Invertebrate Chordates  

 

Using your vocabulary words, make notecards/flashcards that you can use outside of class to study. 

  

 

Work on your Directed Reading in class every chance you get.  Remember that I will assist you on the harder questions.  This assignment is due the day we take the test on this chapter.  Use the information in this assignment as a study guide for your test. 

 

This study guide is to help you study for your test.  It should not be the only item you use to study w hen preparing for the test.  Remember to complete your Directed Reading for every chapter along with writing your vocabulary words on notecards to help you remember them.  And always remember to listen carefully in lecture because all of the information in this study guide will be covered.

 

Section 32-1:  Echinoderms

 

Animal Development

 

If you have been to a saltwater aquarium, you’re sure to have seen echinoderms, which are spiny invertebrates that live on the ocean bottom. How could echinoderms like the brittle star shown on the first page of this chapter be related to animals such as chordates, which are primarily vertebrates? The answer lies in their early development.  As an embryo develops, it goes through a gastrula stage. A gastrula has an opening to the outside called the blastopore. In acoelomate animals, the mouth develops from or near the blastopore. This pattern of development also occurs in some coelomate animals, such as annelids, mollusks, and arthropods.  Animals with mouths that develop from or near the blastopore are called protostomes.

 

Some animals follow a different pattern of development. In phylums Echinodermata and Chordata, the anus—not the mouth—develops from or near the blastopore. (The mouth forms later, on another part of the embryo.) Animals with this pattern of development are called deuterostomes, also shown in Figure 1. If you know the origin of these two terms, it’s easy to remember the differences between the two developmental patterns. The term protostome is from the Greek protos, meaning “first,” and stoma, meaning “mouth.” The prefix deutero- is from the Greek deuteros, meaning “second.” In deuterostomes, the anus develops first and the mouth develops second.

 

The first deuterostomes were marine echinoderms that evolved more than 650 million years ago. They were also the first animals to develop an endoskeleton. Today, most people are familiar with echinoderms known as “starfish,” which are not really fish and are more properly called sea stars. In addition to sea stars, many other animals commonly seen along the sea shore—sea urchins, sand dollars, and sea cucumbers—are echinoderms. All are marine, and all are radially symmetrical as adults.

 

Chordates, as well as a few other small phyla, are also deuterostomes.  (Humans and all other vertebrates are chordates.) Like the echinoderms, chordates have an internal skeleton. This developmental similarity unites these seemingly dissimilar animal phyla. It also leads scientists to believe that chordates and echinoderms derived from a common ancestor.  The identity of the ancestral deuterostome is not known. The fossil record indicates that echinoderms, such as the sea lily, were abundant in the ancient seas. 

 

•         As an embryo develops, it goes through a gastrula stage. A gastrula has an opening to the outside called the blastopore.

•         Animals with mouths that develop from or near the blastopore are called protostomes. 

•         In phylums Echinodermata and Chordata, the anus—not the mouth—develops from or near the blastopore. (The mouth forms later, on another part of the embryo.) Animals with this pattern of development are called deuterostomes.

 

 

Patterns of Embryonic Development

 

 

•         The first deuterostomes were marine echinoderms that evolved more than 650 million years ago. They were also the first animals to develop an endoskeleton. 

•         Like the echinoderms, chordates have an internal skeleton. This developmental similarity unites these seemingly dissimilar animal phyla. 

•         The identity of the ancestral deuterostome is not known.

 

 

This phylogenetic tree shows the relationship of the major chordate and echinoderm groups.

 

Characteristics of Echinoderms

 

 

Modern Echinoderms

•         Many of the most familiar animals seen along the seashore—sea stars, sea urchins, sand dollars—are echinoderms. 

•         Echinoderms have a calcium-rich endoskeleton composed of individual plates called ossicles. 

•         All echinoderms are bilaterally symmetrical as larvae. During their development into adults, the larvae’s body plan becomes radially symmetrical.

 

Pentaradial Symmetry

 

 

•         Echinoderms have a waterfilled system of interconnected canals and thousands of tiny hollow tube feet called a water-vascular system. 

•         The echinoderm body cavity functions as a simple circulatory and respiratory system. 

•         Skin gills are small, fingerlike projections that grow among the echinoderm’s spines. These projections create an increased surface area through which respiratory gases can be exchanged.

 

Water Vascular System

 

 

Anatomy of a Sea Star

 

 

 

Types of Echinoderms

 

 

Echinoderm Diversity

Sea Stars 

•         Almost all species of sea stars are carnivores, and they are among the most important predators in many marine ecosystems. 

•         The ossicles of many species of sea stars produce pincerlike structures called pedicellaria. 

•         Pedicellaria contain their own muscles and nerves, and they snap at anything that touches them. This action prevents small organisms from attaching themselves to the surface of the sea star.

 

Sea Stars

 

Brittle Stars 

•         The sea star’s relatives, the brittle stars and sea baskets, make up the largest class of echinoderms. 

•         Brittle stars have slender branched arms that they move in pairs to row along the ocean floor. 

•         Although a few species are predators, most brittle stars are filter feeders or feed on food in the ocean sediment.

 

         

Brittle Stars

 

 

Sea Lilies and Feather Stars 

•         The sea lilies and feather stars are the most ancient and primitive living echinoderms. 

•         They differ from all other living echinoderms because their mouth is located on their upper, rather than lower, surface. 

•         Sea lilies are sessile and are attached to the ocean floor by a stalk. Feather stars use hooklike projections to attach themselves directly to the ocean bottom or a coral reef.

 

     

Sea Lily    

 

 

The humble sea lily (Endoxocrinus parrae) is an ocean animal closely related to the starfish, sea cucumber and sea urchin. With a ring of feathery fingers and a stalk 50 centimeters long, it resembles an ocean garden flower. And it has a sophisticated method for avoiding danger as caught in the video after the jump which shows a sea lily crawling slowly across the ocean floor on its fingers, dragging its broken stem behind it.  Other footage and photographs captured suggest that the sea lily makes its ocean floor dash to escape the attentions of sea urchins, which have been seen lurking on the sea bed behind the traveling sea lilies - "It's the lizard's tail strategy," Baumiller says. "The sea lily just leaves the stalk end behind. The sea urchin is preoccupied going after that, and the sea lily crawls away."

 

Feather Star

 

Sea Urchins and Sand Dollars 

•         The sea urchins and sand dollars lack distinct arms but have the basic five-part body plan seen in other echinoderms. 

•         Both sea urchins and sand dollars have a hard, somewhat flattened endoskeleton of fused plates covered with spines protruding from it. 

•         Sea urchins are found on the ocean bottoms while sand dollars live in sandy areas along the sea coast.

 

               

 

Sea Cucumbers 

• Sea cucumbers are soft-bodied, sluglike animals without arms. They differ from other echinoderms in that their ossicles are small and are not fused together. 
• The sexes of most sea cucumbers are separate, but some species are hermaphrodites. 
• Sea cucumbers feed by trapping tiny organisms present in the sea water. Their mouth is surrounded by several dozen tube feet modified into tentacles.

 

Sea Cucumber

 

Sea Daisies 

• In 1986, a new class of echinoderm was discovered: strange diskshaped little animals called sea daisies. 
• Sea daisies have five-part radial symmetry but no arms. 
• Their tube feet are located around the edges of the disk rather than along the radial lines, like they are in other echinoderms.

 

 

 

Section 32-2:  Invertebrate Chordates

 

The Chordate Skeleton

• The second major group of deuterostomes are the chordates. Chordates have a very different kind of endoskeleton from that of echinoderms. 
• During the development of the chordate embryo, a stiff rod called the notochord develops along the back of the embryo. 
• Using muscles attached to this rod, early chordates could swing their backs from side to side, enabling them to swim through the water.

 

Other Chordate Characteristics 

• Chordates have a single, hollow, dorsal nerve cord with nerves attached to it that travel to different parts of the body. 
• Chordates also have a series of pharyngeal pouches that develop in the wall of the pharynx, the muscular tube that connects the mouth to the digestive tract and windpipe. 
• Another chordate characteristic is a postanal tail, which is a tail that extends beyond the anus.

 

Parts of a Chordate

 

 

 

Lancelet Interior

 

 

  

Invertebrate Chordates

• The vast majority of chordate species belong to subphylum Vertebrata. 
• Two other subphyla, Urochordata (the tunicates) and Cephalochordata (the lancelets), contain a small number of species. 
• Because members of these two subphyla are chordates that do not have backbones, they are called invertebrate chordates.

  

 

Tunicates 

• Most adult tunicates are sessile, filter-feeding marine animals. A tough sac, called a tunic, develops around the adult’s body and gives tunicates their name. 
• Cilia beating within the tunicate cause water to enter the incurrent siphon. As water passes through the slits in the pharynx, food is filtered from it and passed into the stomach. 
• All tunicates are hermaphrodites, and some are also able to reproduce asexually by budding.

  

Adult Tunicate

  

 

Lancelets 

• Lancelets receive their name from their bladelike shape. Although lancelets may resemble fish, they are not fish. 
• The lancelets V-shaped bundles of muscles are arranged in a series of repeating segments. 
• Lancelets feed on microscopic protists that they filter out of the water. Unlike tunicates, the sexes are separate in lancelets.