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Zoology Chapter 30

Page history last edited by Shelly Turner 10 years, 9 months ago


Chapter 30:  Mollusks and Annelids


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.


A True Coelom

         Snails, slugs, oysters, clams, scallops, octopuses, and squids are all mollusks. 

         Mollusks and annelids were probably the first major groups of organisms to develop a true coelom. 

        Another feature shared by mollusks and annelids is a larval stage called a trochophore which develops from the fertilized egg.



         In some species, the trochophore is free-swimming and propels itself through the water by movement of cilia on its surface.



  • The presence of a trochophore larva in mollusks and annelids suggests that they share a common ancestor.





Key Characteristics of Mollusks

  1. The body cavity in mollusks is a true coelom, although in most species it is reduced to a small area immediately surrounding the heart.  
  1. Most mollusks exhibit bilateral symmetry.  
  1. Mollusks have organ systems for excretion, circulation, respiration, digestion, and reproduction.  
  1. The body of every mollusk has three distinct parts: the visceral mass, the mantle, and the muscular foot.





         The visceral mass is a central section that contains the mollusk’s organs. 

         The mantle is a heavy fold of tissue that forms the outer layer of the body. 

         Finally, every mollusk has a muscular region called a foot, which is used primarily for locomotion.


5.  Many mollusks have either one or two shells that serve as an exoskeleton, protecting their soft body. 

6.  All mollusks except bivalves have a radula, a tongue-like organ located in their mouth. The radula has thousands of pointed, backward-curving teeth arranged in rows.






Mollusk Body Plan


Organ Systems: Excretion 


         A mollusk’s coelom is a collecting place for waste-laden body fluids. 

         The beating of cilia pulls the fluid from the coelom into tiny tubular structures called nephridia. 

         The nephridia recover useful molecules (sugars, salts, and water) from the coelomic fluid.


Organ Systems: Circulation 

         In a circulatory system, blood carries nutrients and oxygen to tissues and removes waste and carbon dioxide. 

         Most mollusks have a three-chambered heart and an open circulatory system. 

         Octopuses and squids are exceptions because they each have a closed circulatory system.


Organ Systems: Respiration 

         Most mollusks respire with gills, which are located in the mantle cavity. 

         Most terrestrial snails have no gills. Instead, the thin membrane that lines their empty mantle cavity functions like a primitive lung. 

         Sea snails also lack gills, and gas exchange takes place directly through their skin.


Organ Systems: Reproduction 

         Most species of mollusks have distinct male and female individuals, although some snails and slugs are hermaphrodites. 

         Certain species of oysters and sea slugs are able to change from one sex to the other and back again. 

         Many marine mollusks are moved from place to place as their trochophore larvae drift in the ocean currents.




Body Plans of Mollusks


         Gastropods—snails and slugs—are primarily a marine group that has successfully invaded freshwater and terrestrial habitats. 

         Most gastropods have a pair of tentacles on their head with eyes often located at the tips. 

         Gastropods display varied feeding habits. Many are herbivores that scrape algae off rocks using their radula.  Some are predators like the sea slug.






         All bivalves have a two-part hinged shell. The valves, or shells, of a bivalve are secreted by the mantle. 

         Two thick muscles, the adductor muscles, connect the valves. When these muscles are contracted, they cause the valves to close tightly. 

         Bivalves are unique among the mollusks because they do not have a distinct head region or a radula.


         Many bivalves use their muscular foot to dig down into the sand.



         Once there, the cilia on their gills draw in sea water through hollow tubes called siphons.









         Squids, octopuses, cuttlefish, and nautiluses are all cephalopods. Most of their body is made up of a large head attached to tentacles. 

         Cephalopods are the most intelligent of all invertebrates. They have a complex nervous system that includes a well-developed brain. 

         The structure of a cephalopod eye is similar in many ways to that of a vertebrate eye, and some species have color vision.






The First Segmented Animals

         Annelids are easily recognized by their segments, which are visible as a series of ringlike structures along the length of their body. 

         Some of the segments are modified for specific functions, such as reproduction, feeding, or sensation. A well-developed cerebral ganglion, or primitive brain, is located in one anterior segment. 

  • Internal body walls, called septa, separate the segments of most annelids.




Characteristics of Annelids 

  1. The fluid-filled coelom is large and is located entirely within the mesoderm.  
  1. The organ systems of annelids show a high degree of specialization and include a closed circulatory system and excretory structures called nephridia.  
  1. Most annelids have external bristles called setae. Some annelids also have fleshy appendages called parapodia.





Annelid Groups

Marine Worms 


         Marine segmented worms are members of class Polychaeta, the largest group of annelids. Polychaetes live in virtually all ocean habitats. 

         A distinctive characteristic of polychaetes is the pair of fleshy, paddle-like parapodia that occur on most of their segments. 

         The parapodia, which usually have setae, are used to swim, burrow, or crawl.

         Nereis, a polychaete worm, grasps its prey in its jaws, which open when it thrusts out its pharynx.




         Earthworms and some related freshwater worms are members of the class Oligochaeta. Oligochaetes have no parapodia and only a few setae on each segment. 

         Earthworms lack the distinctive head region of polychaetes and have no eyes. 

         Earthworms are highly specialized scavengers. They literally eat their way through the soil, consuming their own weight in soil every day. 


Anatomy of the Earthworm





Hydrostatic Skeleton 


         The fluid within the coelom of each body segment creates a hydrostatic skeleton that supports the segment. 

         Each segment contains muscles that pull against this hydrostatic skeleton. 

         Circular muscles wrap around the segment, while longitudinal muscles span its length.






         A leech has suckers at both ends of its body. Most species are predators or scavengers, but some are parasites of vertebrates and crustaceans. 

         Leeches are the only members of class Hirudinea. Leeches lack both setae and parapodia. 

         The body of a leech is flattened, and unlike other annelids, its segments are not separated internally. 

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