1. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu A True (body cavity in mesoderm) Snails, slugs, oysters, clams, scallops, octopuses, and squids are all Mollusks and annelids were probably the first major groups of organisms to develop a true Another feature shared by mollusks and annelids is a larval stage called a which develops from the fertilized egg. Section 1 Mollusks

2. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu A True Coelom, continued In some species, the trochophore is free- swimming and propels itself through the water by movement of on its surface. The presence of a trochophore larva in mollusks and annelids suggests that they share a common Section 1 Mollusks

3. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 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 2.Most mollusks exhibit 3.Mollusks have organ for excretion, circulation, respiration, digestion, and reproduction. Section 1 Mollusks

4. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Key Characteristics of Mollusks, continued 4. The body of every mollusk has three distinct parts: the mass, the mantle, and the muscular Section 1 Mollusks

5. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Key Characteristics of Mollusks, continued The visceral mass is a central section that contains the mollusk’s The is a heavy fold of tissue that forms the outer layer of the body. Finally, every mollusk has a muscular region called a, which is used primarily for locomotion. Section 1 Mollusks

6. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Key Characteristics of Mollusks, continued 5. Many mollusks have either one or two that serve as an exoskeleton, protecting their soft body. 6. All mollusks except bivalves have a, a tongue-like organ located in their mouth. The radula has thousands of pointed, backward-curving teeth arranged in Section 1 Mollusks

7. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Body Plan Section 1 Mollusks

8. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Key Characteristics of Mollusks, continued Organ Systems: Excretion A mollusk’s is a collecting place for waste-laden body fluids. The beating of cilia pulls the fluid from the coelom into tiny tubular structures called The nephridia recover useful molecules (sugars, salts, and water) from the Section 1 Mollusks

9. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Key Characteristics of Mollusks, continued Organ Systems: Circulation In a system, blood carries nutrients and oxygen to tissues and removes waste and carbon dioxide. Most mollusks have a three-chambered heart and an circulatory system. Octopuses and squids are exceptions because they each have a closed circulatory system. Octopuses move by (squeezing water out.) Section 1 Mollusks

10. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Key Characteristics of Mollusks, continued Organ Systems: Respiration Most mollusks respire with, which are located in the mantle cavity. Most terrestrial snails have no gills. Instead, the thin membrane that lines their empty cavity functions like a primitive lung. Sea snails also lack gills, and gas exchange takes place directly through their Section 1 Mollusks

11. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Key Characteristics of Mollusks, continued Organ Systems: Reproduction Most species of mollusks have distinct male and female individuals, although some snails and slugs are Certain species of oysters and sea slugs are able to change from one to the other and back again. Many marine mollusks are moved from place to place as their trochophore larvae drift in the ocean Section 1 Mollusks

12. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Body Plans of Mollusks Gastropods—snails and slugs—are primarily a marine group that has successfully invaded freshwater and habitats. Most gastropods have a pair of 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 Section 1 Mollusks

13. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Body Plans of Mollusks, continued Bivalves All bivalves have a two-part hinged shell. The valves, or shells, of a bivalve are secreted by the Two thick muscles, the 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 region or a radula. Section 1 Mollusks

14. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Body Plans of Mollusks, continued Bivalves Many bivalves use their foot to dig down into the sand. Once there, the cilia on their gills draw in sea water through hollow tubes called Section 1 Mollusks

15. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Body Plans of Mollusks, continued Cephalopods Squids, octopuses, cuttlefish, and nautiluses (with shells) are all. 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 The structure of a cephalopod eye is similar in many ways to that of a vertebrate eye, and some species have Section 1 Mollusks

16. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu The First Segmented Animals Characteristics of 1.The fluid-filled is large and is located entirely within the 2.The organ systems of annelids show a high degree of specialization and include a circulatory system and excretory structures called 3.Most annelids have external bristles called Some annelids also have fleshy appendages called Annelids

17. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Annelid Groups Worms Marine segmented worms are members of class, 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 The parapodia, which usually have setae, are used to swim,, or crawl. Annelids

18. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Annelid Groups, continued Earthworms Earthworms and some related freshwater worms are members of the class. Oligochaetes have no parapodia and only a few setae on each segment. Earthworms lack the distinctive head region of polychaetes and have no. Earthworms are highly specialized scavengers. They literally eat their way through the soil, consuming their own weight in soil every day. They move using circular muscles and they through the skin. Annelids

19. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Anatomy of the Earthworm ( is used in reproduction, gizzard grinds food) Annelids Draw and know parts

20. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Annelid Groups, continued A leech has suckers at both ends of its body. Most species are predators or scavengers, but some are of vertebrates and crustaceans. Leeches are the only members of class Hirudinea. Leeches lack both setae and The body of a leech is flattened, and unlike other annelids, its are not separated internally. Section 2 Annelids