15.4 Mollusks and Annelids

15.4 Mollusks and Annelids

• You will be able to describe the unique features of mollusks by the end of this section.
• They have a variety of predatory forms, ranging from large squid and octopus, some of which show a high degree of intelligence, to small grazing forms with elaborately sculpted and colored shells.
• The annelids include the earthworms and leeches, the polychaetes, and two other smaller classes.

• It is the second most diverse group of animals.
• The earliest descriptions of mollusks came from observations of unshelled, soft-bodied cuttlefish.
• The mollusk body forms share some of the same characteristics, such as a muscular foot that is used for locomotion, and a flap of tissue over the internal organs of the animal.

• There is a shell of calcium carbonate in the mantle.

• The muscular foot varies in shape and function depending on the type of mollusk.
• It is extendable and retractable, used for locomotion and anchorage.
• The OpenStax book is available for free, but only in adult animals.
• It is a multi-purpose space, housing the gills, anus, and organs for detecting food particles in the water, and an outlet for gametes.
• Most mollusks have an open circulatory system with a heart that circulates the hemolymph in open spaces around the organs.
• The squid and octopuses have a closed circulatory system with two hearts that move blood through the gills and a third, systemic heart that pumps blood through the rest of the body.

• There are many types of mollusks and they all share many of the same characteristics.

• There is a radula for food.

• The mollusks have nerve cords.

• There are seven classes in this phylum: Aplacophora, Monoplacophora, Polyplacophora, Bivalvia, Gastropoda, and Scaphopoda.

• Class Aplacophora includes worm-like animals living on deep ocean bottoms.
• These animals have aragonite spicules on their skin.
• Members of class Monoplacophora have a single, cap-like shell around their body.
• Scientists have identified nearly two dozen living species.

• The animals in the class Polyplacophora are known as "chiton" and have an armor-like shell that extends beyond the shell.
• These animals have radulas.
• There is a single pair of nephridia.

• The class Polyplacophora has a shell indicative of it.
• They are found in the water.
• Bivalves are enclosed in a pair of shells that are hinged at the side of the body.
• The body is not straight.
• A radula is absent from the food they feed.
• They exchange gases using a pair of ctenidia.
• In some species, the edges of the mantle may form two siphons that exhale and inhale.
• The property is being used to produce pearls.

• You can watch animations of clams and learn more about bivalves.

• Well-known mollusks include snails, slugs, and sea hares.

• There are shell-bearing species as well as species with a reduced shell.

• The foot is modified for crawling and the mass in the shell is twisted.
• The head of most gastropods supports the eyes.
• A radula is used to remove food particles.
• The ctenidia and a pair of nephridia can be found in the mantle cavity.

• The class includes squids, cuttlefish, and nautilus.
• The shelled and reduced-shell groups are included.
• They display vivid colors which are used for camouflage.
• One of the more awe-inspiring feats of these animals is their ability to rapidly adjust their colors to mimic a background pattern.
• The animals in this class have beaklike jaws.
• A well-developed nervous system, complex eyes, and a closed circulatory system is what all cephalopods have.
• The foot develops into a funnel and tentacles, which are used for movement.
• There are suckers on the squid.
• Ctenidia are serviced by large blood vessels, each with their own heart.

• A stream of water can be forcefully ejected from the mantle cavity by contracting it.
• The females of some species care for the eggs for an extended period of time.
• In squid and cuttlefish, the shell is much reduced and internal, but not in octopus, which has a spiral, multi-chambered shell filled with gas or water.

• The front of the shell is usually exposed to water and the reduced head end is usually buried in the sand.

• Approximately 16,500 species have been described.
• There are earthworms, polychaete worms, and leeches.
• Annelid development is similar to that of mollusks.

• Annelids have a worm-like appearance.
• Their body plan results in repetition of internal and external features.

• The evolutionary benefit of a body plan is that it allows for the evolution of independent modifications in different segments that perform different functions.
• The body can be divided into head, body, and tail.

• The ecdysozoans need to be molted for growth, but the skin of annelids does not.
• A defining character of annelids is the chaetae.
• Polychaete worms have limbs that are unjointed and used for movement and breathing.
• There are two layers of muscle, one running around its circumference and the other running the length of the worm.

• Annelids have organs that are distributed and bathed in coelomic fluid.
• A crosssectional view of a body segment of an earthworm is limited by a membrane that divides the body into compartments.

• Annelids have a closed circulatory system with muscular pumping "hearts" in the anterior segments, and blood vessels that run the length of the body with connections in each segment.
• The moist body surface is where gas exchange occurs.
• Excretion is carried out by pairs of primitive "kidneys" called metanephridia that consist of a convoluted tubule and an open, ciliated funnel.
• Annelids have a well-developed nervous system with two nerve cords and a ring of fused ganglia.

• In this schematic, the stomach is green, the nervous system is yellow, and the circulatory system is red.

• Annelids can be monoecious with permanent gonads or dioecious with temporary or seasonal gonads.

• A close-up look at annelid anatomy is provided in this video and animation.