Bivalvia class: Mollusca Phylum

Phylum Mollusca

Class Bivalves (Bivalvia)

• Bivalve bodies are compressed from the sides

• Strong muscles pull the two shells (valves) together for protection, working against the hinge ligament

• The umbo is the oldest part of the shell, and the shell grows outwards in rings from this point

• The body consists of a visceral mass containing organs, a muscular foot attached at the front-bottom, and gills (ctenidia) on each side covered by a fold of the mantle

• Some bivalves possess one gill per side; most possess two

• The mantle sticks to the inside of the shell and forms two openings at the back

• The incurrent (inhalant) opening brings water in

• The excurrent (exhalant) opening pouches water out

• Bivalves move using their muscular foot

  • The foot extends out between the shells
  • Blood fills the foot, expanding it to anchor in sand or mud
  • The foot then contracts to pull the animal forward

• Tiny food particles are trapped by gills and moved to the mouth by mucus and cilia

• Food enters the stomach where cilia sort it, a rotating crystalline style breaks it down

• Digestive enzymes break down food, with absorption happening in the digestive glands

• Processed food moves through the intestine and waste is expelled as feces through the anus into the exhalant water current

• Bivalves have a three-chambered heart inside the pericardial cavity, containing two auricles and one ventricle

• The heart beats only six times per minute in some instances

• Oxygenated blood goes through sinuses, then the kidneys, then gills to get oxygen, before finally returning to the auricles

• Gills and mantles facilitate breathing

• Each gill has two thin walls (lamellae) joined at the bottom, containing tiny vertical gill filaments supported by chitin rods

• Water flows through pores in the gill walls, aided by cilia

• Partitions inside the gills create small water tubes directing water upward into a larger suprabranchial chamber, then out through the excurrent opening for gas exchange

• Freshwater bivalve females use water tubes as brood pouches to hold eggs and larvae during breeding season

• The nervous system includes 3 pairs of ganglia (nerve clusters) connected by nerves

  • Statocysts in the foot help with balance
  • Osphradia are located in the mantle cavity, but their exact function is unknown
  • Touch-sensitive cells and simple pigmented cells are also on the mantle

• Two U-shaped kidneys (nephridia) behind the heart filter waste, which is then emptied into the mantle cavity for removal

• Bivalves typically have separate sexes, releasing eggs and sperm into the water with external fertilization

• Marine bivalves (oysters) undergo fertilization outside the body in the water

  • The fertilized egg goes through trochphore larva, veilger larva, and spat (juvenile) stages
  • Oyster larvae swim for around 2 weeks before attaching to a surface and becoming spats
  • Oysters take around 4 years to become harvestable

• Freshwater bivalves (clams) undergo fertilization inside the female body

  • Eggs stay inside special tubes in the gills and are fertilized by sperm brought in via the water current
  • Eggs develop into glochidium larva, a type of veliger larva
  • These larvae attach to fish gills or skin for a few weeks, living as parasites before dropping off and settling, beginning an independent life
  • This hitchhiking aids species distribution since bivalves cannot move far on their own

Class Gastropoda

• Gastropod bodies are originally bilaterally symmetrical

• A twisting process (torsion) during development causes internal organs to become asymmetrical

• If they have a shell, it is a single piece (univalve) that can be coiled or uncoiled

• In coiled shells, the smallest and oldest part is at the apex

• Each new whorl grows larger around a central column called the columella

• Gastropods breathe using gills in their mantle cavity

• Aquatic species without gills breathe through the mantle and skin

• Pulmonate snails have a special area in the mantle for breathing air, similar to a lung

• A well-developed nervous system includes a nerve ring around the esophagus and paired nerve cords extending through the body

• Gastropods have one or two nephridia (kidney-like organs) that filter waste from the blood

• Some gastropods have separate sexes (dioecious), while others have both male and female organs (monoecious)

• Many engage in courtship rituals before mating, while monoecious species exchange sperm

• Land snails may shoot a "love dart" before mating to increase excitement

• After mating, both partners release eggs and sperm into the ocean for fertilization

• Most gastropods undergo fertilization inside the body, and the baby gastropods hatch as free-swimming larvae

Class Cephalopoda

• Cephalopods live in the ocean and are active hunters, a name derived from the involvement of their foot in the arm and tentacles around the head

• Their tentacles possess suction cups to catch prey

• They use a modified foot as a funnel for jet propulsion

• Statocysts (balance organs) are more complex than in other mollusks

• Their eyes are highly developed and can form images, except for the Nautilus, which has simpler eyes

• Arms are sensitive with touch and chemical receptors

• Cephalopods lack a sense of hearing

• Protection is provided by ink production in most cephalopods (except nautiluses)

• The ink sac connects to the rectum and holds sepia, a dark fluid with pigment

• When threatened, the cephalopod releases a cloud of ink. This ink can float in the water or get shaped by water currents, distracting predators and allowing escape

• They have a complete digestive system with a beak-like jaw and radula for tearing food

• Digestion takes place in the stomach and cecum, aided by digestive glands

• All have a closed circulatory system

• Breathing is facilitated by gills located in the mantle cavity, with water drawn in and expelled through the funnel for oxygen exchange

• The largest brain of any invertebrate is found in most cephalopods (except nautiluses)

• Many ganglia (nerve clusters) mainly concentrate in the head

• Millions of nerve cells connect these ganglia

• A neurosecretory system helps control body tone

• Waste is filtered by nephridia (kidney like organs) that remove nitrogenous waste and expel it through the mantle cavity

• Male cephalopods pack sperm into spermatophores (sperm bundles), storing them in a sac that connects to the mantle cavity

• One of the male’s arms modifies into a hectocotylus, transferring a spermatophore into the female’s mantle cavity near the oviduct

• Both distinct males and females exist as separate sexes

• Paper nautiluses (Argonauta) create special shell capsules for brooding

• Males often attach to rocks or other surfaces, displaying colors before mating

• Some octopuses care for their eggs

Phylum Arthropoda

Class Crustaceans

• Crustaceans have gills, two pairs of antennae, and mandibles to crush food

• Move side to side for protection, two compound eyes, and 5 pairs of walking legs

• The first pair of legs may be modified into pincers for defense

• Divided into two main parts: the cephalothorax (head and thorax combined) and the abdomen

• The cephalothorax is covered by a hard carapace (shield) that extends between the eyes to form a rostrum (pointed front)

• The first 13 pairs of appendages are attached to the cephalothorax

• The remaining 6 pairs of appendages are attached to the abdomen, which ends in a telson (horizontal fin)

• The abdomen is divided into segments, which includes dorsal tergites (top plates), lateral pleurae (side plates), and ventral sternites (bottom plates)

  • Anterior and posterior gastric muscles are in the stomach region
  • Gastric teeth aid in digestion
  • Mandibular muscle

• Abdominal extensor muscle- extends the abdomen

• Abdominal flexor muscle- curls the tail

• The pyloric stomach and antennal gland aid in secretion

• Complete digestive system, with modified mouthparts for varied feeding methods

• Open circulatory system with a dorsal heart, arteries, and hemocoel

• Breathe through different methods like the body surface, gills, trachea (air tubes), or book lungs

• The nervous system is similar to that of annelids, with a dorsal brain connected by a ring around the throat to a ventral nerve chain

• Some species possess fused ganglia and well-developed sensory organs

• Paired excretory glands, such as coxal, antennal, or maxillary glands, function similarly to the nephridia in annelids

• Some arthropods also possess Malpighian tubules for excretion

• Most arthropods have separate sexes and paired reproductive organs

• Fertilization is usually internal, and they can be either oviparous (laying eggs) or ovoviviparous (eggs hatch inside the body)

• Some undergo metamorphosis, while a few reproduce through parthenogenesis (asexual reproduction)

Class Arachnids

• Arachnids have 2 main body parts, the cephalothorax, possessing chelicerae (mouthparts), pedipalps (appendages for sensing/feeding), and four pairs of walking legs, and the abdomen

• They lack antennae or mandibles (jaws)

• Predatory arachnids possess claws, fangs, poison glands, or stingers to subdue their prey

• Many use sucking mouthparts or a robust sucking pharynx to consume the soft tissues/fluids of their prey

• Spiders digest food outside their bodies, releasing digestive enzymes that turn the prey's tissues to liquid, which they then consume using a sucking stomach

• Behind the stomach is the gut, which extends into the spider's legs, and the midgut branches into the abdomen, surrounding other organs

• An open circulatory system means they lack true blood/veins

• Their bodies contain hemolymph, a fluid that acts like blood

• The heart pumps hemolymph via arteries into sinuses, spaces around their internal organs

• Unique book lungs made up of thin, air-filled layers extending into a blood-filled chamber facilitate respiration

• Air enters this chamber through a tiny slit

• Their nervous system includes two nerve cords running along the gut, with paired ganglia as control centers in each segment

• The brain is formed by fused ganglia from head segments, surrounding the esophagus

• Remove waste using Malpighian tubules, working together with the rectal gland for efficient excretion

• Spiders have separate sexes and reproduce sexually with internal fertilization

• Instead of direct sperm insertion, males use pedipalps to transfer it

• Life cycle includes three stages: Egg, Spiderling (young spiders), and Adult

• Depending on the species, spiders can lay up to 3,000 eggs in one or more silk sacs

Life Cycle of Ticks and Mites

Two-host ixodid ticks usually take two years to complete their life cycle. These ticks go through the following stages:

1. Gravid females (pregnant females) drop from their second host after feeding and lay their eggs during the fall
2. The eggs will then hatch into six-legged larvae that stay dormant during the winter stage
3. In the spring season, the larvae attach to the first host (usually a rodent or rabbit).
4. The larvae will then molt into nymphs while on the first host.
5. The engorged nymphs then drop off in late summer or fall and overwinter in their nymphal stage
6. In the spring, the nymphs molt into adults and attach to the second host (usually a larger herbivore like a deer or cow).
7. Adults feed on the second host in the summer season, where they mate, and the females drop again from the host to lay eggs where they start the cycle all over again

Humans can sometimes serve as a host in this cycle, either as the first or second host

• The second host doesn’t always have to be a different species or individual from the first
• Life Cycle of Ticks and mites:
  • Humans can sometimes serve as a host in this cycle, either as the first or second host
  • The second host doesn’t always have to be a different species or individual from the first

Class Insects

• Insect bodies are divided into three sections: head, thorax, and abdomen

• They have 3 pairs of legs (6 in total)

• Many insects have one or two pairs of wings

• Possess digestive and circulatory systems

• Open circulatory system with a dorsal heart (located along their back)

• Breathe through a system of tracheae, tiny tubes that carry oxygen throughout the body

• Air enters through spiracles (small openings on the sides of the body)

• They have a nervous system to regulate their bodily functions

• They excrete waste via Malpighian tubules

• Separate sexes and fertilization internally or some can produce without fertilization

• Life cycle includes two types of metamorphosis

Complete Metamorphosis

• Four stages: egg, larvae, pupa, and adult
• Larvae look different from the adult, going through a pupal stage before becoming fully grown
• Beetles, flies, bees, butterflies, ants, lacewings, and caddisflies are all examples of insects that undergo complete metamorphosis

Incomplete Metamorphosis

• Three stages: egg, nymph, and adult
• The nymph looks similar to the adult but is smaller and lacks fully developed wings
• Plant bugs, crickets, true bugs, termites, grasshoppers, and cockroaches are examples of insects that undergo incomplete metamorphosis and have no pupa stage.

Overview of the Bivalvia class within the Mollusca phylum. Details include their compressed bodies, strong protective muscles, shell structure with the umbo, and the functions of the incurrent and excurrent openings. It also highlights their movement using a muscular foot.