Protostomes: A major group of animals characterized by the development of the mouth from the blastopore during embryonic development. They are coelomate, meaning they possess a body cavity derived from mesoderm.
Clades:
Lophotrochozoa:
Characteristics include: embryos develop using spiral cleavage.
Most members live in aquatic environments.
Movement is facilitated by cilia or body muscle contractions.
Defined by the presence of:
Trochophore: A free-living larval stage.
Lophophore: A horseshoe-shaped crown of ciliated tentacles surrounding the mouth, used for filter feeding.
Ecdysozoa:
Contains animals that undergo ecdysis, or molting.
Characterized by an exoskeleton, which serves as a hard external skeleton.
Major groups include arthropods and nematodes.
Micrognathozoa
Chaetognatha
Platyhelminthes
Cycliophora
Mollusca (e.g., snails, clams, squids)
Annelida (e.g., earthworms, sea worms)
Bryozoa
Brachiopoda
Nemertea
Loricifera
Nematoda
Kinorhyncha
Arthropoda (e.g., insects, spiders, centipedes)
Onychophora
Tardigrada
Echinodermata
Chordata
Mollusca
Phylum Mollusca
Diversity: Second in diversity only to arthropods.
Examples: Include gastropods (snails, slugs), cephalopods (octopuses, squids), and bivalves (clams, oysters).
Shell: Some have a shell, some do not.
Features of the Phylum Mollusca
Size Range: Varies from microscopic forms to giant clams weighing up to 270 kg.
Habitat: Most groups evolved in oceanic environments and continue to inhabit them.
Human Relevance: Important food source and economically significant. Pearls produced in oysters and mother-of-pearl created by abalones.
Pests: Zebra mussel (Dreissena polymorpha).
Mollusk Body Plan
Mantle:
A thick epidermal sheet that bounds the mantle cavity and secretes the shell if present.
Foot:
Serves as the primary means of locomotion for many mollusks and can be divided into arms or tentacles in cephalopods.
Internal Organs
The coelom is highly reduced and is limited to small spaces adjacent to the excretory organs, heart, and part of the intestine.
Visceral Mass: Concentration of digestive, excretory, and reproductive organs.
Ctenidia: Gills in aquatic mollusks, which also function in food filtration in most bivalves.
Mollusk Shell Characteristics
Protects against predators and harsh environments, though not essential as many species have reduced or lost their shells.
The typical shell structure consists of two layers of calcium carbonate with an internal layer possibly being nacre (mother-of-pearl).
Formation of Pearls: Occurs via coating a foreign object with nacre to alleviate irritation.
Radula
A rasping, tongue-like structure employed for feeding, found in most mollusks (except bivalves).
Functions include scraping algae in herbivorous mollusks and drilling in predatory gastropods, modified into a harpoon with venom glands in Cone snails.
Circulatory and Excretory Systems
Nitrogenous Waste Removal: Achieved via nephridia, consisting of cilia-lined openings called nephrostomes that lead to the excretory pore.
Circulatory System: Most mollusks have an open circulatory system where hemolymph circulates within a hemocoel; cephalopods possess a closed circulatory system.
Reproduction and Development
Most mollusks are gonochoric (individuals are either male or female) while a few, like some oysters, are hermaphroditic.
External fertilization is common, though gastropods typically exhibit internal fertilization.
Development includes a zygote undergoing spiral cleavage, with two larval stages:
Trochophore: A free-swimming larval stage.
Veliger: The second free-swimming larval stage, unique to bivalves and most marine snails.
Classes of Mollusks
There are 7 or 8 recognized classes, focusing on four:
Polyplacophora (Chitons): Marine mollusks with a shell composed of 8 overlapping plates and are primarily herbivorous grazers.
Gastropoda: Includes limpets, snails, and slugs, with most having a single shell and notable for unique coiling and torsion in development.
Bivalvia (Bivalves): Comprising clams, oysters, and mussels, characterized by two hinged shell valves without a radula.
Cephalopoda: Includes squids, octopuses, and chambered nautilus, known for their highly developed nervous systems and closed circulatory systems.
Annelida
Annelids: Segmented worms whose body structure comprises repeat units that allow for specialization.
Annelid Features
Body Plan: Includes a well-developed cerebral ganglion, a ventral nerve cord, internal segmentation with septa, and a closed circulatory system.
Includes specialized body parts for digestion, locomotion via coelomic fluid, and chaetae (chitin bristles).
Annelid Clades
Grouped into two clades:
Errantia: Characterized by active movement and paired parapodia for locomotion and gas exchange.
Sedentaria: Generally less mobile, live in tubes, and may utilize tentacles for feeding.
Earthworms and Leeches
Earthworms: Comprising 100-175 segments, they lack eyes but have senses for light and touch, are hermaphroditic, and engage in cross-fertilization.
Leeches: Mostly freshwater species, flattened dorsoventrally, with a reduced coelom without segmented division, using suckers for attachment and feeding.
Arthropoda
Characteristics: Most successful group of animals with over 1,200,000 species. Comprised of:
Chelicerata: Spiders, scorpions, ticks, with an anterior pair of chelicerae.
Crustacea: Crabs, shrimps, with biramous appendages and two pairs of antennae.
Hexapoda: Insects, characterized by three body regions; has over half of all named species.
Myriapoda: Millipedes and centipedes, with numerous segments and paired appendages.
Arthropod Success Factors
Segmentation: Allows for specialization of body regions.
Exoskeleton: Protects against water loss and must be shed during molting (ecdysis).
Jointed Appendages: Adaptable for various functions (antennae, mouthparts, wings).
Circulatory and Nervous Systems
Generally, an open circulatory system with a double chain of segmented ganglia controlling various activities.
Eyes and Sensory Systems
Compound eyes made of ommatidia or simple eyes (ocelli) that help in light detection and orientation.
Respiratory Systems
Gas Exchange in Marine Arthropods: Often via gills.
Terrestrial Arthropods: Use tracheae branching into tracheoles, with spiracles for airflow control, along with book lungs in certain species.
Excretory Systems
Aquatic arthropods use diffusion across gills while terrestrial insects excrete nitrogenous wastes efficiently via Malpighian tubules.
Classifications of Arthropods
Each class has specific traits and contains many grouped taxa, e.g., different types of insects (beetles, moths, flies, etc.) with respective life stages and reproductive strategies.