Biology Chapter on Invertebrate Phyla

Phylum Ctenophora ("Comb Bearer")

• Radially symmetrical, gelatinous marine animals
• Have eight rows of comb plates (ciliated bands) for locomotion
• Possess a pair of tentacles with colloblasts (adhesive cells) to capture prey
• Are bioluminescent

Anatomical Features

• Have a simple, but complete digestive system with a mouth and anus
• Lack a defined brain, but have a nerve net
• No specialized circulatory or respiratory organs
• Have a pair of canals that function as a hydrostatic skeleton

Nutrition

• Are carnivores, primarily feeding on zooplankton
• Capture prey with their tentacles, which are covered in colloblasts (sticky cells)

Reproduction

• Most are hermaphroditic (possessing both male and female reproductive organs)
• Can reproduce both sexually and asexually

Invasive Species Example

• Mnemiopsis leidyi is an invasive species that has caused ecological problems in the Black Sea and other regions

BIOL 312: Introduction to Bilateria

Eumetazoa

• A clade within the animal kingdom that includes all animals with true tissues.

• The clade consists of Ctenophora and Bilateria

• Bilateria are triploblastic animals with bilateral symmetry.

Morphology-Based Phylogeny

• Based on morphological features, the Bilateria were traditionally seen as a clade with three major groups:
• Deuterostomia: includes echinoderms, hemichordates, chordates
• Protostomia: includes mollusks, annelids, arthropods
• Lophotrochozoa: includes flatworms, rotifers, bryozoans

Characteristics of Bilateria

• Bilaterally symmetric
• Triploblastic (possessing three germ layers: ectoderm, mesoderm, endoderm)
• Cephalization (development of a head region with specialized sensory organs)

Nervous System

• Possess a centralized nervous system with a brain and nerves
• Ventral nerve cord (in most bilaterians)

Body Organization

• Have a body plan with three axes:
• Anterior-Posterior axis: head to tail
• Dorsal-Ventral axis: back to belly
• Left-Right axis: sides

Functions of Body Organization

• The body plan enables organismal development and efficient movement
• Efficient sensory perception
• Specialized functions for different body regions

BIOL 312: Introduction to Bilateria

Circulatory Systems (Fluid-Transport Systems)

• Transport nutrients, oxygen, and waste products

• Can be open or closed:

  • An open circulatory system includes a heart and blood vessels, but blood is not contained within vessels except in the heart
  • A closed circulatory system includes a heart and blood vessels, and blood is confined to vessels throughout the body.

• Different mechanisms of blood circulation exist (e.g., muscular contractions, hydrostatic pressure)

• May contain hemoglobin or other respiratory pigments to facilitate oxygen transport

Blood-Vascular (Hemal) System

• A fluid-filled space within the body cavity (coelom) that functions in circulation

Excretory Organ Types

• Remove waste products from the body
• Include:
  • Protonephridia: found in flatworms and some other invertebrates. Simple tubular structures with flame cells
  • Metanephridia: found in annelids, mollusks, and some other invertebrates. Tubular structures with nephrostomes that open into the coelom. These are usually more complex than protonephridia.
• Malpighian tubules: found in insects and other arthropods. Tubular structures that extend from the digestive tract and collect waste products.

Metanephridial System

• A type of excretory organ that utilizes a network of tubules to filter waste products out of the coelom.
• The filtration process involves:

1. Selective reabsorption of nutrients from the tubules
2. Active secretion of waste.
3. Elimination of waste products through a nephridiopore (opening)

Protonephridial System

• A type of excretory system common in flatworms
• Works by filtering coelomic fluid through specialized cells called flame cells.
• The filtered fluid passes through a network of tubules and exits the body through excretory pores.

Digestive System

• A system that breaks down food and absorbs nutrients.
• It is often organized into a mouth, esophagus, stomach, intestines, and anus
• The digestive system can be complete or incomplete
• Complete systems have a separate mouth and anus
• Incomplete systems have a single opening that serves as both mouth and anus

Embryology

• The development of an organism from a fertilized egg
• Involves a series of stages:
  • Cleavage (rapid cell division)
  • Gastrulation (formation of germ layers)
  • Organogenesis (development of organs)

Germ Layer Formation

• The formation of three germ layers:

  • Ectoderm (outer layer): gives rise to skin, nervous system, sensory organs
  • Mesoderm (middle layer): gives rise to muscles, bones, circulatory system, and excretory system
  • Endoderm (inner layer): gives rise to digestive system, respiratory system, and liver

• The formation of mesoderm differentiates bilaterians from other animal taxa.

Modes of Mesoderm Formation

1. Schizocoely: Mesoderm develops as a solid mass of cells that splits into two layers
2. Enterocoely: Mesoderm forms as outpocketings from the archenteron (primitive gut)

Fate of the Blastopore

• The blastopore is the opening of the archenteron (primitive gut) in a developing embryo.
• In bilaterians, the blastopore can develop either into the mouth (protostomes) or the anus (deuterostomes).

Germ Layer Derivatives

• Ectoderm: skin, nervous system, sensory organs
• Mesoderm: muscles, bones, circulatory system, excretory system
• Endoderm: digestive system, respiratory system, liver

Introduction to Bilateria

Bilateral Cleavage

• Cell division during early development occurs in a spiral pattern, with cells rotating relative to one another.
• This leads to a characteristic pattern of cell arrangement in the embryo.

Three Germ Layers

• The presence of three layers is a defining feature of bilaterians.
• Each germ layer gives rise to specific tissues and organs in the developing organism.

Major Modes of Mesoderm Formation

1. Schizocoely: Mesoderm formation where a solid band of cells splits to form a space. Common among protostomes.
2. Enterocoely: Mesoderm originates as pouches from the primitive gut. Common among deuterostomes.

Fate of the Blastopore

• Protostomes: The blastopore develops into the mouth
• Deuterostomes: The blastopore develops into the anus

Germ Layer Derivatives

• Ectoderm: Skin, nervous system, sensory organs
• Mesoderm: Muscles, bones, circulatory system, excretory system
• Endoderm: Digestive system, respiratory system, liver

P-Xenacoelomorpha

Overview

• A group of bilaterians that includes flatworms (Xenoturbella) and acoel flatworms (Acoela).
• They have been historically difficult to place in the animal tree of life.

Key Taxa

1. Xenoturbella: Simple, worm-like bilaterians with a simple body plan.
2. Acoela: Small, free-living flatworms that lack a through gut.

Molecular and Morphological Characteristics

• Lack a through gut (acoels have a simple sac-like gut)
• Simple nervous system
• Absence of a circulatory system and specialized excretory organs

Locomotion

• Move by cilia (hair-like structures) located on their ventral surface

Nutrition and Digestion

• Xenoturbella are carnivorous and feed on mollusks
• Acoels are omnivorous or carnivorous.
• Some species have a pharynx that can be extended to feed.

Nervous System

• Have a simple nervous system with a nerve net and scattered sensory cells
• Lack a central brain

Excretion

• Waste products are removed through diffusion across the body surface or by protonephridia.

Reproduction

• Sexual reproduction is common, and many species are hermaphroditic.

• Some acoel species can also reproduce asexually through fission.

• Xenacoelomorpha are a relatively new group, and research into their biology is ongoing.

Deuterostome Characteristics

General Features

• Bilateral symmetry in their larval stages
• Triploblastic (possess three germ layers)
• Deuterostomic development (blastopore develops into the anus)

Mesoderm Formation

• Enterocoely: Mesoderm forms from pouches of the primitive gut.

Phylum Hemichordata ("Half Notochord")

General Characteristics

• Marine animals with a worm-like body

• Possess a dorsal, hollow nerve cord (although not as prominent or complex as a notochord)

• Have "gill slits" (pharyngeal slits) used for filter feeding

• They are considered a sister group to chordates

• Exhibit a tripartite body plan:

• 1. Proboscis: anterior, muscular structure used for feeding, burrowing, locomotion
• 2. Collar: a distinct, short region that encircles the proboscis
• 3. Trunk: the main body region containing the digestive system and other organs.

• They are bilaterally symmetrical with a well-developed coelom

Class Enteropneusta ("Intestine Breathers")

• A diverse group of hemichordates called acorn worms
• They are burrowing, filter-feeding animals
• Possess a long, worm-like body

Body Structure

• Body is divided into three segments:
• Proboscis: anterior, muscular structure used for feeding and burrowing
• Collar: short region surrounding the proboscis
• Trunk: the main body region containing the digestive system and other organs

Coelomic Organization

• Possess three coelomic compartments:

• Proboscis coelom: located in the proboscis

• Collar coelom: located in the collar

• Trunk coelom: located in the trunk

• The trunk coelom is further subdivided into dorsal, ventral, and lateral compartments.

• The coelomic compartments are filled with fluid and help support the body

Musculature and Body Wall

• Have a muscular body wall with circular and longitudinal muscle fibers.

• The muscle fibers are arranged in layers and enable locomotion, burrowing, and feeding

• The body wall also includes a cuticle (a non-living protective layer) and epidermis (an outer layer of cells).

Circulatory System

• They have a closed circulatory system with a heart and blood vessels.
• The heart is located in the ventral region of the trunk.

Excretory System

• Have a pair of metanephridia in the trunk region that remove waste products.

• Metanephridia are tubules that filter waste from the coelomic fluid and expel it through nephridiopores

• The following steps summarize how the metanephridia function:

1. Coelomic fluid is filtered into the metanephridia
2. Selective reabsorption of useful substances occurs
3. Waste products are concentrated
4. Waste is expelled through nephridiopores

Nervous System in Hemichordata (Class Enteropneusta)

• Have a dorsal, hollow nerve cord.
• The nerve cord runs along the dorsal side of the body.
• It is not as well-developed or complex as the nervous system of vertebrates.
• The nervous system includes sensory cells and ganglia
• The ventral nerve cord, which is well-developed in most bilaterians, is absent in Enteropneusta.

Nutrition

• They are filter feeders and use their proboscis to collect food particles.

• The proboscis is covered with cilia that trap food particles, which are then transported to the mouth.

• Food is passed through the digestive tract.

• The digestive tract is lined with a layer of mucus-secreting cells, which helps move food through the system.

• They have gills slits, which are openings in the pharynx.

• The gills slits allow water containing oxygen to pass through them

• At the same time, water passes out of the body, carrying waste products with it.

Reproductive Strategies

• They are dioecious, which means that individuals are either male or female.

• Some hemichordates have specialized reproductive organs, while others release gametes directly into the water.

• Fertilization is external and the larvae undergo metamorphosis

• Some hemichordates can reproduce asexually through budding.

Class Pterobranchia ("Wing Gill")

• Smaller, sessile (attached) hemichordates

• They have a colonial lifestyle

• They have a U-shaped gut and a pair of "arms" with tentacles that are used for filter feeding.

• They have two distinct structures:

• 1. Zooids: individuals that live in a chitinous (hardening substance) tube.
• 2. Stolons: connecting tubes that link the zooids together.

Vascular and Excretory Systems

• Circulatory System: Closed, similar to enteropneusts, but with a simpler heart
• Excretory System: A pair of protonephridia (simpler than metanephridia)

Phylum Echinodermata ("Spiny Skin")

Basic Characteristics

• Marine animals with a distinctive radial symmetry in the adult body.
• Have a unique water vascular system that is used in locomotion, feeding, and gas exchange
• Possess a hard exoskeleton made from calcium carbonate plates (ossicles)
• Show a wide range of body forms, including starfish (sea stars), sea urchins, sand dollars, and sea cucumbers.

Coelomic Organization

• Have a complex coelomic system with fluid-filled compartments (coeloms).

• The three coeloms:

• Radial canals (radial coeloms): extend from the ring canal outwards, forming a network of compartments

• Perivisceral coelom: surrounds the internal organs

• Perihemal coelom: surrounds the circulatory system

• The coeloms perform various functions including hydrostatic support, circulation, and transport.

Larval Organization

• Exhibit distinct larval stages (e.g., bipinnaria, brachiolaria).
• The larval stages show bilateral symmetry, which gives evidence of their evolutionary ancestry.
• Bilaterally symmetrical larvae transition into radially symmetrical adults.

Metamorphosis

• The developmental transition from a larva to an adult.
• In echinoderms, metamorphosis includes significant changes in body shape, symmetry, and the development of adult structures.

Phylum Echinodermata: Overview

Body Structure

• Radially symmetrical in adults
• Typically, the sea star design has five arms, but other echinoderms have variations in body form.
• Have a central disc (where the vital organs are found) with arms or rays extending outwards
• The arms are often coated in spines, which are used for defense, feeding, or movement.
• Their endoskeletons are made of calcium carbonate plates that are covered in spines and tubercles.

Coelomic Organization

• The coelomic system involves fluid-filled cavity compartments within the body:
• Hydrostatic support (using the coelomic fluid)
• Circulation (transporting nutrients and oxygen)
• Gas exchange (oxygen absorption)

Water Vascular System (WVS)

• A unique, fluid-filled system of canals and tube feet that is used in locomotion and feeding.

• The system is filled with seawater that is pumped in and out through a structure called the madreporite.

• The canals are connected to tube feet.

• The tube feet are hollow extensions of the WVS that function in adhesion.

• The WVS is involved with:

• Locomotion by extending and retracting tube feet.

• Feeding by use of tube feet in capturing prey and transporting food to the mouth.

• Gas exchange by assisting in the movement of oxygen and carbon dioxide.

Nervous System

• A decentralized nervous system with a nerve ring around the mouth and radial nerves extending into the arms.
• Echinoderms lack a centralized brain but have a network of sensory cells that detect stimuli.

Nutrition and Feeding

• Most echinoderms are carnivorous (eating mollusks or crustaceans).

• Sea stars pry open bivalve shells to access soft tissues inside

• Sea urchins use their jaw- like structure called an Aristotle's lantern to scrape algae off surfaces.

• Some sea cucumbers are detritivores, which means they feed on dead organic matter on the seafloor.

Reproduction

• Most are dioecious (having separate sexes) and external fertilization occurs.

• Fertilization takes place when sperm and eggs are released into the water

• Larvae are often free-swimming and undergo metamorphosis into adults

• Some echinoderms can reproduce through asexual means by fission (dividing into two or more individuals).

Key Classes

• The phylum Echinodermata is divided into five classes:
• Asteroidea
• Ophiuroidea
• Echinoidea
• Holothuroidea
• Crinoidea

Class Asteroidea (Sea Stars)

• Have five or more arms connected to a central disc.

• Have a central disc containing the vital organs and tube feet on the ventral side.

• Features:

• Water vascular system is well-developed and used in locomotion

• They are carnivores, feeding primarily on bivalve mollusks

• Have a regenerative ability, whereby they can regrow lost arms.

Class Echinoidea (Sea Urchins, Sand Dollars, Sea Biscuits)

• They have a spherical or disc-shaped bodies

• They lack arms, but have movable spines for protection and locomotion.

• Sea urchins have a jaw-like structure called Aristotle's lantern used for scraping algae.

• Features:

• A well-developed water vascular system

• A biting mouth, called Aristotle's lantern

• Used for feeding on algae and other organisms.

Class Holothuroidea (Sea Cucumbers)

• Have a soft, elongated body

• They have a reduced radial symmetry

• They are elongated along the oral-aboral (mouth-anus) axis.

• Features:

• Possess a distinct ring of tentacles around the mouth used for feeding.

• A unique system of muscles enable them to contract their bodies into a sphere shape for defense

• They are often detritivores, feeding on sediment and organic matter

• They play a key role in the marine ecosystem by breaking down organic matter

Ecological Roles and Adaptations

• Echinoderms are important predators, grazers, and detritivores in marine ecosystems.
• They play a role in controlling populations other organisms
• They are well-adapted to life on the seafloor, where they utilize burrowing, crawling, and clinging abilities.

Deuterostomia: Chordata - Tunicata (Urochordata)

• General Characteristics*:
• Marine animals
• Typically sessile (attached to surfaces or rocks)
• Possess a tunic (sac-like covering) made of cellulose
• Have a chordate characteristic, a notochord (dorsal supporting rod), which is only present in the larva.
• Have a pharynx with pharyngeal slits (the notochord is also present in the pharynx)
• In some species, the tunic contains chemicals like vanadium and iodine, these chemicals are used by the tunicates for feeding
• Classes within Tunicata*:

1. Ascidiacea: Sea squirts: Sessile filter feeders in the adult stage. Have two siphons (incurrent and excurrent)

Feeding Mechanism

1. They are filter feeders

2. They draw water in through the incurrent siphon

3. The water passes through the pharyngeal slits, where food particles are captured by mucus.

4. The food-laden mucus is transported to the stomach and intestines, where digestion takes place.

5. Waste products are then expelled through the excurrent siphon.

6. Some tunicates have a squirting mechanism: When threatened, they can expel water through their excurrent siphon, thus squirming away or scaring the predator

7. Some tunicates may be bioluminescent

Test your knowledge on invertebrate phyla, focusing on Ctenophora, Bilateria, Echinodermata, and Hemichordata. This quiz covers fundamental concepts such as body organization, mesoderm formation, and feeding mechanisms. Ideal for biology students looking to strengthen their understanding of invertebrate classification.