Survey of Major Invertebrate Phyla: Porifera, Cnidaria, Platyhelminthes, Annelida, Mollusca, and Rotifera

Phylum Porifera (Sponges)

General Characteristics

Phylum Porifera includes sponges, which are simple, multicellular, sessile animals found mainly in marine environments. They are characterized by their porous bodies, lack of true tissues and organs, and unique feeding system based on water flow.

• Symmetry: Asymmetrical

• Tissue Organization: No true tissues; incipient epithelium (pinacocytes)

• Body Cavity: None (acoelomate)

• Digestive System: Intracellular digestion; no true digestive tract

• Feeding: Filter feeders using choanocytes (collar cells)

• Reproduction: Sexual (hermaphroditic) and asexual (budding)

• Support System: Spicules (calcium carbonate or silica) and/or spongin

• Segmentation: None

• Nervous/Circulatory System: Absent

Cell Types

• Pinacocytes: Flattened cells forming the outer layer (epidermis-like)

• Choanocytes: Flagellated cells that create water currents and capture food particles

• Amoebocytes: Mobile cells in the mesohyl that produce spicules and distribute nutrients

Body Plans

• Asconoid: Simple, tubular body; choanocytes line the spongocoel

• Syconoid: Body wall folded into canals; choanocytes line canals

• Leuconoid: Most complex; choanocytes in chambers connected by canals

Classification

Class	Common Names	Distinguishing Features
Calcarea	Hard sponges	Calcium carbonate spicules, no spongin, all body types
Demospongiae	Soft sponges	Silica spicules, spongin present, leuconoid
Hexactinellida	Glass sponges	Silica spicules, no spongin, leuconoid, deep sea

Comparisons

• Asconoid vs. Leuconoid vs. Syconoid: All use choanocytes for feeding and lack organs; differ in complexity and location of choanocytes.

• Hexactinellida vs. Demospongiae vs. Calcarea: Demospongiae has spongin; Hexactinellida and Demospongiae have silica spicules; Calcarea has calcium carbonate spicules.

Phylum Cnidaria (Hydras, Jellyfishes, Sea Anemones, Corals)

General Characteristics

Cnidarians are mostly marine, radially symmetric, diploblastic animals with specialized stinging cells (cnidocytes). They have two main body forms: polyp (sessile) and medusa (motile).

• Symmetry: Radial

• Tissue Organization: Diploblastic (epidermis and gastrodermis, separated by mesoglea)

• Body Cavity: Gastrovascular cavity (coelenteron)

• Digestive System: Single opening (mouth/anus)

• Feeding: Predatory (using tentacles and nematocysts), some filter feeders

• Reproduction: Sexual and asexual (budding); alternation of generations in some classes

• Support System: Hydrostatic skeleton

• Nervous System: Nerve net

Classification

Class	Common Names	Distinguishing Traits
Hydrozoa	Hydras, siphonophores	Polyp dominant, some colonial, both sexual and asexual stages
Scyphozoa	True jellyfish	Medusa dominant, umbrella-shaped, always marine
Cubozoa	Box jellies	Medusa dominant, box-shaped, highly toxic, complex eyes
Anthozoa	Sea anemones, corals	Polyp only, sessile, often colonial

Key Structures

• Cnidocytes: Specialized stinging cells containing nematocysts for prey capture and defense

• Gastrovascular cavity: Central cavity for digestion and circulation

• Mesoglea: Gelatinous layer between tissue layers

Comparisons

• Metridium vs. Aurelia: Both are cnidarians; Metridium is a sessile polyp (cylindrical), Aurelia is a free-swimming medusa (umbrella-shaped).

• Choanocyte vs. Cnidocyte: Both are feeding cells; choanocytes are in sponges (Porifera), cnidocytes are in cnidarians and contain stinging nematocysts.

• Cnidarian vs. Poriferan: Both aquatic; cnidarians have tissues and stinging cells, poriferans lack true tissues and organs.

• Coral vs. Leuconoid Sponge: Both sessile filter feeders; corals have calcium carbonate skeletons and tentacles, leuconoid sponges have silica spicules and filter feed exclusively.

Phylum Platyhelminthes (Flatworms)

General Characteristics

Platyhelminthes are bilaterally symmetric, triploblastic, acoelomate invertebrates. They include free-living and parasitic species with simple organ systems and a flattened body for diffusion of gases and nutrients.

• Symmetry: Bilateral

• Tissue Organization: Triploblastic (three germ layers)

• Body Cavity: Acoelomate

• Digestive System: Incomplete (single opening)

• Feeding: Varies by class; ciliated pharynx (planaria), suckers/scolex (flukes/tapeworms)

• Reproduction: Sexual (hermaphroditic) and asexual (budding, fission)

• Support System: None; flattened body

• Nervous System: Simple brain (ganglia) and nerve cords

Class	Common Names	Distinguishing Features
Turbellaria	Planaria	Free-living, ciliated body, high regeneration
Trematoda	Flukes	Parasitic, suckers, complex life cycle
Cestoda	Tapeworms	Parasitic, segmented, no digestive tract, scolex for attachment

Comparisons

• Planaria vs. Fluke vs. Tapeworm: All are acoelomate, incomplete digestive system, can reproduce sexually/asexually. Flukes and tapeworms are parasitic with suckers/scolex; planaria are free-living with ciliated pharynx.

• Cnidarian vs. Flatworm: Both have a single digestive opening; cnidarians are diploblastic and radially symmetric, flatworms are triploblastic and bilaterally symmetric.

• Gastrovascular cavity vs. Complete digestive tract: Both absorb nutrients; gastrovascular cavity has one opening (cnidarians, flatworms), complete tract has two (mouth and anus; annelids, mollusks).

Phylum Annelida (Segmented Worms)

General Characteristics

Annelids are segmented, coelomate, triploblastic invertebrates with a closed circulatory system and a complete digestive tract. They inhabit marine, freshwater, and terrestrial environments.

• Symmetry: Bilateral

• Tissue Organization: Triploblastic

• Body Cavity: Coelomate

• Digestive System: Complete (mouth and anus)

• Feeding: Varies; pharynx, cilia, sticky pads, or proboscis

• Reproduction: Sexual (hermaphroditic or separate sexes), some asexual

• Support System: Cuticle, hydrostatic skeleton

• Nervous System: Brain, ventral nerve cord, segmental ganglia

Class	Common Names	Distinguishing Features
Polychaeta	Sandworms, bristle worms	Many chaetae, parapodia, marine, separate sexes
Oligochaeta	Earthworms	Few chaetae, no parapodia, terrestrial/freshwater, hermaphroditic
Hirudinea	Leeches	No chaetae/parapodia, suckers, hermaphroditic, some parasitic

Comparisons

• Sandworm vs. Leech vs. Earthworm: All segmented, complete digestive tract; leeches lack chaetae, earthworms have few, sandworms have many and parapodia.

• Annelid vs. Flatworm: Both soft-bodied, triploblastic; annelids are segmented, coelomate, and have a complete digestive tract; flatworms are acoelomate, unsegmented, and have an incomplete digestive system.

Phylum Mollusca

General Characteristics

Mollusks are coelomate, triploblastic invertebrates with a soft body, often protected by a hard shell. They have a muscular foot, mantle, and visceral mass, and display a wide variety of body forms and lifestyles.

• Symmetry: Bilateral

• Tissue Organization: Triploblastic

• Body Cavity: Coelomate (reduced)

• Digestive System: Complete; radula present in most

• Feeding: Herbivores, carnivores, filter feeders

• Reproduction: Sexual (mostly separate sexes, some hermaphroditic)

• Support System: Shell (calcium carbonate), cuticle, or none

• Nervous System: Brain, nerve cords

• Circulatory System: Open (most), closed (cephalopods)

Class	Common Names	Distinguishing Features
Gastropoda	Snails, slugs	Single coiled shell (or none), radula, head
Bivalvia	Clams, oysters	Two hinged shells, filter feeders, no head
Cephalopoda	Squid, octopus	Head, tentacles, beak, closed circulatory system, complex eyes
Polyplacophora	Chitons	Shell of 8 plates, marine, herbivorous

Comparisons

• Chiton vs. Snail vs. Squid vs. Clam: All have a muscular foot and are coelomate; differ in shell structure, feeding, and locomotion. Chiton: 8 plates; snail: coiled shell; clam: two shells; squid: no shell, tentacles.

• Annelid vs. Mollusk: Both triploblastic, coelomate; annelids are segmented, mollusks are not. Mollusks often have shells, annelids do not.

• Closed vs. Open Circulatory System: Closed (annelids, cephalopods): blood always in vessels; open (most mollusks): blood bathes organs directly.

Phylum Rotifera (Rotifers)

General Characteristics

Rotifers are microscopic, aquatic, pseudocoelomate animals with a ciliated corona for feeding and locomotion. They are important members of freshwater plankton communities.

• Symmetry: Bilateral

• Tissue Organization: Triploblastic

• Body Cavity: Pseudocoelomate

• Digestive System: Complete; ciliated mouth, muscular pharynx (mastax), stomach, intestine, anus

• Feeding: Suspension feeders; eat detritus, bacteria, algae, protozoans

• Reproduction: Sexual or parthenogenetic

• Support System: Flexible cuticle

• Nervous System: Cerebral ganglion, simple nerves

Body Structure

• Corona: Ciliated structure for feeding and movement

• Mastax: Muscular pharynx with jaws for grinding food

• Foot: Posterior structure for attachment

Summary Table: Key Features of Major Invertebrate Phyla

Phylum	Symmetry	Tissues	Body Cavity	Digestive System	Support System
Porifera	Asymmetrical	None	None	Intracellular	Spicules/spongin
Cnidaria	Radial	Diploblastic	None	Gastrovascular cavity	Hydrostatic
Platyhelminthes	Bilateral	Triploblastic	Acoelomate	Incomplete	None
Annelida	Bilateral	Triploblastic	Coelomate	Complete	Cuticle
Mollusca	Bilateral	Triploblastic	Coelomate	Complete	Shell/cuticle
Rotifera	Bilateral	Triploblastic	Pseudocoelomate	Complete	Cuticle