Animal Classification: Sponges and Other Phyla

Porifera (Sponges)

• Characterized as sponges
• Sponges do not have advanced organization beyond the cellular level

Demospongiae

• Includes Heteroscleromorpha which contains 5341 species
• Includes Haploscleromorpha which contains 1084 species
• Includes Myxospongiae which contains 138 species
• Contains bath sponges, including the Keratosa, which have 558 species

Hexactinellida

• Includes glass sponges
• The Hexasterophora contain 513 species
• The Amphidiscophora contain 177 species

Homoscleromorpha

• Consists of flesh sponges
• The Oscarellidae contain 19 species
• The Plakinidae contain 68 species

Calcarea

• Includes calcareous sponges
• The Calcinea contain 167 species
• The Calcaronea contain 514 species

Placozoa

• Contains only one species

Ctenophora

• Contains 192 species of comb jellies

Cnidaria

• Radiate animals
• Cnidaria and Ctenophora make up the radiate animals
• Cnidaria and Ctenophora have not advanced beyond tissue-level organization
• Other eumetazoans have bilateral symmetry, or their radial symmetry is derived from a bilateral ancestor
• Exhibit two well-defined germ layers, the ectoderm and endoderm.
• Mesoderm is derived from ectoderm
• Possess an internal body cavity, the gastrovascular cavity
• Extracellular digestion occurs in the gastrovascular cavity
• Gastrodermal cells accomplish cellular digestion
• Most have tentacles for food capture
• Simplest animals with nerve cells, but no central nervous system
• Use sense organs like statocysts and ocelli
• Locomotion achieved by muscular contraction or ciliary comb plates
• Come in polyp and medusa forms, allowing for wider ecological possibilities
• Unique features are cnidocytes (nematocysts), colloblasts, and ciliary comb plates
• The simplest animals, radiates possess statocysts and ocelli as their sense organs
• Muscular contraction or ciliary comb plates provide locomotion
• Both polyp and medusa forms allow wider ecological possibilities
• Cnidocytes (nematocysts), colloblasts, and ciliary comb plates are key features
• A variety of support mechanisms are exhibited, these include: hydrostatic skeletons, mesoglea, axial skeletons and calcareous skeletons
• Exhibit the most primitive type of musculature
• Modified epidermal (epitheliomuscular) and gastrodermal (nutritive muscle) cells are a part of musculature
• Sessile polyp movements are limited to extending, contracting, or bending
• Well-developed pedal disc muscles (as seen in sea anemones)
• Medusae are dominated by epidermal muscles
• Coronal muscles (circular sheets of epidermis) are around the bell margin and over subumbrellar surface
• Nervous system has neurosensory cells (neurons) arranged as a nerve net
• Nonpolar neurons can send impulses in all directions, reducing intensity distally
• Medusa forms have an epidermal nerve net near the bell margin
• Nerves control both the tentacles and sense organs, and the bell pulsations
• Have two categories of neurons: ganglion (intermediary neural connections) and sensory cells (receive stimuli)
• Sensory structures are relatively few in polyps
• Minute hair-like projections are distributed across the general body surface (mechanoreceptors & chemoreceptors) for tentacle and general body movement
• Medusae forms have more varied structures
  • Ocelli are pits of photoreceptor cells
  • Statocysts are pits of vesicles containing statoliths that orients the organism to direction of gravitational pull
• Generally carnivorous
• Food is taken to the mouth and digested extracellularly
• Secretions from mucous gland cells and enzymatic gland cells digest food extracellularly
• Complete digestion occurs in the nutritive cells within the food vacuoles
• Cnidocytes have stinging cells produced by cnidoblasts
• Cnidocytes have a thread released upon stimulation of cnidocil (mechanoreceptor)
• Cnidocytes function is for food collection, defense, locomotion, and attachment
• Nematocysts are harpoon-like tubes connected to toxin sacs that paralyze victims
• Spirocysts are sticky tubes or loops that wrap around and stick to the victim
• Ptychocysts are sticky long threads; found only in burrowing anemones
• Discharged Cnidocytes are replaced by interstitial cells within 48 hours

Medusozoa

• A clade within the phylum Cnidaria that includes the classes Scyphozoa, Cubozoa, and Hydrozoa

Hydrozoa

• Includes 3395 species of Hydroidolina
• Includes 157 species of Trachylina
• Cnidarians that have colonial species and freshwater representatives
• Possess varied forms of nematocysts (approx. 30), including venomous types (=penetrants)
• Exhibits two body forms: polyp (hydropolyp) and medusa
  • Polyp (hydropolyp) - possess a pedicel (stalk) and a hydranth (flower)
    • Hydra are solitary hydropolyp
    • Colonial forms (sessile, benthic species = hydroids)
  • Pelagic species are siphonophores
• Hydroid colonial forms have the following zooids:
  • Stolonate – zooids with a tubular body arising from stolons
  • Coenosarc – zooids arise from a single continuous tissue, joined inside by solenia
  • Fruticose - zooids with bushy, feathery arrangement
• Pelagic colonial species (Siphonophores)
  • Polymorphic colonies that form trailing clusters called cormidia
  • Modified medusae may have nectophores(jet propulsion) or leaflike defensive modules called bracts or phyllozooids
  • Possess floating pneumatophores used for buoyancy
  • Polymorphic colonies include: gastrozooids (feeding zooid), gonozoids (holds medusa buds), and dactylozooids (food-catching zooids)
• Medusae are bell-shaped with an exumbrella and subumbrella
  • Subumbrellar margin has velum (creates water jet)
  • Mouth has a manubrium, dilates as a stomach, and extends as four radial canals
  • Statocyst or ocelli are present on bell margin
  • Mesoglea lacks cells

Scyphozoa

• Exhibit a medusa form
• Tetramerous symmetry is shown with a manubrium that divides into four oral arms
• Septa divides the gastrovascular cavity into four gastric pouches
• Margins of septa bear gastric filaments extending freely into the stomach
• Umbrella margin has scalloped into rounded lobes (=lappets) with sensory lobes called rhopalia placed between lappets
• Mesoglea contains amebocytes
• Asexual reproduction involves a polyp form that divides transversely (strobilation) to form a juvenile medusa, known as the ephyra

Anthozoa

• Anthozoa comprises 70% of all Cnidarian species
• All are marine and lack a medusa form
• Development on the polyp structure focuses on increasing the surface area of the gastrovascular cavity
• Mouth connects to a tubular pharynx and siphonoglyphs
• Gastrovascular cavity is partitioned by mesenteries or septas
• Incomplete mesenteries of the gastrovascular cavity are trilobed, ciliated, and equipped with nematocysts
• Other species are equipped with acontia and acrorhagi used for food capture, offense, and defense
• Subclass Octocorallia
  • Possess 8 tentacles and 8 primary septa and a single siphonoglyph
  • Tentacles are pinnate having lateral outfoldings called pinnules
  • Includes Gorgonacea, Alcyonacea, Telastacea, Stolonifera, Coenothecalia and Pennatulacea
• Subclass Hexacorallia (Zoantharia):
  • Sea anemone and true (stony) corals
  • Unique feature: 6 pairs of primary mesenteries plus multiples of 6 tentacles
  • Scleractinian corals (Order Scleractinia) are reef-building which secrete a calcium carbonate exoskeleton
• Sea Anemones
  • Attach to shells, rocks, and timber etc with pedal discs and some burrow in mud or sand
  • The flat oral disc has a crown of tentacles
  • The slit-shaped mouth leads into a pharynx
  • The siphonoglyph is a ciliated groove that creates the water current into the pharynx
  • Currents bring carry oxygen and remove wastes to maintain fluid pressure for a hydrostatic skeleton
  • In danger, water is rapidly expelled through pores as the contracted anemone shrinks
• Zoantharian Corals
  • Scleractinia are also known as true or stony corals and are miniature sea anemones.
  • Secrete a limey skeletal cup with sclerosepta to project into the polyp rather than a pedal disc
• The surfaces form a sheet of living tissue that connects all gastrovascular cavities