Major Non-Chordate Phyla and their characteristics

Phylum Porifera

  • Porifera is derived from the Latin words porous, meaning “pore or opening,” and -fer meaning “bearing.” So, Porifera means “bearing numerous pores or openings.” The organism of this phylum bears numerous pores which perforate the body wall.
  • Habitat: There are about 8000 species of Porifera, of which more than 98% are marine and fewer than 2% are freshwater. None of them are terrestrial. They remain attached to the substratum, I.e., sessile or non-motile.
  • Symmetry: They are amorphous and asymmetrical. However, some mono-oscular species (like Haliclona) are radially symmetrical.
  • Level of organization: They have many morphologically distinct types of cells. The cells function independently, i.e., the cells do not form tissue and have the cellular level of organization.
  • Body structure: Their body is a perforated bag-like structure. The space present in the body is called spongocoel. The spongocoel is lined with funnel-shaped flagellar cells called choanocytes or collar cells. The choanocytes generate and maintain seawater circulation within the sponge, capture small food particles and trap incoming sperm for fertilization. The outer body wall (epidermis) is made of pinacocytes. A non-living, non-cellular layer called mesohyl is present between the epidermis and choanocyte layer, which contains archaeocytes, support elements like calcareous or siliceous spicules (secreted by sclerocytes), collagenous protein fibers called spongin (secreted by spongocytes).
  • Water enters through Ostia, passes through the spongocoel, and is released through the osculum. Thus, Ostia, spongocoel and osculum form water canal system.
  • Reproduction: They are hermaphrodite (bisexual) and reproduce asexually and sexually. Asexual reproduction occurs by fragmentation. Many freshwater species and a few marine species reproduce by gemmule formation. The gemmule is a dormant structure formed of archaeocytes and the surrounding capsule. Under the appropriate conditions, the living cells leave the gemmule (hatch) through a narrow opening and differentiate into a functional sponge. They also reproduce asexually by bud formation. Sexual reproduction occurs by gamete formation and is internally fertilized in choanocytes where eggs are already present. The gametes (egg and sperm) are produced by archaeocytes or by modification of the choanocytes.
  • Development: The development is indirect and involves the amphiblastula larval stage.
  • Examples: Sycon (Scypha), Spongilla (Freshwater sponge), Euspongia (Bath sponge), Cliona (Boring sponges), Euplectella (Venus’s flower basket), etc.

Phylum Cnidaria (=Coelenterata)

  • The term Cnidaria is derived from the Greek word knidē, “nettle” from the stem of knizein, “to scratch scrape.” Coelenterata is also derived from the Greek words koilos + enteron meaning “hollow gut.” So, this phylum includes organisms having hollow guts and stinging thread.
  • Habitat: More than 99% of the Cnidarians are marine, and only about 0.2% are freshwater. They are sessile (polyp form) or free-living (medusa form).
  • Symmetry: They have radial
  • Level of organization: They have only two layers of living tissue ectoderm (epidermis) and endoderm (gastrodermis). Therefore, they are diploblastic. A gelatinous layer, the mesoglea, is present between these two layers, which contain cells from the ectoderm that play a role in digestion, nutrient transport, storage, wound repair, and antibacterial defense. They have a tissue level of organization.
  • All cnidarians have tentacles surrounding the mouth on the hypostome and only a single opening to the hollow gastro-vascular cavity.
  • All cnidarians secrete cnidae (nettle or stinging thread). The cnidae are secreted by cnidoblast cells which are discharged with explosive force for various functions like anchorage and capturing prey.
  • Digestion occurs intracellularly (in amoebocytes) or extracellularly.
  • Some cnidarians, like corals, have a skeleton composed of calcium carbonate.
  • They show an alternation of generation or metagenesis. Two different body forms, medusa (swimming form) and polyp (sessile form), alternate with each other.
  • Reproduction: It occurs by asexual and sexual means. The medusa form produces polyp sexually, and the polyp produces medusa asexually. Sexual reproduction happens by gamete formation in gonads which develop within the gastro-vascular cavity.
  • Cnidarians are primarily carnivorous, although some soft-coral species will also eat phytoplankton. In many reef-building (hermatypic) corals, individuals obtain additional nutrients through the photosynthesizing activities of unicellular algae living symbiotically in their tissues.
  • Examples: Physalia (Portuguese man-of-war), Adamsia (Sea anemone), Pennatula (Sea-pen), Gorgonia (Sea-fan), and Meandrina (Brain coral).

Phylum Ctenophora

  • Ctenophora is derived from the Greek ctene, “a comb.” They have eight rows of cilia fused to form ctene and spaced equally. The ctene extends from oral to aboral end. It is used for locomotion. For the presence of the comb, they are also called comb jellies.
  • Habitat: they are exclusively marine.
  • They possess radial symmetrydiploblastic organisms, and have tissue level of organization. They are transparent.
  • Digestion: Both extracellular and intracellular.
  • Bioluminescence (the property of a living organism to emit light) is well-marked in ctenophores.
  • Sexes are not separate; that is, they are hermaphrodites. Reproduction takes place only by sexual means. Fertilization is external with indirect
  • Examples: Pleurobrachia, Ctenoplana, Mertensia, Callianira

Phylum Platyhelminth

  • The term Platyhelminth is derived from the new Latin Platyhelmintha from Platy, meaning “flat,” and -helmins meaning “worm.” The members of this phylum are commonly called flatworms as they are dorso-ventrally flattened.
  • Habitat: Aquatic (marine and freshwater); most are parasitic, and many are free living.
  • All flatworms are acoelomatetriploblasticbilaterally symmetrical and have an organ-system level of organization.
  • Most platyhelminths have an incomplete digestive system (having only one opening serving as the mouth and anus). Parasitic forms lack an alimentary canal.
  • They lack specialized respiratory and circulatory systems, although very few species possess hemoglobin. Gas exchange is accomplished by simple diffusion across the body surface.
  • Hooks and/or suckers are present in the parasitic forms used to maintain their position within the host’s gut.
  • The body is externally covered with cilia or cuticles.
  • Excretion: Metabolic wastes are removed by diffusion across the general body surface. Additionally, they have flame cells (protonephridia) to remove waste products.
  • Reproduction: They are hermaphrodites and reproduce sexually by cross-fertilization. They also reproduce asexually by regeneration. The development occurs through many larval stages.
  • Nervous system: Brain ganglion is present in the head from which a pair of ventral nerve chords runs posteriorly. The ventral nerve chords are connected by transverse nerves giving it a ladder-like appearance.
  • Examples: Taenia (tapeworm), Fasciola (liver fluke), Planaria, Echinococcus,

Phylum Aschelminthes

  • The term Aschelminthes is derived from the Greek words askos, meaning “sac,” and helminth meaning “worm,” from the former belief that the members of this group contain fluid-filled sac. The body of the Aschelminthes is circular in cross-section, hence, the name
  • Habitat: They are aquatic or terrestrial, free-living, or parasitic in plants and animals.
  • Roundworms are triploblasticpseudocoelomatebilaterally symmetrical, and have an organ-system level of organization.
  • Aschelminths grow by increasing cell size rather than by increasing cell number. This phenomenon is called eutely.
  • Digestive system: The digestive system is complete with a distinct mouth, pharynx, intestine, and anus. The muscular pharynx helps the parasitic forms to suck blood from the host.
  • Excretory system: Excretory pores are present to remove metabolic wastes.
  • Reproduction: They have separate sexes (dioecious) and show sexual dimorphism (often females are longer than males). Fertilization is internal, and development may be direct or indirect.
  • Examples: Ascaris (Round Worm), Wuchereria (Filaria worm), Ancylostoma (Hookworm), Enterobius (pinworm), etc.

Phylum Annelida

  • The term Annelida is derived from the Latin anulus meaning “a ring.” Each segment is externally divided into many parts by rings, hence, the name Annelida.
  • Habitat: They are restricted to moist environments.
  • The annelids are triploblasticcoelomate, bilaterally symmetrical, and have an organ-system level of organization.
  • All adult annelids (except sipunculans) possess at least one pair of chitinous bristles, called setae, or chaetae, which help to grip the ground during locomotion. Polychaetes possess parapodia for swimming.
  • All are vermiform (worm-shaped), soft-bodied, reasonably circular in cross-section, and longer than they are wide.
  • They show metamerism or metameric segmentation. i.e., The serial repetition of segments and organ systems (skin, musculature, nervous, circulatory, reproductive, and excretory systems). The individual segments are separated to a large degree by septa (mesodermally derived peritoneal tissue).
  • Nervous system: Neural system consists of paired ganglia (brain) from which nerve cords arise and run ventrally. They bear ganglia in each segment and give off the segmental nerves.
  • Circulatory system: The circulatory system is closed (blood contained within vessels); The dorsal blood vessel carries blood anteriorly, and the ventral vessel carries blood posteriorly. Capillaries interconnect the dorsal and ventral blood vessels. Only a few (siboglinid polychaetes) have specialized hearts.
  • Gas exchange: They have a moist body surface and are required for gas exchange. The epidermis secretes a protective cuticle; the cuticle remains permeable to both water and gases. Hence, gas exchange takes place through the general body surface. They have oxygen-carrying blood pigments (Haemoglobin) found in the circulatory fluid of most annelids. In some other annelids, chlorocruorin and hemerythrin are also seen.
  • Excretion: Although some wastes are excreted across the general body surface, excretion occurs through structures called nephridia (“little kidneys”) open at both ends. This type of nephridium is called a metanephridium.
  • Reproduction: They are hermaphrodites with cross-fertilization. In many annelid species, nephridium plays a role in discharging gametes and urine. Some annelids (like Nereis) are dioecious. Fertilization may take place externally (oligochaetes) or internally (leeches)
  • Examples: Nereis, Pheretima (Earthworm), and Hirudinaria (Bloodsucking leech).

Phylum Arthropoda

  • Arthropoda is derived from the Greek words arthron, “a joint,” and podos, “feet.” In modern Latin, it means “those with jointed feet.” The phylum Arthropoda is the largest phylum of the animal kingdom, comprised of articulated invertebrates.
  • They have segmented, jointed, and hardened (sclerotized) chitinous exoskeleton produced by the epidermis, with intrinsic musculature between individual joints of appendages.
  • Habitat: They are present in all environments. They may be free-living, ectoparasite, or endoparasite.
  • They are triploblasticcoelomatebilaterally symmetrical, and have an organ-system level of organization.
  • They have metameric However, serial repetition of like segments is masked by fusion and modification of different body regions. The specialization of groups is called tagmatization. The fused pieces are called tagma or tagmata, which perform highly specialized functions. The three distinct tagmata are the head, thorax, and abdomen. The tagmata bear appendages.
  • Circulatory system: Open type circulatory system is present. The main body cavity hemocoel is also a part of the circulatory system. The blood leaves the heart through the closed blood vessels but enters the heart from the hemocoel through the pores in the heart (Ostia).
  • Respiratory system: Respiration occurs through the tracheal system (interconnected tubules). They may have book lungs (internalized book gills) in primitive arachnids open outside by the spiracles. Some other arthropod has book gills for gas exchange. The blood of many terrestrial species lacks respiratory pigment. However, hemocyanin (HCy) and sometimes hemoglobin (Hb) are found in others.
  • Sensory system: Simple photoreceptors called ocellus is present (such simple photoreceptor is also present in Platyhelminthes, Annelida, and Mollusca). Among insects and crustaceans, compound eyes (made up of many ommatidia) are also present in addition to the ocellus. They (except Chelicerates) have a pair of antennae (sensory in function). They may possess a statocyst (gravity sensor for balancing).
  • Nervous system: An elaborated ganglion called the brain is present. Behind the brain is a double ventral nerve with a pair of ganglia in each segment. They receive sensory innervation and give off motor nerves.
  • Reproduction: They are mostly dioecious (gonochoristic), but some sedentary and parasitic species are hermaphroditic. They reproduce by sexual reproduction. Fertilization is internal in most species, but external fertilization also occurs in some. Some insects, brachiopods, and copepods reproduce asexually by parthenogenesis (production of offspring from unfertilized eggs). They are mostly oviparous (egg-laying).
  • Excretory system: The major excretory organs are long, slender, blind-ending tubes called Malpighian tubules in the insects. Coxal glands are present in the class Merostomata and Arachnida. Antennal glands or green glands are present in the Crustaceans.
  • Development: The development may be direct or indirect. They may have one or more larval stages. Wingless species lack a larval stage. With the increasing size of an individual, they shed their cuticle. The removal of the cuticle is called
  • Examples: Spiders, Scorpions, Apis (Honey bee), Bombyx (Silkworm), Laccifer (Lac insect), Mosquitoes, Limulus (King crab), Termites, Beetles, etc.

Phylum Mollusca

  • The term Mollusca is derived from Latin Molluscus “thin-shelled,” or from Mollis “soft.”
  • Dorsal epithelium forms a mantle, which secretes calcareous spicules or one or more shells. The secretions of the mantle may form a pearl if a foreign particle (like sand) is trapped between the mantle and the shell’s inner surface.
  • Habitat: They are aquatic (marine and freshwater) and terrestrial.
  • They are triploblasticcoelomatebilaterally symmetrical, and have an organ-system level of organization.
  • The body is unsegmented, but the head, muscular foot, and visceral hump are present. The main body cavity is a hemocoel.
  • Digestive system: Cuticular band of teeth (radula) in the esophagus, used for feeding (not present or lost in the bivalves). The gut bears a digestive gland; the anus opens into the mantle cavity.
  • Ventral body wall muscles develop into a locomotory or clinging foot.
  • Respiratory system: A cavity between the mantle and the viscera called the mantle cavity, usually houses the comb-like gills known as ctenidia—an exit site for digestive, excretory, and reproductive systems. The ctenidia may have purely respiratory functions. Hemocyanin is dissolved in the blood, which increases affinity for oxygen.
  • Circulatory system: In cephalopods (octopus), the circulatory system is closed type; in others, it is an open type with blood flowing in the sinuses.
  • Nervous and sensory system: The brain is a simple ring around the esophagus; two longitudinal nerve cords arise. Sensory tentacles are usually present, and there may be eyes.
  • Many cephalopod species possess numerous photophores (light-producing organs).
  • Reproduction: They are usually gonochoristic, but some species are hermaphrodites. Fertilization of eggs is external (in the Scaphopoda) or internal (in terrestrial and freshwater species).
  • Development: They are oviparous with indirect development.
  • Excretory system: They have one or more pairs of nephridia. The urine is discharged into the mantle cavity through a renal pore (nephridiopore) and carried away by water currents.
  • Examples: Pila (Apple snail), Aplysia (Sea hare), Sepia (Cuttlefish), Loligo (Squid), Pinctada (Pearl oyster), Octopus (Devil fish), Dentalium (Tusk shell), and Chaetopleura (Chiton).

Phylum Echinodermata

  • Echinodermata is derived from the Greek ekhinos “sea urchin,” originally “porcupine, hedgehog” (see echidna) + derma “skin.” Their endoskeleton of calcareous ossicles produced as spines on the skin justifies its name.
  • Habitat: They are marine.
  • They are triploblasticcoelomateradially symmetrical (larva is bilaterally symmetrical), and have an organ-system level of organization.
  • Water vascular system: It is a series of fluid-filled canals (derived from one of three coelomic compartments) extending into tubular structures called podia, or tube feet. It helps in gas exchange, locomotion, excretion (echinoderms lack a specialized excretory system), chemoreception, and circulation.
  • Nervous system: They do not have a brain or ganglia. The nervous system is a diffuse network of three nerves condensed into nerve fibers.
  • They have mutable connective tissue or catch tissue which can change its stiffness in a fraction of a second.
  • Digestive system: It is complete with the mouth on the oral and anus on the aboral side.
  • Reproduction: They are generally gonochoristic and reproduce by sexual reproduction. Asexual reproduction is often encountered among asteroid and ophiuroid echinoderms. Fertilization is typically external, but it is internal in Concentricycloids (sea daisies).
  • Development involves free-living, bilaterally symmetrical larvae.
  • Examples: Asterias (Star fish), Antedon (Sea lily), Echinus (Sea urchin), Ophiura (Brittle star), Clypeaster (sea biscuits), Mellita (sand dollars), and Cucumaria (Sea cucumber).

Phylum Hemichordata

  • The term Hemichordata is derived from the Greek words hemi, half, + chorda, string,cord. They are previously considered as a subphylum within the phylum chordata because they possess gill slits, a rudimentary notochord like structure (stomochord), and a dorsa nerve cord. Later, it is found that their “notochord” is not homologous to the chordate notochord, it is an evagination of their mouth cavity and so the hemichordates are considered a separate phylum.
  • Habitat: All are marine. They are bottom dwellers, live usually in shallow waters. Some are colonial and live in secreted tubes. Many are sedentary or sessile.
  • Size: The length ranges from 5 mm to 14 mm in the class Pterobranchia (pterobranchs) and 20 mm to 2.5 m in the class Enteropneusta (acorn worms).
  • Body structure: The body is covered by mucus; divided into a tongue like proboscis, a short collar, and a long
  • A row of gill pores extends dorsolaterally on each side of the trunk just behind the collar. These gill pores open from a series of gill chambers that in turn connect with a series of gill slits in the sides of the pharynx. The primary function of these structures is not respiration, but food gathering
  • They are triploblasticcoelomatebilaterally symmetrical, and have an organ-system level of organization.
  • Reproduction: They are dioecious. Frtilization is external and the development is indirect.
  • Examples: Balanoglossus, Saccoglossus, Ptychodera, Glossobalanus, and Harrimania.

References:

  • Ereskovsky, A. V. (2019). In search of the ancestral organization and phylotypic stage of Porifera. Russian Journal of Developmental Biology50(6), 317–324. https://doi.org/10.1134/s1062360419060031
  • Pechenik, J. A. (2014). Biology of the invertebrates(7th ed.). McGraw Hill Higher Education.
  • Moore, J. (2006). An introduction to the invertebrates(2nd ed.). Cambridge University Press.
  • Hickman, C. P., Roberts, L. S., & Larson, A. L. (2002). Animal Diversity(3rd ed.). McGraw Hill Higher Education.
  • Etymonline – Online Etymology Dictionary
  • Dictionary by Merriam-Webster: America’s most-trusted online dictionary

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