Animal Diversity: Sponges, Cnidarians, Molluscs, Annelids, Arthropods and Echinoderms
Animal Diversity: Key Invertebrate Groups
Sponges (Porifera)
Sponges have radial symmetry or are asymmetrical. Their body is a sack inside which an atrial cavity opens to the exterior through the osculum. The body wall contains pinacocytes and pores (ostia) through which water enters. Choanocytes (collar cells) create a continuous stream of water through the sponge. Sponges have a non-cellular intermediate layer (mesohyl or mesoglea) with amoebocytes that digest food, produce gametes, and produce spicules.
Sponges reproduce both asexually and sexually.
Types of Sponges
- Calcareous sponges: calcium carbonate spicules
- Hexactinellid sponges: silica spicules
- Demosponges: silica spicules and spongin fibers
Cnidarians
Cnidarians exist as polyps and medusae. Their epidermis contains cnidoblasts (stinging cells). The mesoglea contains amoeboid cells, and the gastrodermis has pseudopodia used to capture food.
Molluscs
Gastropods
Gastropods live on land, in marine and freshwater environments. They are asymmetrical and have a spiral shell and a large muscular foot for movement. They possess sensory tentacles with eyes and a radula, and breathe through gills or lungs. Fertilization is internal and development is external. Examples include snails, slugs, limpets, and sea slugs.
Bivalves
Bivalves live in seas or rivers and show bilateral symmetry. They have a shell made of two valves, and an burrowing foot. Their small head has no tentacles and no radula. They breathe through gills and fertilization is external. Examples include mussels and clams.
Cephalopods
Cephalopods are large marine molluscs with bilateral symmetry. They have an internal shell or none at all. Their foot is modified into tentacles, and their head is highly developed. They breathe through gills and fertilization is internal. Examples include cuttlefish, squid, octopuses, and nautiluses.
Annelids
Annelids have a segmented external body, and their internal tissues are repeated in each segment (metamerism). Main groups include:
- Polychaetes: marine worms
- Oligochaetes: earthworms
- Hirudinea: leeches
Arthropods
Main Characteristics
- A rigid exoskeleton made of chitin. To grow, they must molt the exoskeleton.
- A direct respiratory system that delivers oxygen to cells.
- Specialized sensory organs that are well developed.
- Complex behavior.
- Reduced intraspecific competition due to metamorphosis.
The body is divided into three regions: head, thorax, and abdomen.
Arthropod Groups
- Chelicerates: cephalothorax and abdomen without segmentation, with a pair of appendages called chelicerae.
- Mandibulates: head with jaws. Includes myriapods, insects, and crustaceans.
Types of Arthropods
- Arachnids: terrestrial, bilaterally symmetrical, with a separate cephalothorax and abdomen, no antennae, chelicerae, four pairs of legs, tracheal or book lungs, separate sexes, and no metamorphosis. Examples include spiders and scorpions.
- Crustaceans: aquatic, bilaterally symmetrical, with four pairs of antennae, chewing mouthparts, and variable legs. Examples include barnacles and crabs.
- Insects: terrestrial, bilaterally symmetrical, with a head, thorax, and abdomen, oral appendages, two antennae, three pairs of legs, tracheal respiration, and separate sexes. Examples include butterflies and wasps.
- Myriapods: terrestrial, bilaterally symmetrical, with two antennae, two jaws, numerous legs, and tracheal respiration. Examples include centipedes and millipedes.
Echinoderms
Echinoderms move using hundreds of tube feet (ambulacral feet) that operate through a water vascular system. Their skeleton is made of calcareous plates beneath the skin. Types include:
- Asteroids: sea stars
- Echinoids: sea urchins