Key Characteristics of Major Invertebrate Phyla

Major Invertebrate Phyla

Mollusks

Mollusks are soft-bodied animals whose anatomical plan typically includes a head, a foot, a visceral mass, and a mantle, often secreting an external shell. Notable examples include gastropods, bivalves (characterized by two flexible shells united by a hinge), and cephalopods, marine predators whose foot has evolved into tentacles.

Arthropods

Arthropods are the most abundant animals on the planet. Their body is typically divided into segments, forming a head with sensory and feeding structures, a thorax, and an abdomen. They periodically need to shed their chitinous exoskeleton, which is rigid and does not expand as the animal grows. The main classes are:

• Insects: Possess three pairs of legs and one or two pairs of wings.
• Arachnids: Have eight walking legs and are almost all carnivores.
• Crustaceans: Live in water and are characterized by each of their segments being equipped with a pair of appendages that perform diverse functions.

Echinoderms

Echinoderms are animals whose adult deuterostome stages have radial symmetry. They can be predators, scavengers, or filter feeders. Beneath their skin, they possess a calcareous endoskeleton that projects outwards as bumps or spines. They have a unique characteristic: an aquiferous system, a pressure system of water-filled tubes involved in locomotion, respiration, and food capture.

Simpler Invertebrate Phyla

Porifera (Sponges)

Porifera are truly ancient animals. They have a body riddled with holes, called pores, and an opening at the top end, the osculum. Their cells retain independence and do not form true tissues. They live in water. Some sponges have an internal skeleton.

Cnidaria (Jellyfish, Corals)

Cnidaria are aquatic animals with a sac-like body and radial symmetry, possessing a single opening that serves as both mouth and anus. Their body plan is diploblastic and has remained at the gastrula stage. There are different types, but all are variations of two basic forms: the tubular polyp, which lives attached to rocks and has tentacles to catch prey, and the medusa (jellyfish), which drifts with ocean currents.

Worm-like Invertebrate Phyla: Triploblastic Animals

Platyhelminthes (Flatworms)

Platyhelminthes are flatworms, lacking a coelom. They are triploblastic animals: they have a skin, internal organs, and some have a digestive tube. Although there are free-living flatworms, many are parasites.

Nematodes (Roundworms)

Nematodes are thin, round, whitish worms with a pseudocoelom filled with fluid that acts as a hydrostatic skeleton.

Annelids (Segmented Worms)

Annelids are characterized by the repetition of body segments, which facilitates locomotion, allowing them to perform more complex movements than unsegmented worms. This phylum includes three classes:

• Polychaetes: Marine worms with many setae that swim or burrow in the sand.
• Oligochaetes: With few setae, like earthworms.
• Hirudinea: Or leeches.