These triploblastic and bilaterally symmetric organisms can have 3 different body plans:
i. Worm-like: These types of protostomes are characterized by a cavity inside their body that is well developed and is known as coelom that helps in the circulation of body fluids and functions like a hydrostatic skeleton that employs fluid pressure for movements.
Based on their feeding system they can be further classified:
a. Spoon worms or Echiura utilize a structure known as proboscis to acquire and move food. They are present in front of the mouth region and trap food and then using cilia move it to the mouth region.
b. Penis worms or Pripulida are characterized by the presence of a toothed throat that can be extended inside out to capture the prey and once the prey is captured they retract its back for consumption.
c. Ribbon worms or Nemertea employ proboscis that is present inside the body above the gut. The proboscis has a barbed tip and it is extended to capture prey by entangling it or by using venom.
ii. Arthropod Body Plans: Arthropods have segmented bodies that have 3 parts that include head, thorax, and abdomen. They comprise crustaceans, all insects, myriapods, and arachnids. They also have jointed appendages and are the most abundant taxonomic group.
They are named after the presence of jointed limbs and have a chitinous exoskeleton that also contains sclerotin proteins that harden them. The muscles are attached directly to the exoskeleton for movement. The coelom is reduced and is involved in providing space for organs and the circulation of fluids.
iii. Mollusk Body Plan: These marine groups comprises Chitons, Bivalves, Gastropods, and Cephalopods. Their body plan has 3 major parts:
a. The foot: this is responsible for movement and is constitutes a large muscle at the base of the animal.
b. The visceral mass: it contains external gills and internal organs. Thye help foot in movement.
c. The mantle: forms the layer that envelops visceral mass and secretes calcium carbonate to form shells.Â