AP BIOLOGY

Detailed explanation-1: -Organisms in phylum Platyhelminthes and Aschelminthes are also triploblastic but they do not have a true coelom. Tapeworm and Planaria are flatworms and are a part of phylum Platyhelminthes. They are acoelomates, i.e., body cavity is absent.

Detailed explanation-2: -Flatworms (Platyhelminthes) are acoelomates i.e. they lack a coelom. Molluscs and insects (Arthropoda) are coelomates as they have a true coelom derived from the germ layer mesoderm. Roundworms (Aschelminthes) are pseudocoelomates as the coelom is not derived from the mesoderm.

Detailed explanation-3: -Spongilla, Sea anemone, Planaria, Liver fluke, Wuchereria, Ascaris, Nereis, Earthworm, Scorpion, Birds, Fishes, Horse.

Detailed explanation-4: -Flatworms, lacking a coelom, were traditionally thought to represent leftovers from the early days of animal evolution, before any animals had a coelom. The author suggests that modern flatworms descended from a coelomate ancestor, which lost its coelom (and its anus!) through the course of evolution.

Detailed explanation-5: -The true coelom is a body cavity formed during embryo development from the three germinal layers. The body cavity meaning a fluid filled space that can accomodate organs. The coelom is lined by mesodermal epithelium cells. Presence or absence of coelom is one of the criteria for classifying animals.