Page 2 - 2.Lesson Notes-Animalia 1

Diploblastic and Triploblastic Organisation

Based on the number of embryonic layers, animals are classified into two-
diploblastic and triploblastic animals.

• Diploblastic animals: In diploblastic animals, the cells form two embryonic layers,
i.e., an outer layer- ectoderm and an inner layer – endoderm, e.g. Phylum
Coelenterata.

• Triploblastic animals: In triploblastic animals, cells are arranged in three
embryonic layers- ectoderm, endoderm and a middle layer – mesoderm. Animals
belong to Phyla Platyhelminthes to Chordata have a triploblastic organisation.

Coelom

• Coelom is a key feature for classification.

• Coelom is a cavity between the body wall and gut wall, lined by mesoderm.

• Depending on presence/absence of coelom, animals have been classified into
three categories.

Coelomate, Pseudocoelomate and Acoelomate.

Porifera

• This phylum consists of the sponges.
• They are mostly marine, very few are freshwater.

• All the animals are sessile (fixed in one place).
• Cells are loosely arranged (cellular grade of the organization).
• Animals are diploblastic with outer ectoderm and inner endoderm held with
jelly-like mesoglea.
• Exoskeleton in the form of spicules made up of silica or calcium carbonate.
• Sponges have pores all over the body. Body pores are called Ostia.
• Water from outside enters the body through Ostia and leaves through one
large opening called the osculum.

Coelenterata

• These animals are aquatic, mostly marine.
• They are solitary or colonial.
• Animals are radially symmetrical.