Good morning, all.

Day 2 Summary

Coelom plays a crucial role in the organization and functionality of animal bodies, especially in more advanced organisms such as Organ Development & Compartmentalization, Movement & Flexibility, Circulatory System Efficiency, Shock Absorption & Protection, and Space for Complex Structures. Based on type of Coelom, animals are categorized as 

Acoelomates - Body cavity is entirely filled with tissue as in Platyhelminthes (flat worms)

Pseudocoelomates - Mesoderm occurs as scattered pouches as in Aschelminthes (round worms)

True Coelomates - Body cavity lined by mesoderm as in Annelida and higher phyla

Again, true coelomates can be Schizocoelomates - coelom formed by splitting of mesoderm (Annelida, Arthropoda and Mollusca) or Enterocoelomates - coelom formed from out pouches of enteron. (Echinodermata, Hemichordata and Chordata). 

** All body cavities are not coeloms as they are all not lined by mesoderm. Coelom is a body cavity lined by mesoderm.

** In Porifera, body cavity is called Spongocoel and in Coelenterata, it is called Gastrovascular cavity.

** In organisms with open kind of circulatory system, body cavity is filled with blood and is called Haemocoel. 

In metameric segmentation (metamerism or true segmentation), body id divided into segments with serial repetition of at least some organs.

Complete segmentation (both internal and external) is found in Annelida whereas we can find external segmentation in Arthropoda and internal segmentation in Chordata.

** Pseudo segmentation is found in Tapeworms, the members belonging to phylum Platyhelminthes.

All Chordates shows presence of Notochord - a rod like skeletal element derived from mesoderm, during their embryonic development. In some members of Chordates, it is replaced by bony Vertebral column, and they are called Vertebrates.

"All Vertebrates are Chordates, but all Chordates are not Vertebrates"