Lesson 3 of 12 · 10 min
Coelom, segmentation and notochord
NCERT §4.1.4–§4.1.6, Figure 4.4
Ammu's teacher slices a boiled prawn, a roundworm specimen and a preserved earthworm across the middle. In the earthworm, a fluid-filled gap sits between the gut and the body wall. In the flatworm there is no gap at all.
The lesson in notes
In short
Classification leans heavily on the body cavity: is there a space between the gut and the body wall, and what lines it?
A coelom is a body cavity lined by mesoderm. Coelomates: annelids, molluscs, arthropods, echinoderms, hemichordates and chordates.
In a pseudocoelom the cavity is not lined by mesoderm; the mesoderm lies as scattered pouches between ectoderm and endoderm. Aschelminthes are pseudocoelomates.
Acoelomates have no body cavity at all; Platyhelminthes are the example.
Metameric segmentation (metamerism): the body is divided externally and internally into segments, with at least some organs repeated in series, as in the earthworm.
The notochord is a rod-like structure of mesodermal origin formed on the dorsal side of the embryo. Animals that form it are chordates; those that do not (Porifera to Echinodermata) are non-chordates.
Figure 4.4 sorts the phyla step by step: cellular level (Porifera) versus tissue or higher; radial versus bilateral symmetry; then no coelom, pseudocoelom or true coelom; then segmented or not; then notochord or not.