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A Handbook of Biology

3. GËRMÏÑÅL LÅÝËRS (ËMBRÝØÑÏÇ LÅÝËRS)

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These are layers of embryo from which all the body organs are formed.

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Based on the number of germ layers, animals are of 2 types- Diploblastic

and Triploblastic.

Diploblastic animals: 2 germ layers- outer ectoderm and inner

endoderm. In between these layers, an undifferentiated jelly-like

layer called mesoglea is present. E.g. Porifera, Cnidaria & Ctenophora.

Triploblastic animals: 3 germ layers- Outer ectoderm, middle

mesoderm and inner endoderm. E.g. Platyhelminthes to

Chordata. (AIPMT 2010)

A

B

Mesoglea

Ectoderm

Endoderm

Diploblastic organisation

Endoderm

Mesoderm

Ectoderm

Triploblastic organisation

4. ÇØËLØM (BØDÝ ÇÅVÏTÝ)

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It is the cavity lined by mesoderm. It is

seen between body wall and gut wall.

Coelom separates the muscles of gut

and body wall.

Based on the nature of coelom, animals

are of 3 types:

(a) Acoelomate: No coelom. The space

between body wall and digestive

cavity is filled with matrix. E.g. Porifera

to Platyhelminthes.

(b) Pseudocoelomate:

False

coelom.

Here, the body cavity is not lined by mesoderm. Mesoderm is present

as scattered pouches in between the ectoderm and endoderm. E.g.

Aschelminthes. (AIPMT 2010)

(c) Coelomate: True coelom. Here, the coelom arises from the mesoderm.

Coelom is lined by peritoneal layer and filled with coelomic fluid.

E.g. Annelida to Chordata.

Coelom

(a)

(b)

(c)

Pseudocoelom