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EVOLUTION OF (LECTURE 1) Part I: Semester 2 CC3: Non 11-Coelomates Zooa-CC2-3-th Dr Adity Sarbajna, Department of Zoology, Surendranath College BODY PLAN

In the cnidarians, the space between the and tissue layers is filled with an acellular mesoglea. In the platyhelminthese () fills the space between mesoderm and the endoderm

The advantage of the body plan, is the superior pull muscles can make using solid mesoderm as a lever, versus gel-like mesoglea.

Problems:

1. Body organs cannot move freely but are embedded in solid mesoderm tissue.

2. 2. It is more difficult for materials to move from the gut to the body wall.

A solution to both problems is to have some sort of , called a coelom, in which the body organs lie bathed in body fluid. Movement of the body wall would not squeeze the organs, organs could grow without being pressed by the body wall, and diffusion of nutrients and wastes would be easier of Coelom lecture notes by Dr Adity Sarbajna 3/28/2020 COELOM

¨ The term coelom was suggested by Haeckel in 1872. A coelom (Greek: coel = hollow cavity) is a fluid-filled cavity between the alimentary canal and the body wall lined on all sides by mesoderm.

¨ Features of a True Coelom

¨ It is a secondary body cavity formed by the splitting of mesoderm during . ¨ It is also bounded on all sides by a definite coelomic epithelium. ¨ It consists of colorless coelomic fluid also the excretory organs open into it. ¨ Reproductive organs arise from its walls.

Evolution of Coelom lecture notes by Dr Adity Sarbajna 3/28/2020 Characteristics of coelom

¨Developmentally coelom arises as a split in the mesoderm which becomes bifurcated into two layers, a somatic layer lying next to the epidermis and a splanchnic layer around the endoderm. ¨Coelom becomes bounded on all sides by coelomic epithelium which secretes coelomic fluid. ¨The greater part of the coelom forms the perivisceral cavity or splanchnocoel. ¨It is a fluid-filled space inside which is lodged the viscera. ¨Because of this packing the viscera remains independent of the move•ments of the muscles of body wall. ¨In some higher part of the perivisceral cavity is kept separate to form restricted cavities whose coelomic nature can and only be realised if their developmental history is followed. ¨In all probabilities the ancestral coelomate animals had segmentally arranged mesodermal pouches. ¨From these pouches gametes were formed by the process of proliferation of the epithelial lining. ¨Later on, these pouches became modi•fied in structure and function. ¨The evolution of mesodermal pouches is evident in present day coelomates.

Evolution of Coelom lecture notes by Dr Adity Sarbajna 3/28/2020 Organization of based on coelom

¨ Acoelomata: Acoelomates do not have a coelom or body cavity. The space between the body wall and gut is filled up by and muscle fibres.

¨ Eg: Porifera, , Platyhelminthes and Nemertinea

Evolution of Coelom lecture notes by Dr Adity Sarbajna 3/28/2020 Pseudocoelomata:

¨ The space between the body wall and gut encloses a fluid filled cavity which is not lined by mesodermal epithelial cells. It is known as pseudocoel or false coelom and the animals possessing it are known as pseudocoelomates. ¨ Embryologically, the pseudocoelom is derived from the of the . The internal organs are free within the space, as there is no bounding the cavity ¨ Eg: phylum Nematoda, , , Rotifera, and Gastrotricha

Evolution of Coelom lecture notes by Dr Adity Sarbajna 3/28/2020 Coelomata:

¨ In rest of the bilateria, the body-cavity present between the body wall and gut is lined by a peritoneum. Peritoneum consists of epithelial cells derived by embryonic mesoderm. This type of cavity is known as true coelom and the animals possessing it are known as Eucoelomates ¨ Eg: phylum Annelida, Arthropoda, , Echinodermata, Hemichordata and Chordata. ¨ True coelomates are of the following types. ¤ Schizocoelomates ¤ Enterocoelomates

Evolution of Coelom lecture notes by Dr Adity Sarbajna 3/28/2020 Schizocoelomates

Schizocoelomates are true coelomates in which the body cavity originates by splitting of mesodermal tissue at the time of . This method of coelom formation is called schizocoelous (Greek: sch izo = split), and occurs in animals like , and mollusks. Sometimes the schizocoelom is filled with blood and is called haemocoel as in arthropods and mollucs.

Evolution of Coelom lecture notes by Dr Adity Sarbajna 3/28/2020 Enterocoelomates

Enterocoelomates: In most , such as chordates and , the coelom originates by out-pouching of the during gastrulation. Each pouch then expands and its mesoderm lines the gut on the inner side and body wall on the outer side. This method of coelom formation is called enterocoelous

Evolution of Coelom lecture notes by Dr Adity Sarbajna 3/28/2020 Significance of Coelom:

¨ The coelomic fluid content facilitates smooth transportation of particles or materials in solution. ¨ Coelom affords flexibility to the body and extends room for the movement of the gut which remains suspended. ¨ Gonads which develop from coelomic epithelium are housed in the cavity of the coelom. So also are the nephridial tubules, which connect the coelom to the exterior and in some cases allow the passage of eggs and sperms. ¨ The coelom filled with incompressible coelomic fluid acts as a hydrostatic and helps in locomotion

Evolution of Coelom lecture notes by Dr Adity Sarbajna 3/28/2020 Further reading:

Evolution of Coelom lecture notes by Dr Adity Sarbajna 3/28/2020