Page 29 - 11-Bio-14 Transport

14. Transport eLearn.Punjab

Transportation in Planaria

The body of Planaria is flat, so the most of its cells are exposed to the outer water. Diffusion is the
process involved in the movement of materials into and out of the cells.

There is no special transport system in Planaria. The reasons are:

(i) The body of Planaria is flat, and provides greater surface area for the exchange of materials,
between the body and the environment.

(ii) Planaria is acoelomate i.e. there is no body cavity and the mesodermal layer or mesenchyme is
composed of loosely packed cells between ectoderm and endoderm. Whatever materials, such
as 02, diffuse in the ectoderm, pass to mesoderm cells and then diffuse into endoderm cells.
For the removal of wastes the same route is reversed. Intestinal caecae reach near almost every
cell of the body and digested food is provided to the cells by diffusion. The endoderm cells, can
also acquire food, water, dissolved minerals, and to some extent 02. and remove wastes into the
gut.

CIRCULATORY SYSTEM

In the body of larger and complex animals, there is very little exposed surface area to volume ratio.
Most of the cells are not exposed to the external environment directly and it becomes very difficult
to transport materials by simpl diffusion. Complex animals have evolved transport systems in the
form of blood vascular system or circulatory system.

Characteristics of Circulatory System

A circulatory system accounts for rapid mass flow of materials from one part of the body to the
other, where diffusion would be too slow.

There are three characteristics of a circulatory system.
(A) A circulatory fluid - the blood.
(B) A contractile pumping device - which may be the modified blood vessel or a heart.
(C) Tubes, which can transport the circulatory fluid (blood) to and from cells of the body These

tubes are the blood vessels. Materials must be exchanged between the circulatory fluid and
other body cells.

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