Feeding, Digestion and Assimilation in Animals

248 Pages·1972·18.08 MB·English

by  J. B. Jennings (auth.)

Feeding, Digestion
and Assimilation in Animals
Macmillan Studies in Comparative Zoology
General Editors:
J. B. Jennings and P. J. Mill, University of Leeds
Each book in this series will discuss an aspect of modern zoology in
a broad comparative fashion. In an age of increasing specialisation
the editors feel that by illustrating the relevance of zoological
principles in a general context this approach has an important role
to play. As well as using a wide range of representative examples,
each book will also deal with its subject from a number of different
viewpoints, drawing its evidence from morphology, physiology
and biochemistry. In this way the student can build up a complete
picture of a particular zoological feature or process and gain an
idea of its significance in a wide range of animals.
Feeding,
Digestion and Assimilation
in Animals
J. B. JENNINGS
Reader in Invertebrate Zoology,
The University of Leeds
MACMILLAN EDUCATION
© J. B. Jennings 1965, 1972
All rights reserved. No part of this publication may be
reproduced or transmitted, in any form or by any means,
without permission
First published 1965 by The Pergamon Press
Second edition 1972
Published by
THE MACMILLAN PRESS LTD
London and Basingstoke
Associated companies in New rork Toronto
Melbourne Dublin Johannesburg and Madras
SBN 333 13391 9 (board)
333 13622 5 (paper)
ISBN 978-0-333-13622-5 ISBN 978-1-349-15482-1 (eBook)
DOI 10.1007/978-1-349-15482-1
Library of Congress catalog card number 72-90021
Preface
THIS book is intended as a general introduction to the study of
animal nutrition. The subject is approached from a zoological
rather than a biochemical viewpoint and consequently as much
attention is given to the various structures and organs concerned
with the capture, ingestion, digestion and assimilation of food by
animals as there is to the more chemical aspects of these processes.
Wherever possible specific examples are used to illustrate
important points in the text and these have been carefully
selected to ensure that the majority will be familiar to any reader
with an elementary knowledge of Zoology. Chapter l deals with
the essential components of the animal's diet, their properties
and the uses to which they are put. Chapters 2 and 3 review
the various means used by animals to obtain food from their
environment, and the different types of feeding mechanisms
described are illustrated by examples drawn from the major
phyla of the animal kingdom. Chapter 4 describes the variety
of alimentary systems found in animals, and Chapter 5 deals
with the fundamental features of the process of digestion, the
properties of the digestive enzymes and the sequence in which
these operate. Finally, Chapter 6 describes in detail feeding
and digestion in a restricted number of animals, which includes
representatives from the Protozoa, the acoelomate phyla, the
Arthropoda, the fish and the mammals.
A nuniber of the text figures are reproduced from other works
and I wish to thank theW. B. Saunders Company, the McGraw
Hill Book Co. Inc., Academic Press Inc., Methuen & Co. Ltd.,
J.
M. Dent & Sons Ltd., Macmillan & Co. Ltd., Oxford Univer
sity Press, Robert Hale Ltd., Cambridge University Press and
the Editorial Boards of the Biological Bulletin and the Journal of
VI Preface
Cell Biology for their kindness in granting permission for such
reproduction. Acknowledgements to the individual authors
concerned are given at the appropriate point in the text and a
full reference to each source is given in the bibliography.
June 1964 ]. B.].
Preface to the Second Edition
IN this second edition much of the original text dealing with the
functional morphology of animal feeding mechanisms and alimen
tary systems, the chemical nature of foodstuffs and basic digestive
physiology, remains unchanged. Many new line drawings and
photographs have been added, however, in the hope that they
will convey more information than those they replace and illustrate
the text more effectively. Other additions include new material on
the digestive physiology of lower invertebrates and the mammal,
and an outline classification of the animal kingdom. The latter
shows the systematic position of those orders and genera cited in
the text in connection with some particular aspect of a feeding
mechanism, alimentary system or digestive process and is included
to demonstrate the relationships of the various types of animals
cited and to facilitate, therefore, the comparative study of animal
nutrition which is the central theme of the book.
April 1972 ]. B.].
Contents
1. Essential Components of the Diet
2. Animal Feeding Mechanisms. I 18
3. Animal Feeding Mechanisms. II 71
4. Alimentary Systems 94
5. Digestion 144
6. Digestion and Absorption in Selected Animal Types 171
APPENDIX: An outline classification of the Animal
Kingdom, showing the systematic position of Orders
and genera cited in the text 215
BIBLIOGRAPHY 229
INDEX 235
1
Essential Components of
the Diet
ANIMALS, in common with all other living organisms, maintain
themselves by taking in materials from the external environment
and incorporating at least a proportion of these into their own
structure. The substances obtained from the environment,
however, must satisfy two basic requirements if they are to form
an adequate diet which allows the animal to develop to its full
potential.
First, they must provide the raw materials necessary for all
the synthetic processes of the body. The animal is constantly
synthesizing new materials which are needed for its growth to
the size and shape characteristic of the particular species, for the
regeneration of ageing or damaged tissues, the production of
germ cells and, often, for the provision of a food supply for the
next generation until this is capable of independent existence as
either a larval or juvenile form.
Secondly, the food materials must supply the energy needed
to carry out these processes and all the other normal activities
of the animal such as locomotion, osmoregulation and excretion.
To meet these requirements the diet, whatever its form or
origins, must possess the following components:
Proteins Water
Carbohydrates Inorganic salts
Lipids Vitamins
A2
2 Feeding, Digestion and Assimilation in Animals
PROTEINS
The proteins are complex organic compounds contammg
carbon, hydrogen, oxygen, nitrogen and, in many cases, sulphur
and phosphorus. They form the structural basis of the soft parts
of the animal body and consequently the protein content of the
diet is used primarily for synthesis, but under certain circum
stances it may be used also as a source of energy.
The proteins are built up from simpler compounds, the
amino acids. The amino acid molecule contains both a car
boxyl (-COOH) and an amino (-NH2) group and has the
general formula:
R
I Where R represents an aliphatic,
H-C-NH aromatic, heterocyclic or other
2
I radical.
COOH
The presence of both a carboxyl and an amino group enables
individual molecules to combine with other molecules of either
the same or different amino acids and as a result several different
amino acids may enter into the structure of any one protein. The
union between two amino acid molecules is effected by the
carboxyl group of one linking with the amino group of another
to form a more complex molecule called a dipeptide. The
reaction is termed a condensation, since one molecule of water
is split off, and the linkage formed between the carboxyl and
amino groups is a peptide bond.
This process can be illustrated by reference to the simple
amino acid aminoacetic acid or glycine. In this substance R of
the general formula given above is a hydrogen atom.
H
I
cH2-NH[fi·····Haioc CH -N-C=0 + H20
I :. ................ I 2
.! I I
COOH CH2·-NH2 COOH CH2-NH2
glycine glycine glycyl-glycine
(dipeptide)