Animal Diversity:
(Non-Chordates)
Phylum : Metazoa
Ranjana Saxena
Associate Professor,
Department of Zoology,
Dyal Singh College,
University of Delhi
Delhi
e-mail: ranjanasaxena1957@gmail.com
24th September 2007
CONTENT:
INTRODUCTION
SYMMETRY
Asymmetry
Radial symmetry
Bilateral symmetry
Spherical symmetry
COELOM
Introduction
Functions of coelom
Origin of coelom
Evolution of coelom
METAMERISM
General account
Origin and evolution of metamerism
Significance of metamerism
ORIGIN OF METAZOA
BIBLIOGRAPHY
METAZOA
INTRODUCTION:
Metazoans may be defined as motile, multicellular, heterotrophic organisms that develop
from embryos. However, metazoans like sponges and some cnidarians have become sessile,
but they still have motile larvae in their life-cycle.
The differentiation of cells is a distinctive feature of metazoans. Their body cells are
generally differentiated into tissues and organs (exception being the poriferans which have a
cellular grade of organization and the cnidarians that have a tissue grade of organization).
The metazoans are diploid organisms in which the haploid gametes are formed by meiosis.
The gametes are produced within multicellular sex organs.
There is a great diversity in metazoans and according to the most widely accepted groupings
there are 29 phyla of which only one, viz. the chordata, contains animals which are not
invertebrates.
The important features on the basis of which the metazoan hierarchy is decided are:
symmetry, coelom and metamerism.
SYMMETRY:
Division of the body into equal parts by lines or planes is called symmetry. Theoritically
there are four types of symmetrical patterns (Fig. 1) and the animals can show any one of
them. The four symmetrical patterns are:
¾ Asymmetry
¾ Radial symmetry
¾ Spherical symmetry
¾ Bilateral symmetry
ASYMMETRY:
An animal cannot be divided into similar halves by sections passing through any plane. These
animals appear to be so highly irregular in shape that they lack symmetry of any kind e.g.
Amoeba and among metazoans some sponges (Fig.1).
RADIAL SYMMETRY:
Certain animals have a cylindrical body. One end of their long axis that bears the mouth is
called the oral end, while the opposite end is the aboral end e.g. Cnidarians (Hydra, Obelia
etc.). All planes passing through the centre are called radial planes. If the body is cut along
any radial plane, it results in two similar halves or antimeres and the symmetry is called
radial symmetry (Fig.1). Radial symmetry occurs when a body is constructed around a
central (oral-aboral) axis.
Biradial symmetry is a variation of radial symmetry. In this, there are only two planes of
symmetry: one through the longitudinal and saggital axis and the other through the
longitudinal and at right angles to the saggital axis that divides the animal into similar halves.
Each half of the body is identical to the opposite side e.g. sea- anemone, and comb jellies.
The radial as well as the biradial symmetry is best suited for floating life.
BILATERAL SYMMETRY:
In higher animals, the longitudinal axis of the body runs from the anterior to the posterior
end and the body parts are arranged similarly on either side of this axis so that the right and
left halves are mirror images of each other (Fig.1). Thus, there is a single plane (the median
longitudinal plane) through which the body can be divided into two similar halves. Bilateral
symmetry is a characteristic of active, crawling or swimming animals e.g. all higher
invertebrates and vertebrates. The first phylum of animal kingdom to exhibit bilateral
symmetry is phylum Platyhelminthes.
SPHERICAL SYMMETRY:
This type of symmetry is found in animals whose body is in the form of a sphere ( Fig.1). All
planes that pass through the centre will cut the body into similar halves e.g. Volvox and
Radiolaria. This type of symmetry is suited for free- floating forms or animals showing
rolling movements with almost all points on the surface having an equal opportunity to
perceive the stimulus.
The protistans show all four types of symmetry while the metazoans show only two, viz.
radial and bilateral symmetry.
The type of symmetry is related to the kind of life an animal leads and their needs in relation
to the type of environment in which they live. Sessile or free floating animals require an all
round knowledge of the environment and are thus radially symmetrical. The sense organs are
peripherally present in them i.e. equally distributed all around the edge of the animal. This
enables them to receive stimuli equally from all directions. On the other hand, the bilaterally
symmetrical animals move in a particular direction and their sense organs are crowded at the
end which reaches the new environment first. This has led to cephalization, i.e.
differentiation of a distinct specialized head at the anterior end and formation of anterioposterior axis. The opposite end is the posterior or the tail end. This concentration of sense
organs at the anterior end permits more rapid detection of and reaction to stimulus. These
bilaterally symmetrical animals also have an upper or dorsal surface and a ventral surface
facing the substratum.
Thus, on this basis the Eumetazoa was divided into two super- groups:
¾ Radiata which includes Phylum Cnidaria and Phylum Ctenophora.
¾ Bilateria that includes all other Eumetazoa phylum.
COELOM
INTRODUCTION:
The term coelom was suggested by Haekel in 1872. Coelom is defined as a fluid filled
perivisceral cavity lined by epithelial cells derived from the embryonic mesoderm.
Depending on the presence or absence of coelom, the division bilateria shows three grades of
organization:
1. Among the lowest bilaterians (triploblastic), the space between the bodywall and gut
is filled up by parenchymatous cells derived from the mesoderm. Such animals are
known as Acoelomates (Fig. 2) e.g. phylum Platyhelminthes and Nemertini.
2. In some bilaterians, the space between the bodywall 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 (Fig. 2).
Embryologically, the pseudocoelom is derived from the blastocoel of the embryo. The
internal organs are free within the space, as there is no peritoneum bounding the
cavity e.g. phylum Nematoda, Acanthocephala, Entoprocta, Rotifera, and
Gastrotricha.
3. In rest of the bilateria, the body-cavity present between the bodywall and gut is lined
by a peritoneum (epithelial cells derived by embryonic mesoderm). This type of
cavity is known as true coelom and the animals possessing it are known as
Eucoelomates (Fig. 2) e.g. phylum Annelida, Arthropoda, Mollusca, Echinodermata,
Hemichordata and Chordata.
The coelom is lined externally by a parietal epithelium (parietal peritoneum) and
internally by a visceral or splanchnic epithelium (visceral peritoneum). The peritoneum
surrounds all the internal (visceral) organs, including the alimentary canal. In other words, all
the internal organs are retroperitoneal i.e. they are present behind the peritoneum. Thus,
coelom is also called as the perivisceral cavity.
The coelom contains coelomic fluid and constitutes one or more perivisceral spaces around
the heart, alimentary canal and other organs. Coelom is divided into fluid filled compartments
by transverse partition called septa which extend from the annuli of the bodywall to the
alimentary canal. The wall of the septa is perforated, through which coelomic fluid
communicates from one compartment to another. The coelom communicates with the
exterior by two sets of ducts namely nephridia and coelomoduct. Nephridia are ectodermal
tubes that remove water and excretory waste while coelomoduct are mesodermal tubes
which usually open into the coelom at one end while the other end communicates with the
exterior.
However, in some coelomates like arthropods and molluscs, the coelom is reduced or absent
in adult, but is present in the embryo. The space containing blood and lymph is in the form of
tubes through which fluid is made to circulate by the heart, and this space is called a
haemocoel. The visceral organs are surrounded by the haemocoel. The haemocoel reduces
the coelom to small cavities around the heart (pericardium), gonads and kidney. In these
animals, the haemocoel is called the primary body-cavity while the coelom is called the
secondary body cavity.
FUNCTIONS OF COELOM:
1.
Coelom surrounds the internal organs and thus protect them from external shocks.
2.
It serves as a hydrostatic skeleton to assist in locomotion and maintaining shape of
the body.
3.
It provides flexibility to the body. The internal organs in coelomate animals
become large and are able to perform movements freely of their own.
4.
It helps in removing excretory waste from the body.
5.
Helps in the transportation of gases and nutritive materials from one part of the
body to the other.
6.
In some animals, it keeps the body wall moist to facilitate respiration and to
destroy bacteria and other harmful micro-organisms.
7.
Acts as a site for gamete maturation and brooding of embryos.
ORIGIN OF COELOM:
According to embryological studies, there are two basic ways in which the coelom can
arise.
1. SCHIZOCOEL HYPOTHESIS: (Gr. Schizen: to split; koilos: hollow)
The coelom arises from the splitting up of the mesoderm (Fig.3) e.g. phylum Annelida,
Arthropoda, and Mollusca.
2. ENTEROCOEL HYPOTHESIS: (Gr. Enteron:gut; koilos: hollow)
Coelom arises by the outpocketing of the archenteron in the initial separation of the
mesoderm from the endoderm. In simple words, pouches arise from the archenteron, which
fuse together to form a coelom (Fig.3) e.g. phylum Echinodermata, Hemichordata and
Chordata.
EVOLUTION OF COELOM:
The evolution of coelom is uncertain. Many theories have been put forward but only four of
them are of importance.
1. SCHIZOCOEL THEORY: According to this theory, the coelomates evolved from
an ancestral acoelomate like flatworms by hollowing out of the parenchymal cells of
the mesenchyme. Some of these cells would form the peritoneum. According to this
theory, the acoelomate body plan is primary and ancestral to the coelomate plan. The
acoelomate flatworms, thus, form the basic group in the evolution of bilateral animals.
The schizocoel mode of coelom formation in the embryonic development of annelids
and molluscs would claim as supporting evidence of this theory. However, the
evolution of coelom is not related to gonads or endodermal pouches of lower forms.
2. ENTEROCOEL THEORY: This theory was first proposed by Lankester (1875)
and later modified by Sedgwick (1884) and Hartman (1963). This theory argues that
the coelom evolved from the gastric pouches of some cnidarian ancestors, such as
anthozoans, or scyphozoans. These gastric pouches separated out from the main
gastric cavity to form the coelomic pouches. This theory proposes that all bilateral
animals are basically coelomate and that acoelomate forms like flatworms are
secondarily derived from coelomate ancestors by the loss of the cavity. The
enterocoelous mode of coelom formation in the embryogeny of echinoderms,
hemichordates and chordates is the main supporting evidence of this theory.
The enterocoelous theory has never gained much importance because it is difficult to
postulate functional steps that have led to a change from coelomate to acoelomate and
from bilateral to radial symmetry. In contrast, the ancestral position of flatworms
among the bilateria and the primitive nature of the acoelomate body plan has been
widely accepted. Moreover, the gastric pouches occur in the more advanced
cnidarians such as Anthozoa and Scyphozoa, which are not suitable for ancestral
types. Also, the enterocoelous theory of coelom origin is associated with the origin of
metameric segmentation, an association imposing severe restrictions on the theory.
3. GONOCOEL THEORY: The theory was first enunciated by Bergh in 1885. It
regards the coelom as the cavity of an expanded gonad and its origin is based on the
common association between the gonads and the coelomic epithelium. Bergh believed
that coelom initially arose in a segmented condition by enlargement and cavitation of
the gonads after the release of gametes (Fig.4).
One of the main drawback of this theory is that it closely links the origin of coelom
with the origin of metameric segmentation and hence it is difficult to account for the
unsegmented coelomates. There is no evidence that the unsegmented coelomates have
originated from the segmented ancestors. This theory has no embryological support
because gonads donot arise before the coelom.
4. NEPHROCOEL THEORY: Proposed by Lankester in 1874. The coelom originated
as an expanded nephridium. This theory however, was never taken seriously because
protonephridia has been described in coelomates and also excretory organs are absent
in some coelomates like echinoderms.
METAMERISM
Gr. Meta: after ; Mere: part
The serial segmentation of the body along an anterior- posterior axis is called metamerism.
Each segment is known as a metamere or somite. True metamerism occurs in annelids,
arthropods and most chordates. A typical segmented animal has a specialized anterior head or
acron and a posterior pygidium or telson with varying number of intermediate segments.
Metamerism is always limited to the trunk region of the body. The head bearing the brain and
sense organs and the pygidium is not a metamere. The growth region in these metamerically
segmented animals is localized to the region in front of the pygidium. Thus, new segments in
them arise from just in front of the pygidium while the oldest ones are present just behind the
head. In these metamerically segmented animals the appendages, nerves, ganglia, muscles,
nephridia, gonads, coelomic cavities and transverse blood vessels are repeated within each
segment. The segments are also marked by constriction of the body wall musculature.
The ideal condition of exactly similar segments is known as homonomous segmentation. It is
not present in any existing animals because a few of the anterior segments are always
specialized to form the head. The closest approach to homonomous condition is seen in
Polychaetes, in which all the segments in between the head and pygidium are similar (Fig.5).
Generally, the segments of particular region are lost or fused together and modified for
special functions. This is known as heteronomous segmentation. The anterior segments are
united together to form the head that bears the nervous, sensory, and feeding structures, a
process called cephalization. The segments of the other region are also specialized resulting
in differentiated parts of the body. The adults of arthropods and chordates show incomplete
or heteronomous metamerism.
There are two types of segmentation:
1. Mesodermal: beginning in the mesoderm and proceeding outwards e.g. annelids,
arthropods, and chordates.
2. Superficial: begins externally from the circular surface, then proceeds inwards, often
involving only the body wall musculature e.g. Acanthocephala.
The kind of segmentation encountered in tapeworms is mesodermal but differs from true
segmentation as seen in annelids. In annelids and arthropods new segments are formed at the
posterior end while in tapeworm the neck is the zone of proliferation.
It is believed that metamerism has evolved twice in the animal kingdom:
1. Annelid-Arthropods. In annelids as an adaptation for burrowing.
2. Chordates. In chordates as an adaptation for undulatory swimming.
ORIGIN AND EVOLUTION OF METAMERISM:
Various hypothesis have been proposed for the origin of metamerism but none of them are
acceptable in the absence of convincing evidence. The main theories are:
¾ PSEUDOMETAMERISM THEORY: According to this theory, metamerism
developed secondarily as a result of serial repeatition of body parts in an individual.
Segmentation in them was probably an adaptation for an undulating mode of
swimming. This theory was supported by Hyman (1951) and Goodrich. They
observed such serial repeatition of organs in some elongated turbellarians and
nemerteans which later crystallized into metameric segmentation.
¾ CYCLOMERISM THEORY: It was originally proposed by Sedgwick (1884) and
was later supported by Remane (1950,1963). Metamerism has evolved from the
fundamental radial organization of the Actinians (class Anthozoa: phylum Cnidaria)
by the separation of four gastric pouches or enterocoelic pouches from the central gut
(Fig. 6). According to the believers of this theory, the gastric pouches in these
cnidarians were linearly arranged and not in a circular fashion. According to this
theory, all bilateral metazoans were originally segmented and coelomate and that the
acoelomate unsegmented groups (Platyhelminthes, and Nemerteans) have secondarily
lost these characters.
¾ CORM OR FISSION THEORY: Perrier (1882) proposed this theory. According to
this theory, some non-segmented ancestors divided repeatedly by transverse fission or
by asexual budding producing a chain of zooids or sub-individuals. These zooids
remained united end to end due to their incomplete separation. With the passage of
time, these sub-individuals gradually became integrated into a complex individual,
both morphologically and physiologically. A segmented animal, thus, is a chain of
completely coordinate sub-individuals (Fig.7).
The major drawback of this theory is that the reproduction by fission is usually
confined to sessile animals whereas the ancestors of metamerically segmented
animals were probably free swimming.
Another objection is that the sequence of zooid formation is never serial, fission
always occurs in the middle of the chain e.g. Platyhelminthes, and strobilae of
Scyphozoans. However, in cestodes, the proglottids are serially arranged but in the
reverse order i.e new segments are budded off at the anterior end while the oldest ones
are present posteriorly.
¾ EMBRYOLOGICAL THEORY: This theory suggested that mechanical stresses in
the mesoderm of the elongating embryo or larva resulted in the fragmentation of the
mesoderm leading to segmental repeatition of mesodermal derivatives in the adult.
The formation of metameric segmentation was mainly as an embryological accident.
¾ LOCOMOTION THEORY: Metamerism evolved as an adaptation to locomotion of
different kinds.
¾ Annelid metamerism probably evolved as an adaptation for burrowing while chordate
metamerism is an adaptation for undulatory, serpentine movement. Clark (1964)
suggested that the ancestors of phylum Annelida were elongated coelomate animals
which were burrowers. The coelom gave a hydrostatic skeleton to these animals while
the development of septa and metameric segmentation allowed only a part of the
body to contract while other parts in the longitudinal axis relaxed. The locomotion can
be more coordinated and controlled if the action of body-wall muscles are localized.
Metamerism permits such localization and also accounts for the evolution of this
condition in annelids. The localization enabled a strong peristaltic wave to propagate
down the body resulting in a much controlled and efficient type of locomotion in
these burrowers.
SIGNIFICANCE OF METAMERISM:
The most important advantage is that the segmentation divides a body into a series of
compartment each of which can be regulated independently. This leads to regional
specialization of segments thus leading to rapid evolution of high grade of organization. The
simplest of this regional specialization is seen in annelids in which the body may be weakly
divided into head, thorax and abdomen. These three divisions are more prominent in
arthropods. In higher vertebrates, even the segmental arrangement of muscles is obscured by
regional specialization. Thus, primitiveness of an animal, to some extend, can be determined
by the degree of segmentation it displays.
Metamerism helps in locomotion in several ways. It facilitates lateral movement of the body.
Since metameric segmentation results in compartmentalization of the body, the coordination
of muscular action can be restricted to a few segments at a time thereby increasing the
locomotory efficiency.
ORIGIN OF METAZOA:
Cnidarians have been regarded as the most primitive eumetazoan and the possession of radial
symmetry has been regarded as a primitive character of cnidarians. The origin of metazoa is
based on this assumption.
COLONIAL THEORY: Most widely accepted theory was first conceived by Haeckel
(1874). Later, the theory was modified by Metschnikoff (1887) and Hyman (1940).
According to this theory the ancestors of the metazoans are the flagellates. The ancestral
metazoan probably arose from a spherical, hollow, colonial flagellate. The colony possessed
a distinct anterior-posterior axis with monoflagellate cells on the outer surface. It moved with
its anterior axis forward. The somatic cells were differentiated from the reproductive ones.
This hypothetical stage was called blastaea. It is believed to reflect, the blastula stage which
occurs in the development of all animals. The blastaea invaginated to produce a doublewalled gastrea which was regarded as the hypothetical metazoan ancestor by Haeckel
(Fig.8). This gastrea was equivalent to the gastrula stage in the embryonic development of
living metazoans. Division of labor of the somatic cells probably led to increasing
interdependence of cells and finally an original colony of unicellular individuals became a
multicellular organism.
Metschnikoff argued that the originally hollow blastaea became transformed into an organism
having a solid structure (gastraea) by the proliferation of the cells from the blastula wall into
the blastocoel.. This hypothetical ancestral metazoan is believed to be radially symmetrical,
ovoid, with no definite mouth. The outer flagellated cells assumed a locomotory function
while the inner ones were nutritive and reproductive in function. This
hypothetical ancestral metazoan resembled the planula larva of cnidarians and has been called
as the planuloid ancestor which has given rise to the lower metazoans (Fig.9). Thus, this
theory clearly suggests that the cnidarians have been derived from the planuloid ancestor and
that the radial symmetry is primary one and that the bilateral symmetry is a later
modification.
CELLULARISATION/ SYNCYTIALTHEORY: Hadzi (1953) and Hanson (1977) are the
chief proponents of this theory. They suggested that the metazoans have evolved by the
cellularisation of a syncytial protistan. According to him the most primitive acoelous
turbellarians e.g. Convoluta were derived from multinucleate ciliated protistans by the
process of cellularisation. Since many ciliates tend towards bilateral symmetry they argued
that the ancestral metazoans were bilaterally symmetrical and gave rise to the acoels which
were regarded as the primitive living metazoans.
POINTS IN FAVOUR:
¾ Size of the acoels is comparable to the ciliates.
¾ Acoels are also ciliated.
¾ Bilaterally symmetrical.
¾ Both lack a hollow digestive cavity.
¾ Imperfect cellularisation is found in the acoels. Syncytial aggregates have been found
in the central region of the parenchyma of the acoels.
However, Hadzis theory also suggests that the Cnidarians, more specifically the anthozoans
were derived from the turbellarians and that their radial symmetry is because of their sessile
mode of existence. These cnidarians retained some of their ancestral bilateral symmetry
internally in the stomodaeum, mesentries and muscle bands. Thus, according to this view, the
Hydrozoans are regarded as advanced rather than being primitive and that their simple
structure is a secondary adaptation to its mode of existence.
CRITICISM:
¾ This theory is based on the adult structure and disregards the embryological
evidences. The development of turbellarians, nemerteans, annelids, molluscs and
some other groups show spiral cleavage which is not found in cnidarians or
ctenophora. The cnidarians show a wide variety of cleavage patterns suggesting a
¾ primitive nature of this group. The mere fact that cnidarians have evolved from the
turbellarians would mean the abandonment of spiral cleavage which seems
improbable.
¾ The primitive groups are generally more variable with a simple body plan than the
highly evolved ones. Hydrozoans are more variable than either Anthozoa or
Scyphozoa or Turbellaria and therefore Hydrozoa should be considered most
primitive. However, according to Hadzi, hydrozoans are regarded as advanced and
their structural simplicity is a secondary development.
¾ According to this theory, the acoels are the most primitive living metazoans. Thus, the
bilateral symmetry becomes primitive for the metazoans. This would mean that the
radial symmetry of the cnidarians has been secondarily derived from the flatworms.
Many zoologist now doubt that the acoels are the most primitive flatworms.
¾ A ciliate ancestory does not explain the occurrence of flagellated sperm in metazoans.
¾ With a few exceptions, anthozoans are not hermaphrodites. However, most sessile
animals are hermaphrodite. Thus, it seems improbable to consider anthozoans as the
descendants of hermaphroditic turbellarian.
Suggested Reading
:
1. Invertebrate Zoology ,
by Robert D. Barnes, Publisher: Saunders College
International Edition (5th Edition)
2. Parker And HaswellText Book Of Zoology, Invertebrates, Volume 1, Edited by
Marshall And Williams (7th Edition), A.I.T.B.S. Publishers And
Distributors
3. Biology Of The Invertebrates, by Jan A. Pechenik, Publisher: Mc Graw Hill Higher
Education (4th Edition)
4. Biology Of Animals, by Ganguly, Sinha And Adhikari, Publisher: New Central Book
Agency
5. Invertebrate Zoology, by E. L. Jordan And P. S. Verma, Publisher: S. Chand And
Company
6. Modern text book of Zoology, Invertebrates by R. L. Kotpal, Publisher : Rastogi
Publications