EVOLUTIONARY SIGNIFICANCE OF CTENOPHORA
Affinities of Ctenophora
(A) Interphylar Relationship:
Relationship with Sponges:
Many authors tried to establish the sponges as to be closely related to Ctenophora.
They put forward the following points to support their contentions:
1. The large central cavity and the osculum of sponges correspond to the coelenteron
and mouth of Ctenophora respectively.
2. Absence of well-formed mesoderm in both,
3. Simpler organisation in both.
But closer examination reveals that these two groups are quite widely apart.
The points of differences are:
1. Developmentally the osculum of sponges does in no way correspond to the mouth of
Ctenophora.
2. Presence of inhalant pores and peculiar collar cells in sponges are lacking in
Ctenophora.
3. Absence of colloblasts in sponges.
4. Absence of specialised nervous and sensory structures in sponges.
Remark:
The above differences are sufficient to separate the two groups from each other, though
the above relationship is emphasised by Leuckart and some other authors.
Relationship with Platyhelminthes:
Similarities between Polycladida (Platyhelminthes) and Ctenophora:
The idea that Ctenophora gave rise to certain bilateria (Polyclad) has been supported by
many zoologists. Platyctenea has been considered to be a connecting link between
Ctenophora and the bilateria.
Similarities:
1. Flat compressed body having a movement by creeping-like on the sole of the foot.
2. General ciliation of the body.
3. The dorsal polar nerve of Turbellaria can be compared with the statocyst of
Ctenophorm.
4. Origin of the so-called mesoderm is more or less similar.
5. Primary locomotor organs in the larva (Muller’s larva) consist of eight ciliated ridges
of ectoderm which can be compared with the ctenophoran meridional comb-plates.
6. Dermal musculature well developed.
7. Branched gastro-vascular cavity.
, 8. Ctenophora exhibits both radial as well as bilateral symmetries.
9. Development of the two groups has a close resemblance.
Dissimilarities:
1. Polyclads have a highly developed brain.
2. Numerous eyes in polyclads.
3. No definite ciliary plates in Turbellaria.
4. Tentacles generally absent in Turbellaria,
5. Presence of flame cells in polyclads.
6. Ctenophores are radially symmetrical
7. Complex reproductive system with a muscular penis in polyclads.
8. Embryological details vary in these two groups.
Remarks:
The view that the primitive bilateria have evolved through Platyctenea has not been
accepted. Because a thorough examination of the Platyctenea reveals that it is a
ctenophore which has become extensively modified for sessile habits. It can further be
suggested that Platyctenea is a tissue-grade diploblastic animal, whereas polyclad is an
organ-grade triploblastic form. Again Lang (1881) advocated that the ctenophores led
directly to the polyclads and that the Platyctenea bridge the gap between the Coelenterata
and Platyhelminthes. Hadzi (1944) and de Beer (1954 & ’58) hold that Ctenophora arose
from Polycladida by neoteny.
Relationship with Nemertines:
Similarities:
The larval form of Nemertine (Pilidium larva) shows some similarities with ctenophores.
1. Locomotion is performed by lobed bands of cilia in Pilidium larva.
2. The aboral end on the body contains a cup-like sense organ.
3. Radial structure without coelom.
4. Gonad ectodermal.
Dissimilarities:
1. Ctenophores lack mesoderm proper.
2. Paired tentacles with basal musculature are present in Ctenophora, absent in
Nemertines.
3. The aboral sense organ in both differs widely because aboral calcareous sense organ
in Ctenophora but absent in Nemertines.
Remarks:
The similarities may best be explained as to be due to distant convergence and perhaps
with no phylogenetic significance.
Affinities of Ctenophora
(A) Interphylar Relationship:
Relationship with Sponges:
Many authors tried to establish the sponges as to be closely related to Ctenophora.
They put forward the following points to support their contentions:
1. The large central cavity and the osculum of sponges correspond to the coelenteron
and mouth of Ctenophora respectively.
2. Absence of well-formed mesoderm in both,
3. Simpler organisation in both.
But closer examination reveals that these two groups are quite widely apart.
The points of differences are:
1. Developmentally the osculum of sponges does in no way correspond to the mouth of
Ctenophora.
2. Presence of inhalant pores and peculiar collar cells in sponges are lacking in
Ctenophora.
3. Absence of colloblasts in sponges.
4. Absence of specialised nervous and sensory structures in sponges.
Remark:
The above differences are sufficient to separate the two groups from each other, though
the above relationship is emphasised by Leuckart and some other authors.
Relationship with Platyhelminthes:
Similarities between Polycladida (Platyhelminthes) and Ctenophora:
The idea that Ctenophora gave rise to certain bilateria (Polyclad) has been supported by
many zoologists. Platyctenea has been considered to be a connecting link between
Ctenophora and the bilateria.
Similarities:
1. Flat compressed body having a movement by creeping-like on the sole of the foot.
2. General ciliation of the body.
3. The dorsal polar nerve of Turbellaria can be compared with the statocyst of
Ctenophorm.
4. Origin of the so-called mesoderm is more or less similar.
5. Primary locomotor organs in the larva (Muller’s larva) consist of eight ciliated ridges
of ectoderm which can be compared with the ctenophoran meridional comb-plates.
6. Dermal musculature well developed.
7. Branched gastro-vascular cavity.
, 8. Ctenophora exhibits both radial as well as bilateral symmetries.
9. Development of the two groups has a close resemblance.
Dissimilarities:
1. Polyclads have a highly developed brain.
2. Numerous eyes in polyclads.
3. No definite ciliary plates in Turbellaria.
4. Tentacles generally absent in Turbellaria,
5. Presence of flame cells in polyclads.
6. Ctenophores are radially symmetrical
7. Complex reproductive system with a muscular penis in polyclads.
8. Embryological details vary in these two groups.
Remarks:
The view that the primitive bilateria have evolved through Platyctenea has not been
accepted. Because a thorough examination of the Platyctenea reveals that it is a
ctenophore which has become extensively modified for sessile habits. It can further be
suggested that Platyctenea is a tissue-grade diploblastic animal, whereas polyclad is an
organ-grade triploblastic form. Again Lang (1881) advocated that the ctenophores led
directly to the polyclads and that the Platyctenea bridge the gap between the Coelenterata
and Platyhelminthes. Hadzi (1944) and de Beer (1954 & ’58) hold that Ctenophora arose
from Polycladida by neoteny.
Relationship with Nemertines:
Similarities:
The larval form of Nemertine (Pilidium larva) shows some similarities with ctenophores.
1. Locomotion is performed by lobed bands of cilia in Pilidium larva.
2. The aboral end on the body contains a cup-like sense organ.
3. Radial structure without coelom.
4. Gonad ectodermal.
Dissimilarities:
1. Ctenophores lack mesoderm proper.
2. Paired tentacles with basal musculature are present in Ctenophora, absent in
Nemertines.
3. The aboral sense organ in both differs widely because aboral calcareous sense organ
in Ctenophora but absent in Nemertines.
Remarks:
The similarities may best be explained as to be due to distant convergence and perhaps
with no phylogenetic significance.
