1. Echinodermata:Phylogenetic Considerations
The echinoderms have Fossil records to the early Cambrian peroid but
Paleontology does not throw much light on the origin & evolution of the
Echinoderms. However there is a great similarity in the embryonic
development of Echinoderms representing the concept of common
ancester of Echinoderms. The term Dipleura,(DIPLEURULA LARVA) was
given by Semon (1888) as a common ancestor. Bather (1900) adapted &
expounded the dipleura.
According to Bather, the dipleura is a soft bodied , elongated,
bilaterally symmetrical without skeletal formation.
The mouth, preceded by a pre-oral lobe, located ventrally near the
anterior end,& the anus is slightly ventral at the posterior end. A simple
digestive tract, with stomachic enlargement runs from mouth to anus.
At the anterior end a nervous center is found bearing a sensory tuft &
giving off nerves in the posterior direction. Internally coelomic sacs of
enteric origin are: axocoel,hydrocoel & somatocoel. The axocoel opens
on the dorsal surface through hydroporic canal by a hydropore, with a
continuity with the hydrocoel by a narrow stone canal. The right
axocoel & hydrocoel are poorely developed & are absent in crinoid &
holothuroid embryos. Hyman,do not agreed with the importance of
dipleura in elucidating the common ancestor of echinoderms.
2. • Bather assumes that the dipleura in the process of evolving into
an Echinoderm,attached itself by the right side of the oral lobe.
The attachement brought about the degeneration of some
structures of the right side, & forced the mouth to move
towards the original posterior end,taking the hudrocoel with it,
thus the looping of the digestive tract.as well as the right
somatocoel becomes dorsal & the left the ventral. The pressure
of the digestive tract on the left hydrocoel force it into a horse-
shoe shape eventually which closes into a ring.
• The process of metamorphosis in the development of
echinoderms shows that the original echinoderm must have
been very different from the present form. To become an
echinoderm ,the creature attaches at the oral end, resulted into
the migration of mouth but anus maintains the same position.
3.
4. • So the echinoderms evolved from a bilateral ancestor, the strange
metamorphosis changes the bilateral symmetry to radial symmetry.
• Embryology shows that the crinoids have evolved from simple stalked
pelmatozoan with a complete pentamerous theca. The crinoids have
not given rise to any other group . No satisfactory evidence to show
that elutherozoan classes originated from pelmatozoan ancestors.
• The holothuroids left no fossil record & regarded as primitive. The
auricularia larva of holothuroids resemble the bipinnaria of
asteroids,probably the holothuroids diverged from the other
eluthrozoan at a very early stage in their evolution.
•
The other three classes have a common ancestors because in all the
same apical skeleton in aboral side of the larva.
