Cnidaria: corals, anemones,
hydroids, jellyfish
The Cnidaria are
characterized by production and use of nematocysts. The nematocysts are complex organelles contained
in certain cells of cnidarians. The
previous post suggests how they may have developed from modifications around
the spicules inherited from ancestral sponges.
Two main body types are found
in the sexual stage of cnidarians. The
polyp or hydroid body type is the original type if the findings of Kazmierczak
(1984) are accepted. Generations of
biologists assumed something like the simple hydra was one of the earliest cnidarians. The new evidence makes an extinct coral the
likeliest candidate for the ancestral origin of cnidarians. Corals and other in the class Anthozoa have the
polyp stage predominating, and no medusa (or jellyfish) stage. Those in the class Hydrozoa usually have both
polyp and medusa stages. In the class
Scyphozoa the jellyfish stage dwarfs other stages. The medusa stage is the sexual adult stage in
cnidarians having a medusa.
Tentacles, furnished with
many nematocysts, and partitions or tubes in the digestive cavity (gastrovascular
cavity) in considerable variety are often part of structural diversity of
cnidarians. The medusa stage was
recognized by early biologists as having a very similar structure to an
inverted polyp stage. So, thinking of a
coral-like polyp stage as the starting point in pre-Cambrian seas, it is easy
to imagine an early extinction event making life for the coral so difficult
that its polyp, released from the coral skeleton, managed to survive a marginal
extinction event to preserve genes enabling such a release. Ultimately, repeating the process eventually
produced forms with medusae dominating the life cycle.
Besides the microscopic
features such as the spicule-nematocyst connection, gross features providing a
base for determining selection for the sponge-cnidarian transition involved a
number of events. (1) The attached bottom dwelling lifestyle was conducive to
retaining radial symmetry. (2) The upward facing osculum of the sponge
provided an opening for gradual evolution of a mouth and transition of the
spongocoel to a gastrovascular cavity as adaptations for acquiring larger
particulate food developed. (3) Spicule
deposition shifted from the generalized sponge skeletal elements to the external
cup-like coral skeleton. Adaptations for
muscles, nerve, and other new and useful soft structure elements optimized for
size made vegetative growth of colonies by budding a suitable solution.
Extinct tetracorals were
common early fossil corals. Prior to or
along with their radiation into the vast range of anthozoan hard corals, soft
corals, and anemones, it is likely that they led to the hydrozoan medusae that were
the ancestral hydrozoans. The four radial
gastrovascular canals and related parts may be due to the square cups of
tetracoral skeletons affect on selection/development of soft parts.
The hydra is one of the
hydrozoan polyps. Several nematocyst
types are found in hydrozoa, about four kinds in hydra. Hydra is specialized for fresh water
existence by loss of the medusa stage.
The freshwater jellyfish retained the medusa stage but the polyp stage does not have
tentacles; their polyps bud from a connection in the sediment, some polyps bud
off medusae but most have a mouth for feeding.
Similar hydrozoan medusae are found in salt water species.
Tracing vertebrate roots
through cnidarians
Just as sponges underwent
much diversification after giving rise to cnidarians, cnidarians gave early
rise to the forerunner of the flatworms.
The prevailing opinion that phylum Cnidaria begin at a rudimentary stage
is incorrect. The structure of the
hydrozoan medusa needed relatively little modification beyond elongation to
produce the ancestor of the common planarian.
The centrally located manubrium of the jellyfish is positioned similarly
to the proboscis of planaria. The four
branches of the gastrovascular cavity are reduced to the three in the planaria;
the fourth was eventually lost due to inability to develop in the compressed
space above the proboscis. The sensory
complexes were lost in all locations except where the head developed at the
anterior or forward branch of the gastrovascular cavity.
Hadzi (1963) noted the
similarities of flatworm and medusa and proposed the flatworms as intermediate
between protozoans and cnidarians, a view that has not been accepted. Ax (1963) seemed to think the long
evolutionary history of existing groups precluded any from being ancestors of
any other major group. But successful
adaptations, in my opinion, could very likely persist as their variations give
rise to great diversification and other phyla.
The spin-off of new groups was easier before adaptations became well
fixed or stable. Such a series of events
in the early history of animal diversification seems more compatible with the
Cambrian “explosion” of animal groups.
As the jellyfish body
flattened and elongated for bilateral and mobile life as a flatworm, the outer
longitudinal muscle fibers and inner circular muscle fibers are now known and
positioned as outer circular and inner longitudinal muscle layers.
The nematocysts took on more degenerated
and/or restricted function as the rhabdites of the flatworm epidermis. Other developments added to the complexity of
flatworms.
References
Ax, P. 1963. Relationships and phylogeny of the
Turbellaria. Pp. 191-224 in E.
Dougherty. The Lower Metazoa. Univ. of
California Press, Berkeley. 478 pp.
Hadzi, Jovan.
1963. The Evolution of the Metazoa.
Macmillan , New York
Kazmierczak, Jozef.
1984. Favositid tabulates: evidence for
poriferan affinity. Science, 225:835-837.
Joseph G. Engemann July 19, 2013
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