make a list, or table, to distinguish among three classes oc cnidarians (Anthozo
ID: 256241 • Letter: M
Question
make a list, or table, to distinguish among three classes oc cnidarians (Anthozoa, Hydrozoa, Cubozoa).
with respect to
1) life cycles,
2) anatomical complexity,
3) morphologicsl specialization,
4) physical integration,
5) extent of coloniality versus solitary lifestyles,
6) any other traits that you think distinguish among them.
Also discuss the pros and cons for the hypothesis that anthozoans are the earliest-evloving lineage. Which class do you think is ancestral and which do you think is most derived and why? Thank you!
Explanation / Answer
Cnidaria is a phylum containing over 10,000 species of animals found exclusively in aquatic environments and can be divided into classes like Anthozoa, Hydrozoa, Cubozoa.
Class: Anthozoa
No. of species are 6,100
Cells found in mesoglea
Nematocysts in exodermis
No Medusa phase in life cycle
The Animals included display only polyp morphology and have cnidocyte-covered tentacles around their mouth opening.
flowers of the oceans
-polyp form only
-gastrodermas divided by septa or mesenteries that bear nematocysts
-tentacles are solid
-may be hexamerous or octamerous
Examples: Sea anemones, corals, sea pens
Class: Hydrozoa
No. of species are 3,600
Cells do not found in mesoglea
Nematocysts are not in exodermis
In some species Medusa phase in life cycle
The Animals included true polymorphs and can be found as colonial or solitary organisms.
polyp form only
-feeding tentacles surround the hypostome, at the center of which is the mouth
-the mouth opens into a single, tubelike gastrovascular cavity
-body ends in a basal disc
The hydra has only a polyp stage, for example, whereas most hydrozoans have a biphasic (two-stage) life cycle that alternates between a sedentary polyp stage and a mobile, bell-shaped medusa stage.
Examples: Hydra, siphonophores
Class: Cubozoa
No. of species are 42
Cells found in mesoglea
Nematocysts in exodermis
Medusa phase in life cycle
The Animals included live as box-shaped medusa.
cubozoans have a cube-shaped medusa.
Their polyp stage is inconspicuous, and in some species, it has never been observed. Most box jellies are only a few centimeters in height, although some are 25 cm (10 in.) tall.
A tentacle or group of tentacles is found at each corner of the “box.” Stings of some species, such as the sea wasp, can inflict severe pain and even death among humans.
bell shaped medusa square in cross section; tentacles hang from corner of umbrella; no velum but velarium. These are the deadliest and can kill fish up to 5 times bigger than they are.
Examples: Box jellies
Hypothesis: anthozoans are the earliest-evloving lineage
Studies regarding the relationships in the phylum Cnidaria led to the conclusion that the medusae producing classes(Scyphozoa, Hydrozoa, Cubozoa) have to be grouped together,and that the Anthozoa have to be regarded as primitive and as the basal group within the phylum. As a consequence, two phylogenetic hypotheses must be regarded as definitely untenable: the ‘scyphozoan-hypothesis’ of coelenterate evolution and the well-known ‘gastraea-hypothesis’ of metazoan evolution. Both are not at least dependent of the assumption that the so-called ascon-sponges and the scyphozoa, or the hydrozoa respectively, would be primitive. The consequences are far reaching. When these well-known and largely taught hypotheses are abandoned, the question arises: how can we explain early metazoan evolution in adifferent way, and what else can be taught. scientists in the United States are routinely trained in the tradition that the class Hydrozoa is primitive obviously with the consequence that there seems to be little doubt about the gastraea-hypothesis and European scientists are trained to regard the class Anthozoa or Scy-phozoa as primitive.
Which class do you think is ancestral and which do you think is most derived and why?
Class Anthozoa is ancestral and Class: Cubozoa is most derived.
the Anthozoa (sea anemones and corals) are regarded as the sister-group of the rest, which suggests that the earliest cnidarians were sessile polyps with no medusa stage.
Class Cubozoa has attributes like possession of complex eyes and associated visual capabilities to extraordinary courtship and mating behavior.
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