28.2D: Class Cubozoa and Class Hydrozoa
- Distinguish between cubozoa and hydrozoa cnidarians
Class Cubozoa
Class Cubozoa includes jellies that have a box-shaped medusa: a bell that is square in cross-section; hence, they are colloquially known as “box jellyfish.” These species may achieve sizes of 15–25 cm. Cubozoans display overall morphological and anatomical characteristics that are similar to those of the scyphozoans. A prominent difference between the two classes is the arrangement of tentacles. This is the most venomous group of all the cnidarians.
The cubozoans contain muscular pads called pedalia at the corners of the square bell canopy, with one or more tentacles attached to each pedalium. These animals are further classified into orders based on the presence of single or multiple tentacles per pedalium. In some cases, the digestive system may extend into the pedalia. Nematocysts may be arranged in a spiral configuration along the tentacles; this arrangement helps to effectively subdue and capture prey. Cubozoans exist in a polypoid form that develops from a planula larva. These polyps show limited mobility along the substratum. As with scyphozoans, they may bud to form more polyps to colonize a habitat. Polyp forms then transform into the medusoid forms.
Class Hydrozoa
Hydrozoa includes nearly 3,200 species; most are marine, although some freshwater species are known. Animals in this class are polymorphs: most exhibit both polypoid and medusoid forms in their lifecycle, although this is variable.
The polyp form in these animals often shows a cylindrical morphology with a central gastrovascular cavity lined by the gastrodermis. The gastrodermis and epidermis have a simple layer of mesoglea sandwiched between them. A mouth opening, surrounded by tentacles, is present at the oral end of the animal. Many hydrozoans form colonies that are composed of a branched colony of specialized polyps that share a gastrovascular cavity, such as in the colonial hydroid Obelia . Colonies may also be free-floating and contain medusoid and polypoid individuals in the colony as in Physalia (the Portuguese Man O’ War) or Velella (By-the-wind sailor). Other species are solitary polyps ( Hydra ) or solitary medusae ( Gonionemus ). The true characteristic shared by all these diverse species is that their gonads for sexual reproduction are derived from epidermal tissue, whereas in all other cnidarians they are derived from gastrodermal tissue.
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- cnidocyte. Provided by : Wiktionary. Located at : en.wiktionary.org/wiki/cnidocyte . License : CC BY-SA: Attribution-ShareAlike
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- Anthozoan. Provided by : Wikipedia. Located at : en.Wikipedia.org/wiki/Anthozoan . License : CC BY-SA: Attribution-ShareAlike
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Key Points
- Cubozoans differ from Scyphozoans in their arrangement of tentacles; they are also known for their box-shaped medusa.
- Out of all cnidarians, cubozoans are the most venomous.
- Hydrozoans are polymorphs, existing as solitary polyps, solitary medusae, or as colonies.
- Hydrozoans are unique from all other cnidarians in that their gonads are derived from epidermal tissue.
Key Terms
- hydroid : any of many colonial coelenterates that exist mainly as a polyp; a hydrozoan