bio_theories1234

Order Semaeostomeae The jellyfishes in this order are among the most familiar members of the class, being those usually seen in temperate waters, and they are also the most typical. There are numerous tentacles round the bell margin and the oral arms are long and frilly. The musculature is usually obvious and there are numerous … Read more

Order Rhizostomeae Members of this order closely resemble the semacostome jellyfishes, the differences being the lack of tentacles round the bell margin and the great increase in size and fusion of the oral arms. These are much-branched and the original grooves of the arms close over to form a canal. The main mouth generally becomes … Read more

In the large jellyfishes or Scyphozoa the medusoid phase is dominant, the polyp, known as the hydra tuba or scyphistoma, being a small tubular organism resembling Hydra. The class comprises five orders as follows: Semaeostomeae, Rhizostomeae, Coronatae. Cubomedusae and Stauromedusae The most noticeable external feature of the class is its four-rayed symmetry. The bell of … Read more

Sea firs (hydroids), hydras and siphonophores (Class Hydrozoa) The class Hydrozoa comprises over 2,700 species and is subdivided into seven orders: Athecata, Thecata, Limnomedusae, Trachymedusae, Narco medusae, Siphonophora and Hydrocorallinae. Both polyp and medusa are generally present, but the latter may be reduced and then does not always leave the parent colony. Many members of … Read more

Reproduction and development in Cnidarians Cnidarians are usually dioecious, that is, the sexes are separate, but may be hermaphrodite. Gametes are usually shed into the seawater, where fertilisation occurs. In general, medusae produce gametes, but in instances where only the polyp occurs, (e.g. in anthozoans) gonads are borne on the projecting internal septa or (e.g. … Read more

Cnidaria structure Musculo-epithelial cells in Cnidarians The major cell types of the Cnidaria are the musculo epithelial cells, nerve cells, and nematoblasts which produce nematocysts. The ‘body’ is composed mainly of musculo-epithelial cells. These cells, found only in the cnidarians, are a means of producing an epithelium combined with a contractile system. Each cell consists … Read more

The phylum Cnidaria is predominantly marine and contains many common shore animals such as sea anemones and sea firs (hydroids), as well as deeper water forms such as the Portuguese Man o’ War. Freshwater forms are less common, but one group, the hydras, is probably the best-known member of the phylum. Classes of Cnidaria There … Read more