Are hydrozoa symmetrical?
Most hydrozoan species are colonial. The medusa is the sexually reproducing stage in most hydrozoans. They are often formed by budding from polyps, and are usually solitary free-swimming organisms. They are similar in structure to an inverted polyp, radially symmetric, and often have four-fold symmetry.
What are the characteristics of class Hydrozoa?
- Internal space for digestion is the gastrovascular cavity.
- Gastrovascular cavity has one opening, the mouth.
- Exoskeleton of chitin.
- Are almoust entirely marine and predators.
- Sexual reproduction produces the planula larvae.
- Two body forms, a polyp and medusea.
- Presence of stinging cells called Cnidocytes.
What kind of symmetry do cnidarians have?
The bilateral symmetry of Cnidaria is thought to be inherited from the common ancestors of both cnidarians and triploblastic bilaterians. The secondary radial symmetry of Cnidaria evidently is a result of the adaptation to the sessile mode of life.
Are platyhelminthes asymmetrical?
The phylum Platyhelminthes, the flatworms, shows bilateral symmetry. In bilateral symmetry, you create two matching halves by drawing a line from the anterior (head) end of the organism to the posterior (tail) end.
Are cnidarians asymmetrical?
Cnidarians in both groups have an external radial symmetry, yet internal asymmetries and bilaterality are displayed in many groups.
How do you identify a hydrozoa?
Marine identification key: Hydrozoa. The Hydrozoa is a class of Cnidaria. They are characterized by having a life-cycle involving sexual and asexual reproduction. Most hydrozoans have a benthic, colonial polyp stage, which reproduces asexually by budding.
Why do Cnidaria have radial symmetry?
Radial Symmetry. The body parts of a radially symmetrical animal are arranged around a central axis so that each part extends from the center. Radial symmetry allows them to reach out in all directions. Cnidarians have two tissue layers.
What class is Scyphozoa in?
The Scyphozoa are an exclusively marine class of the phylum Cnidaria, referred to as the true jellyfish (or “true jellies”). It may include the extinct fossil group the Conulariida, whose affinities are uncertain and widely debated.
How many species of hydrozoans are there in the world?
The Hydrozoa is a subgroup of cnidarians containging approximately 3700 species. It is a diverse group with a variety of life cycles, growth forms, and specialized structures. Like many cnidarians, hydrozoans have both polyp and medusa stages in their life cycle.
What is the defining characteristic of the class Scyphozoa?
Class Scyphozoa includes all the jellies and is exclusively a marine class of animals with about 200 known species. The defining characteristic of this class is that the medusa is the prominent stage in the life cycle, although there is a polyp stage present.
What kind of life cycle does a Hydrozoa have?
Hydrozoans have a complex life cycle, usually with two or three morphologically different stages. The classic cycle starts with fertilized eggs developing into small, free swimming larvae called planulae, which may be able to enter a dormant resting state to resist unsuitable environmental conditions.
What kind of cell structure does a Hydrozoa have?
Like all cnidarians, hydrozoans have special ectodermal cells called cnidocytes, each containing a single intracellular structure called a cnida (aka nematocyst). Cnidae are unique to the Cnidaria. Each cnida, when triggered by a mechanical or chemical stimulus, shoots out a tiny hollow tube at high speed.