O·vip·a·rous

Producing eggs that hatch outside the body. Amphibians, birds, and most insects, fish, and reptiles are oviparous

More specifically:

• Ovuliparity: fertilization is external (in arthropods and fishes, most of frogs)
• Oviparity: fertilization is internal, the female lays zygotes as eggs 
• Ovo-viviparity: or oviparity with retention of zygotes in the female’s body or in the male’s body
• Histotrophic viviparity: the zygotes developed in the female’s oviducts, but find their nutriments by oophagy or adelphophagy (eating the other eggs in the womb, or live young).
• Hemotrophic viviparity: nutriments are provided by the female, often through placenta.

Land-dwelling animals that lay eggs, often protected by a shell, such as reptiles and insects, do so after having completed the process of internal fertilization. Water-dwelling animals, such as fish and amphibians, lay their eggs before fertilization, and the male lays its sperm on top of the newly laid eggs in a process called external fertilization.

Almost all non-oviparous fish, amphibians and reptiles are ovoviviparous, i.e. the eggs are hatched inside the mother’s body (or, in case of the sea horse inside the father’s).

(Source)

Pol·yp

A polyp in zoology is one of two forms found in the phylum Cnidaria, the other being the medusa (see this post). Polyps are approximately cylindrical in shape and elongated at the axis of the body. In solitary polyps, the aboral (surface opposite the mouth) end is attached to the substrate by means of a disc-like holdfast called the pedal disc, while in colonies of polyps it is connected to other polyps, either directly or indirectly. The oral end contains the mouth, and is surrounded by a circlet of tentacles. 

In the class Anthozoa, comprising the sea anemones and corals (see these posts), the individual is always a polyp; in the class Hydrozoa, however, the individual may be either a polyp or a medusa, with most species undergoing a life cycle with both a polyp stage and a medusa stage. In class Scyphozoa, the medusa stage is dominant, and the polyp stage may or may not be present, depending on the family. In those scyphozoans that have the larval planula metamorphose into a polyp, the polyp, also called a “scyphistoma,” grows until it develops a stack of plate-like medusae that pinch off and swim away in a process known as strobilation. Once strobilation is complete, the polyp may die, or regenerate itself to repeat the process again later. With Cubozoans, the planula settles onto a suitable surface, and develops into a polyp. The cubozoan polyp then eventually metamorphoses directly into a Medusa.

The body of the polyp may be roughly compared in a structure to a sac, the wall of which is composed of two layers of cells. The outer layer is known technically as the ectoderm, the inner layer as the endoderm (or gastroderm). Between ectoderm and endoderm is a supporting layer of structureless gelatinous substance termed mesogloea, secreted by the cell layers of the body wall. The mesogloea may be a very thin layer, or may reach a fair thickness, and then sometimes contains skeletal elements formed by cells which have migrated into it from the ectoderm.

The sac-like body built up in this way is attached usually to some firm object by its blind end, and bears at the upper end the mouth which is surrounded by a circle of tentacles which resemble glove fingers. The tentacles are organs which serve both for the tactile sense and for the capture of food. Polyps extend their tentacles, particularly at night, containing coiled like stinging nettle-cells or nematocysts which pierce and poison and firmly hold living prey paralysing or killing them.

(Source)

Did you know?

Chrysaora is a genus of the family Pelagiidae (Jellyfish, see this post) A recent analysis of the genus found there to be 12 valid species (see this post to show the order of taxonomy for clarification)

Sea nettles (see this example) are made up of 95 percent water. The other five percent is protein and salt. Corals and anemones are the closest relatives of sea nettles. They are all part of a group of animals which all have stinging cells. Sea nettles can’t see. They don’t have eyes, but they have rhopalia or light sensing organs. The rhopalia are located around the rim of the bell and help the sea nettle detect changes in light. They can’t make out distinct shapes, colors or details, but these light sensing organs enable the animal to move toward or away from light. Sea nettles have a simple body tissue organization. They have a thin ectoderm or outer skin layer and a thin endoderm or inner skin layer. The ectoderm serves as a protector, while the endoderm helps in the digestion of food. Between the two layers, forming the bulk of the volume of the animal, is the mesoglea or jelly-like layer. The mesoglea helps to keep the sea nettle organized and helps maintain its shape. It provides a place for bell muscles to pull from. Sea nettles lack any true organ systems, and survive without a heart, brain, liver or kidneys. Being almost jelly-like and transparent or clear, sea nettles use this quality as a defense mechanism. Since they don’t swim very well and they don’t have a hard protective body covering, their light-colored, almost transparent bodies keeps them out of sight from their predators. It is eaten by sea turtles, large fish like the ocean sunfish and sea birds.      

(Source)

Nu·di·bran·chi·a

An order of shell-less marine gastropod mollusks (see this post) (order Nudibranchia, class Gastropoda) that comprises the sea slugs.

There are four basic types of nudibranchs:

Doridoidea, Aeolidoidea, Dendronotoidea, and Arminoidea. Doridoidea and Aeolidoidea are the two largest suborders of nudibranchs. Characteristics of Doridoidea include a broad, flat foot, a thick, fleshy mantle, and a circle of gills on the posterior end of the dorsal surface. The gill surrounds the anus. These nudibranchs are also generally the larger species. Aeolidoidea have long, narrow bodies, lack gills, and have a number of projections on the dorsum called cerata. These slugs are generally smaller in size. Dendronotoidea characteristics include rhinophoral sheaths, mid-lateral anus, but resemble aeolid’s. Finally, Arminoidea is made up of a motley group of species. The species in this suborder differ amongst themselves which makes very few externally consistent characteristics. Some features that many of the species in this suborder share are lack of rhinophoral sheaths and, often, oral tentacles, and the anus is far forward on the body. These broad characteristics of the suborders are helpful in determining which suborder an individual belongs to so that it may be identified further.

Nudibranchs move by muscular action or by ciliary action which causes the slug to move quite slowly. In a slug that moves via muscular action, contraction and expansion of the muscles move the slug forward. In ciliary motion, cilia in the foot beat against the substrate causing the slug to move forward.

All nudibranchs are hermaphroditic, but rarely will they fertilize themselves. Normally nudibranchs will copulate and each individual will lay an egg mass, called a nidosome. Egg masses of a number of nudibranchs are quite distinctive and can be found on substrates where nudibranchs feed or are often found. When the eggs hatch, a veliger larva (larva with a protective shell and a ciliated flap-shaped foot used for swimming and feeding) is usually released. The veliger has a shell, but once the larvae has settled out of the plankton, the shell is released and they enter a juvenile stage.

Check out the archive for other information/images of nudibranchs.

(Source)

Did you know?

Medusa is another word for jellyfish, and refers to any free-swimming jellyfish stages in the phylum Cnidaria. They are found in every ocean, from the surface to the deep sea. Jellyfish have several different morphologies that represent several different cnidarian classes including the Scyphozoa, Staurozoa, Cubozoa, and Hydrozoa.

(Source)

Did you know?

The Mariana Trench is the deepest part of the world’s oceans, and the lowest elevation of the surface of the Earth’s crust. It is located in the western Pacific Ocean, to the east of the Mariana Islands. It reaches a maximum-known depth of about 10.91 kilometers or 6.78 miles (over 35,000 feet) at the Challenger Deep, a small slot-shaped valley in its floor at its southern end named after the British survey ship Challenger II, which discovered this deepest location in 1951. If Mount Everest, the highest mountain on Earth at 8,848 meters (29,029 ft), was set in the deepest part of the Mariana Trench, there would be 2,076 metres (6,811 ft) of water left above it.

(Source)