Question

Discuss the different invertebrates that have a body plan similar to that of nudibranchs. Mention and discuss all the proctoscopes with radial cleavage and/or that have a blastopore that forms the anus.

Answer 1

Nudibranchs are a group of soft-bodied, marine gastropod molluscs. They have extraordinary colours and striking forms.

Nudibranchs occur in seas,they are apparently bilaterally symmetrical externally because they have undergone secondary detorsion. In all nudibranchs, the male and female sexual openings are on the right side of the body, reflecting their asymmetrical origins. Some species have venomous appendages on their sides, which deter predators. Many also have a simple gut and a mouth with a radula.

Gastropoda includes all nudibranchs, like sea slugs, sea hares, snails, limpets

A characteristic of most members of this class is their single shell. Nudibranchs are“naked” because they are considered snails without shells at adult stage , and the prominent projections on the top and backs of many species are gills. Snails such as abalones and conchs and gastropods such as cowries which have retained their shells, use them for protection. They can pull inside their shell until danger passes.

Maneuvering a hatch-like plate called the operculum can shut the opening to the shell. The head of gastropods is easily discernible and it is quite common to see a pair of stalked eyes on the ends of protruding tentacles peering from a shell.

Nudibranchs have several means of defense Some rely on secretions that prove noxious to predators.Some animals are able to steal the stinging cells of corals, hydroids and other cnidarians on which they prey and put them to good use. They take the unfired stinging cells and transfer them to projections of their own backs where they provide a formidable defense.Some sea hares are quick to release clouds of dark ink to repel or confuse predators.

They are feed on a variety of sources and display diverse feeding habits. Many graze on a range of algae and other food found along the sea floor. Some are scavengers, and still others are predators. The radula of predatory snails use their radula to bore holes into their prey which can include other shelled mollusks and the leathery eggs of horn sharks and swell sharks. Once the hole is bored, the radula is used to rasp the desired tissues.

In cone shells, the radula has been modified into a harpoon-like device which can be shot rapidly into prey to inject a highly toxic poison. Though it is extremely rare, cone snails can kill humans.

Protostomes

Animals in which the blastopore becomes the mouth are called protostomes; those in which the mouth develops after the anus are called deuterostomes

They make up the Bilateria, mostly comprising animals with bilateral symmetry and three germ layers. The major distinctions between deuterostomes and protostomes are found in embryonic development.

Earthworms, has the embryo that forms a dent on one side, the blastopore, which deepens to become the archenteron, the first phase in the growth of the gut.

Examples : Arthopoda, a fire worm,land snails

In protostomes, the mesoderm originates from a pair next to the blastopore, which then migrate into the blastocoel, the internal cavity of the embryo, to become various internal structures. In coelomates, the mesentoblasts hollow out to become coeloms, cavities lined by a contractile peritoneum, the myoepithelium.

In protostomes, the process of coelom formation is called schizocoely. The archenteron pouches out to form coelomic cavities, in a process called en-terocoely.