Question

The main features of molluscs. How can molluscs be described according to examples of representative species, basic morphology, type of symmetry, germ layers and coelom, digestive system, respiratory system, circulatory system, excretory system, nervous system and types of reproduction?

Answer 1

       Mollusca is the second largest animal phylaum .

         The main features of molluscs are following-

• unsegmented soft body.
• most have internal or external shell.
• organ -system level of body organisation
• body is covered by a calcareous shell with a distinct head muscular foot abd visceral hump.

Snails, pila, octopus, loligo, oyster and slug are examples of representative species.

Morphology- The word “mollusc” means “soft thing”. Molluscs have soft bodies and this feature explains the name of the phylum.

Symmetry- There is bilateral symmetry in molluscus. Animal which can be divided two equal right and left halves by only one plane has bilateral symmetry.

Germ Layer- It has three types of germ layers- ectoderm, mesoderm and endoderm. Molluscus are triploblastic animal.

Coelom - Ceolem is defined as a body cavity lined by mesoderm. Animals having coeloms are called coelomates, and those without are called acoelomates. There are also subtypes of coelom:

schizocoelom: develops from split in mesoderm found in annelids, arthropods and molluscs. haemocoelom: true coelom reduced & cavity filled with blood found from arthropoda and mollusca.

Digestive system- like humans, mollusks have a mouth, esophagus, stomach, and anus. Their mouth, also like humans, contains teeth as part of an oral structure called a radula.

Respiratory system- Basically all molluscs breathe by gills that are called ctenidia (comb-gills) because of their comb-like shape. In terrestrial molluscs this respiration organ is reduced, but still respiration takes place in the pallial cavity. That is why it is also called the snail's respiratory cavity.

Circulatory system- Open circulatory systems pump blood into a hemocoel with the blood diffusing back to the Circulatory system between cells. Blood is pumped by a heart into the body cavities, where tissues are surrounded by the blood.

Excretory system- Kidney.

Nervous system- molluscs have two pairs of main nerve cords organized around a number of paired ganglia.

Types of reproduction- asexual and sexual.

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