Explain the phylum mollusca of non chordates?

The mollusca are also known as the soft body animals. Aristotle described it and Johnston gave the name. They are mainly marine but some of them do occur in the fresh water. They have multiple body forms and their body is unsegmented. The neopilina is the only molluscum which is segmented. They arise from the three germ layers such as ectoderm, mesoderm and endoderm. They are known as triploblastic animals. They have unisexual and bisexual forms and show sexual dimorphism. They have bilateral symmetry. They have an organ system level of organization. The digestive system is complete and has mouth and anus for ingestion and egestion. It includes the fore gut, mid gut and hind gut and has radula which contains the teeth. The blood vascular system is of open type with the presence of irregular spaces known as lacunae. The blood is of blue color due to the presence of copper containing pigment known as haemocyanin. They have reduced coelom and have cavities in the kidneys and gonads. The coelom which contains blood is known as haemocoel. They have shell which is made up of calcium carbonate and is secreted by mantle. It can be external or internal. The external shells are present in the most of mollusca. The internal shells are present in the cuttle fish and are absent in the octopus. They have the epidermis layer which is single layered and have unstriped muscles which occur in bundles. The respiration occurs through the gills known as ctenidia and can occur in semi terrestrial forms by the lung known as pulmonary sac. The kidneys are the main excretory organs. They occur in the pair and have ammonia as their chief excretory product. The nervous system is made up of couple of cerebral, pleural and visceral ganglia which are connected by connectives and commissures. They have no asexual reproduction. Their development can be direct or indirect. In the indirect development a larva is formed. This is known as the trochophore. They have no asexual reproduction form. They have eyes and tentacles as their sense organs. The eyes act as photoreceptors while the tentacles act as tactile receptors. They have the statocyst which acts as a balancing organ. They have an osphradium which helps to test the nature of water.
The mollusca are considered better than annelids as they have a shell and distinct head. They have well developed gills and sense organs for respiration and the better functioning of body respectively.