The second-largest phylum of the animal kingdom after the Arthropod. These are soft-bodied animals. The members of this phylum are called mollusks or mollusks. Around 85000 species of mollusks are recognized. The size of mollusks ranges from microscopic to 20 meters long animals. They show much impact on human lives. These are huge sources of food as well as ornaments.
Characteristics features of phylum Mollusca
- Habitats: The mollusks exist on terrestrial or aquatic (marine or fresh-water habitats). Phylum Mollusca is the largest marine phylum.
- Level of organization: They possess an organ-system level of organization.
- Symmetry: The members having a bilaterally symmetrical body.
- Germinal layer: These are triploblastic
- Body cavity or coelom: The members of this phylum are coelomate (true coelom called schizocoel).
- Shell: Calcareous (calcium carbonate) shell covers the body.
- Segmentation: Unsegmented body, distinct head, muscular foot, and visceral hump is found. The muscular foot helps in locomotion.
- Over the hump, a soft and spongy layer of skin called the mantle.
- The mantle cavity is the space between the hump and mantle, Feather-like gills are present in the mantle cavity.
- These feather-like gills perform the function of excretion and respiration, hemocyanin is respiratory pigment.
- They possess a myogenic heart.
- The organ of Bojanus and Kebers’s organ (Unio)function for excretion.
- The circulatory system is an open type.
- Sensory organs: The anterior part of the head consists of sensory tentacles.
- Feeding: The mouth having a rasping-like organ called the radula, which helps in feeding. Complete digestive tract and ciliary structure present in it.
- Open type circulation with dorsal heart.
- Reproduction: These are mostly dioecious– distinct male and female individuals, and oviparous– They lay eggs.
- Fertilization: may be external or Internal.
- Development: The development is indirect.
- Some examples: Pila (apple snail), Sepia (cuttlefish), Pinctada (pearl oyster), Loligo (Squid), Octopus (Devil-fish), Dentalium (Tusk shell), Aplysia (Sea-hare), Chaetopleura (Chiton).
Scientific Classification of Phylum Mollusca
- Some mollusks like Unio possess green glands that mimic the liver in vertebrates.
- To test the chemical nature of water, they comprise osphradia.
- The body equilibrium is maintained by statocysts.
- They possess hemocyanin in blood, that pigments blue or green color.
- Eight arms of the octopus are without the shell. Ink gland is found in them for protection purpose.
Some common examples of mollusk with a general name
- Pila- Apple snail
- Unio- Freshwater mussel
- Limax- Slug
- Helix- Garden snail
- Octopus- Devilfish
- Loligo- Squid.
Classification of Molluscs
Aplacophora or Solenogasters
- The digestive ceca are not found in these organisms.
- Cuticle with Spicules covers the body.
- These are mostly worm-like mollusks
- Keel or crest is found, dorsal longitudinal
- Example: Neomenia, Chaetoderma
- These are marine animals.
- The tusk shells or tooth shells, technically the Scaphopoda, are members of a class of shelled marine mollusc.
- Distributed worldwide and are the only class of exclusively infaunal marine molluscs.
- Shells of species within this class range from about 0.5 to 15 cm in length. reduced foot.
- Members of the order Dentaliida tend to be significantly larger than those of the order Gadilida.
- The gastropods, commonly known as snails and slugs.
- These belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda.
- Broad, muscular foot
- all snails and slugs including abalone, limpets, conch, nudibranchs, sea hares, sea butterflies
- Monoplacophora, meaning “bearing one plate”,
- It is a polyphyletic superclass of molluscs with a cap-like shell now living at the bottom of the deep sea.
- Respiration through gills (external)
- Major members- an ancient lineage of molluscs with cap-like shells
- Example: Neopilina
- Also called Bivalvia, lamellibranchiate, and pelecypods.
- Marine and freshwater mollusk
- The members have no head and they lack some usual molluscan organs like the radula and the odontophore.
- They include the clams, oysters, cockles, mussels, scallops, and numerous other families that live in saltwater, as well as a number of families that live in freshwater
- The gills have evolved into ctenidia, specialized organs for feeding and breathing.
- They bury themselves in sediments for protection from predation.
- Example: Mussels, Unio
- The shell is composed of eight longitudinal plates
- Chitons are marine mollusks of varying sizes in the class Polyplacophora, formerly known as Amphineura.
- About 940 extant and 430 fossil species are recognized.
- They possess a well-developed radula
- The study of cephalopods is a branch of malacology known as teuthology.
- These exclusively marine animals
- characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot.
- They possess a common ability to squirt ink also called ink fish.
- Example: squid, octopus, cuttlefish, or nautilus.
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