Phylum-Mollusca Questions in English

(A) The $Veliger$ larva is a characteristic larval stage found in the life cycle of many mollusks,particularly gastropods and bivalves.
Other larval forms such as $Trochophore$ and $Glochidium$ are also characteristic of the phylum $Mollusca$.
Therefore,the correct option is $A$.

(D) The $Octopus$ belongs to the class $Cephalopoda$ within the phylum $Mollusca$.
$Cephalopoda$ are characterized by having a prominent head and a set of arms or tentacles modified from the primitive molluscan foot.

(D) $Mytilus$ belongs to the class $Pelecypoda$ (also known as $Bivalvia$) of the phylum $Mollusca$.
This class is characterized by a shell consisting of two valves and a wedge-shaped foot.

(C) In $Unio$,the heart is highly contractile and three-chambered,consisting of a single ventricle and a pair of lateral auricles.

(C) In $Sepia$ (a member of the class $Cephalopoda$),the foot is significantly modified. It is displaced to the region surrounding the mouth and is divided into $8$ to $10$ arms or tentacles,which are used for capturing prey and locomotion. This is a characteristic feature of cephalopods.

(C) The phylum $Mollusca$ is the second-largest animal phylum.
Most members of this phylum are aquatic,specifically marine,although some species are found in freshwater or terrestrial environments.
Therefore,the majority of molluscs are marine.

(A) $Dentalium$ is a genus of marine scaphopod mollusks in the family $Dentaliidae$.
It is commonly known as the 'elephant's tusk shell' due to the shape of its shell,which resembles a miniature elephant's tusk.

(D) . $Xenophora$ (Sea mica) is a mollusc.
$B$. $Doris$ (Sea lemon) is a mollusc.
$C$. $Aplysia$ (Sea hare) is a mollusc.
$D$. $Pennatula$ (Sea feather or Sea pen) belongs to the phylum $Cnidaria$ (Coelenterata),not $Mollusca$.

(C) The phylum $Mollusca$ is divided into several classes such as $Gastropoda$,$Scaphopoda$,$Cephalopoda$,$Bivalvia$,$Polyplacophora$,etc.
$Decapoda$ is not a class of phylum $Mollusca$.
$Decapoda$ is an order belonging to the class $Crustacea$ under the phylum $Arthropoda$.

(C) The correct answer is $C$.
Pearl,a precious jewel,is a secretory product of the pearl oyster (mussel),which belongs to the class $Pelecypoda$ of the phylum $Mollusca$.
$Pelecypoda$ (also known as $Bivalvia$) includes organisms like oysters and clams that are economically significant due to pearl production and their use as a food source.

(A) The class $Bivalvia$ (also known as $Pelecypoda$) is characterized by a laterally compressed body enclosed within a shell consisting of two valves.
One of the most significant and distinctive features of this class is the reduction or complete absence of a distinct head (acephalic condition).
They lack a radula,which is typically present in other mollusks,and they possess a hatchet-shaped foot for burrowing.
Therefore,the absence of a head is a diagnostic character of $Bivalvia$.

(C) The correct answer is $C$. Snail $(Helix)$ belongs to the class $Gastropoda$ of the phylum $Mollusca$. Sea cow $(Dugong)$ is a mammal,Sea squirt $(Herdmania)$ is a urochordate,and Sepia is a cephalopod mollusk.

(C) The characteristics mentioned—mantle,muscular foot,and a coelom restricted to the pericardial cavity,gonads,and kidneys—are diagnostic features of the phylum $Mollusca$.
$1$. The mantle is a specialized fold of the body wall that secretes the shell.
$2$. The muscular foot is primarily used for locomotion.
$3$. In mollusks,the true coelom is greatly reduced and is restricted to the spaces around the heart (pericardial cavity),gonads,and kidneys.

(C) $Neopilina$ is considered a primitive mollusc that exhibits characteristics of annelids,such as internal metameric segmentation and $5$ pairs of nephridia. It is regarded as a living fossil and serves as a connecting link between annelids and molluscs.

(C) The class $Trematoda$ belongs to the phylum $Platyhelminthes$,not $Mollusca$.
$Monoplacophora$,$Gastropoda$,and $Pelecypoda$ are all classes within the phylum $Mollusca$.

(D) The respiratory pigment found in most molluscs is haemocyanin.
Unlike haemoglobin,which is contained within red blood cells in vertebrates,haemocyanin is a copper-containing protein that is dissolved directly in the blood plasma.
Therefore,the correct option is $D$.

(D) The larval form of gastropods is known as the $Veliger$ larva.
Additionally,many molluscs,including some gastropods,pass through a $Trochophore$ larval stage during their development.
$Ephyra$ is the larval form of $Scyphozoa$ (jellyfish).
$Glochidium$ is the larval form of freshwater bivalves.
$Scyphistoma$ is the polyp stage in the life cycle of $Scyphozoa$.

(B) The term $Cephalopoda$ is derived from two Greek words: $cephalo$ (meaning head) and $poda$ (meaning foot). Therefore,in this class of mollusks,the foot is modified into tentacles and is located on the head region.

(A) $Aplysia$ is a marine gastropod mollusk that is commonly known as the 'sea hare' due to the presence of two pairs of tentacles on its head that resemble the ears of a hare.

(B) The larvae of gastropods (like snails) are bilaterally symmetrical.
However,during the process of metamorphosis,they undergo a phenomenon called torsion,which results in the adult becoming asymmetrical.
This type of symmetry is known as secondary asymmetry.
Therefore,the correct answer is $B$ (Snail).

(A) The characteristics mentioned are:
$1$. Open circulatory system: Found in most molluscs (except cephalopods).
$2$. Heart with auricle and ventricle: Molluscs typically possess a heart with one or two auricles and a ventricle.
$3$. Haemocyanin: This is a copper-containing respiratory pigment that gives blood a blue color,which is characteristic of many molluscs.
Therefore,these features collectively describe the phylum $Mollusca$.

(C) The phylum $Mollusca$ exhibits a diverse range of respiratory mechanisms.
Some molluscan animals,such as certain gastropods,show both aquatic and aerial modes of respiration.
These animals possess a gill (ctenidium) for aquatic respiration and a pulmonary sac or lung for aerial respiration.

(A) The Cuttlefish belongs to the phylum $Mollusca$.
It is a marine invertebrate characterized by a soft body,often protected by a shell (though reduced or internal in some cephalopods like the cuttlefish),and a muscular foot.
Therefore,the correct option is $A$.

(B) $Teredo$ is commonly known as the 'shipworm'.
It is a highly specialized marine bivalve mollusc that is very destructive to wood submerged in seawater.
The body is long and slender with a small anterior shell.
The shell is used for burrowing into the wood of ships,piers,or wharves.

(A) The study of molluscan shells is specifically called $Conchology$.
$Malacology$ is the broader study of mollusks as a whole.
$Entomology$ is the study of insects.
$Mastology$ is the study of mammals.

(C) Haemocyanin is a respiratory pigment found in the blood of many molluscs and arthropods. Unlike haemoglobin,which contains iron,haemocyanin contains copper as its metal component. When oxygenated,it gives the blood a characteristic blue colour.

(B) $Unio$ (a freshwater mussel) exhibits filter feeding,which involves straining food particles from large quantities of water using its gills (ctenidia).

(C) In a closed circulatory system, blood is pumped through vessels of varying diameters (arteries, veins, and capillaries).
Among the given options, $Cuttle Fish$ (which belongs to the class $Cephalopoda$ of the phylum $Mollusca$) possesses a closed circulatory system.
$Snail$ (a $Gastropod$) and $Cockroach$ (an $Arthropod$) both possess an open circulatory system where blood flows into open spaces called sinuses.

(B) $Pila$ (snail) can survive long periods of drought by remaining inactive with its shell aperture tightly closed. This state of summer sleep is known as aestivation.

(A) Octopus (Devil fish),Squid (Loligo),and Cuttlefish (Sepia) are members of the phylum Mollusca. Within this phylum,they are classified under the class Cephalopoda. This class is characterized by a well-developed head and a foot that is modified into tentacles or arms around the mouth.

(D) The phylum $Mollusca$ is characterized by the presence of a soft body,which is typically unsegmented.
The body is covered by a fleshy fold of the body wall known as the $mantle$ or $pallium$.
This $mantle$ secretes a calcareous shell in most members.
Therefore,the combination of a $mantle$ and a $nonsegmented$ body serves as a primary diagnostic feature to differentiate $Mollusca$ from other phyla.

(A) In the phylum $Mollusca$,the shell is typically external and calcareous. However,in some cephalopods like $Loligo$ (squid) and $Sepia$ (cuttlefish),the shell is reduced and internal. $Chiton$ (chiton),$Dentalium$ (tusk shell),and $Unio$ (freshwater mussel) all possess external shells.

(A) The $Osphradium$ of $Pila$ is a comb-shaped sensory organ attached to the roof of the mantle cavity near the left nuchal lobe.
It functions as a chemoreceptor,which helps the organism in testing the chemical nature of the water entering the mantle cavity.

(D) The class $Scaphopoda$ is commonly known as tusk shells or tooth shells.
These mollusks possess a tubular,conical shell that is open at both ends,which resembles an elephant's tusk.

(D) The head of a snail bears a pair of short,fleshy,and stump-like optic stalks known as $Ommatophores$.
One stalk is located on either side behind the $2nd$ pair of tentacles.
Each $Ommatophore$ bears a small,black,and somewhat circular eye,situated slightly below its tip on the outer side.

(A) $Pila$,commonly known as the apple snail,belongs to the phylum $Mollusca$.
Within the phylum $Mollusca$,it is classified under the class $Gastropoda$.
$Gastropoda$ is the largest class of mollusks,characterized by a distinct head,a muscular foot,and a spirally coiled shell.

(B) Torsion is a characteristic process in the development of gastropods,where the visceral mass,mantle,and shell rotate $180^{\circ}$ relative to the head and foot during the larval stage. Among the given options,$Pila$ (apple snail) is a gastropod that exhibits torsion during its development. $Lamellidens$ is a bivalve,while $Sepia$ and $Octopus$ are cephalopods,none of which undergo this specific larval torsion process.

(A) The presence of a mantle,a muscular foot,and a shell are diagnostic characteristics of the phylum $Mollusca$.
Among the given options,$Nautilus$ belongs to the class $Cephalopoda$ of the phylum $Mollusca$.
$Echinus$ belongs to the phylum $Echinodermata$.
$Limulus$ (King crab) belongs to the phylum $Arthropoda$.
$Euplectella$ (Venus flower basket) belongs to the phylum $Porifera$.

(C) The correct answer is $C$. The mantle (also known as the pallium) is a significant organ in molluscs that secretes a calcareous shell. This shell is generally external,providing protection,but in some species,it may be internal,supportive,or completely absent.

(A) Torsion is a characteristic developmental process in the class $Gastropoda$.
During the larval stage,the visceral mass,mantle,and shell rotate $180^{\circ}$ relative to the head and foot.
This rotation brings the mantle cavity and the anus to a position above the head,which is a unique feature of gastropods.

(C) The asymmetry in Gastropoda is primarily caused by the process of torsion. Torsion is the rotation of the visceral mass,mantle,and shell $180^{\circ}$ with respect to the head and foot during larval development. This twisting results in the asymmetrical arrangement of internal organs,such as the shifting of the anus to a position above the head.

(C) Osphradium is a chemoreceptor organ found in many mollusks,particularly in aquatic gastropods.
Its primary function is to test the chemical nature of the water entering the mantle cavity,which helps the organism in detecting food or potential predators.

(C) The animals belonging to the phylum $Mollusca$ exhibit the following characteristics:
$1$. They are triploblastic (derived from three germ layers: ectoderm,mesoderm,and endoderm).
$2$. They possess an organ-system level of body organization.
$3$. They are coelomate (possess a true body cavity).
$4$. Their body is generally unsegmented (unlike Annelida or Arthropoda).
Therefore,the correct option is $C$.

(C) Haemocyanin is a copper-containing respiratory pigment that gives blood a blue color when oxygenated.
It is commonly found in the hemolymph of many mollusks,such as cephalopods (e.g.,squids,octopuses) and some gastropods.
While some annelids use haemoglobin or chlorocruorin,haemocyanin is a characteristic feature of many members of the phylum $Mollusca$.

(B) The osphradium is a small, bipectinate sensory organ found in the mantle cavity of $Pila$.
It functions as a chemoreceptor, which is sensitive to chemical stimuli in the water, helping the organism to test the quality of water.

(A) connecting link is an organism that exhibits characteristics of two distinct taxonomic groups,suggesting an evolutionary relationship between them.
$Neopilina$ is a primitive mollusk that possesses segmented body structures,a characteristic feature of $Annelida$,while also exhibiting typical molluscan features like a shell and a mantle. Therefore,it serves as a connecting link between $Annelida$ and $Mollusca$.

(A) The octopus belongs to the phylum $Mollusca$.
Within this phylum,it is classified under the class $Cephalopoda$.
$Cephalopoda$ (meaning 'head-foot') includes organisms like octopuses,squids,and cuttlefish,which are characterized by having arms or tentacles attached to the head region.

(C) The mollusk known as the tusk shell is $Dentalium$.
It belongs to the class $Scaphopoda$ of the phylum $Mollusca$.
These organisms possess a tubular,conical shell that is open at both ends,resembling an elephant's tusk,hence the name 'tusk shell'.

(A) The $Ink$ $gland$ (also known as the ink sac) is a specialized organ found in cephalopods like $Sepia$ (cuttlefish),$Loligo$ (squid),and $Octopus$.
It produces a dark-colored fluid (ink) that is ejected through the siphon to confuse predators and facilitate escape.
$Pila$,$Mytilus$,and $Pecten$ are mollusks but do not possess an ink gland.