Phyllum Mollusca contains soft bodied animals characterized of : a. They are second largest phyllum in animal kingdom b. They are generally bilaterally symmetrical c. They are only Terrestrial animals d. They are having muscular foot Which of the statements given above is/are correct ?
- (1)only a
- (2)a, b and d
- (3)a, b and c
- (4)only a and b
Correct — option (2), 'a, b and d'. Judge the four printed statements one at a time before looking at any option, and the set resolves cleanly. Statement a. is correct: Mollusca is the second largest phylum in the animal kingdom, ranking behind Arthropoda, the insects, crustaceans, arachnids and their relatives, and ahead of every other group in described species. Statement b. is correct as printed, and the word 'generally' is doing real work in it: molluscs are built on a bilaterally symmetrical plan, with a left and a right that mirror one another, and that plan is plainly visible in a bivalve's two matching valves and in the body of a squid. The gastropods complicate it, because during development they undergo torsion, a twisting of the visceral mass through as much as 180 degrees, which leaves the adult snail's shell coiled and its internal organs unequally developed on the two sides; the phylum is therefore bilaterally symmetrical in its ground plan and in most of its members, but not in every one, which is exactly what 'generally' allows. Statement d. is correct: the muscular foot is one of the three defining parts of the molluscan body, the other two being the visceral mass and the mantle. It is the creeping sole of a snail, the burrowing hatchet-shaped organ of a clam, and, modified past recognition, the ring of arms and tentacles of a squid or octopus, whose class name Cephalopoda means head-foot. Statement c. is the only false one: molluscs are overwhelmingly aquatic and chiefly marine, from mussels and oysters on the shore to squid and the nautilus in open water, with a further set living in fresh water; only some of the gastropods, the land snails and slugs, have moved onto land. To call them 'only Terrestrial animals' is therefore the reverse of the truth. Statements a., b. and d. stand and c. falls, so option (2) is the answer. The stem prints 'Phyllum' where the standard spelling is 'Phylum', and the phrase 'characterized of' is likewise as printed.
- (1)only a — This option accepts the ranking of the phylum by size and rejects everything else, which means discarding two statements that are both sound. It throws out bilateral symmetry, although the molluscan body plan is bilaterally symmetrical and the statement is hedged with 'generally' precisely so that the torsion of gastropods does not break it, and it throws out the muscular foot, which is not an incidental feature but one of the three parts every textbook uses to define the phylum. A candidate lands here by treating any statement that admits of an exception as false. In a statement-judgement item the qualifier has to be read as carefully as the claim: 'generally bilaterally symmetrical' and 'always bilaterally symmetrical' are different assertions, and only the second would have been wrong.
- (3)a, b and c — This is the trap the question is built around, because it takes in the one statement that is plainly false while leaving out the one that is most securely true. Statement c. says molluscs are only terrestrial animals, and the phylum is in fact predominantly marine — the oysters, mussels, cowries, squid, cuttlefish and the pearly nautilus are all sea creatures, and freshwater mussels and snails add another large group — with land snails and slugs forming only a minority within a single class. Dropping statement d., the muscular foot, compounds the error, since that character is part of the standard definition of the phylum. Anyone choosing this option has probably assembled it from the first three statements in printed order without testing the third.
- (4)only a and b — This option correctly rejects the claim that molluscs are only terrestrial, so the candidate who picks it has done the hard half of the work, and then it drops statement d. as well. The muscular foot, however, is one of the three components of the molluscan body plan alongside the visceral mass and the mantle, and it is what the classes are largely named around — Gastropoda meaning stomach-foot, Pelecypoda meaning hatchet-foot, Cephalopoda meaning head-foot. The likely reason for dropping it is the sight of an octopus or a squid, in which the foot has been so heavily modified into arms that it is no longer recognisable as a sole; but modification is not absence, and the character still holds across the phylum.
The molluscan body plan is worth learning as three parts and two accessories, because almost every question about the phylum is answerable from it. The three parts are the head-foot, a muscular organ used for creeping, burrowing, swimming or grasping; the visceral mass, which holds the digestive, excretory and reproductive organs; and the mantle, a fold of the body wall that encloses a mantle cavity and secretes the shell. The two accessories are the shell itself, made of calcium carbonate and reduced or lost in several groups, and the radula, a rasping ribbon of chitinous teeth used to scrape food, present in most classes but absent in the filter-feeding bivalves. Molluscs are coelomate and triploblastic, most have an open circulatory system with haemolymph bathing the tissues directly, and the cephalopods are the striking exception with a closed system and the most complex nervous system and eyes in any invertebrate. The phylum's principal classes are Gastropoda, the snails and slugs; Bivalvia or Pelecypoda, the clams, mussels and oysters; Cephalopoda, the squids, cuttlefish, octopuses and the nautilus; Scaphopoda, the tusk shells; and Polyplacophora, the chitons, whose shell is made of eight plates.
The 'which of the statements is/are correct' format is the most common single shape in this paper — thirty-eight questions turn on a statement list — and it is failed procedurally at least as often as factually. The method that works is fixed: read every statement and mark it true or false on its own merits, then find the option that reproduces exactly those verdicts. Reading the options first invites the candidate to look for a familiar-looking combination rather than to test each claim. Two features of this particular item deserve notice. The first is the qualifier: 'generally bilaterally symmetrical' is true where 'always' would not be, and MPSC uses such hedges deliberately, so a statement that is nearly always true and is printed with a hedge is meant to be accepted. The second is that the false statement is an absolute — 'only Terrestrial animals' — and absolutes are the likeliest statements in any list to be false, because a single counter-example destroys them, and here there are thousands of marine ones. Note also that this question labels its statements a. to d. in the English column while printing the four choices as (1) to (4), so the letter a means a statement in the stem and a storage position in the choices at the same time.
- Mollusca is the second largest phylum in the animal kingdom by number of described species, ranking after Arthropoda, and its members are soft-bodied, triploblastic and coelomate.
- The molluscan body consists of a muscular foot, a visceral mass and a mantle; the mantle encloses the mantle cavity and secretes the calcium carbonate shell where a shell is present.
- Molluscs are predominantly aquatic and chiefly marine, with a substantial freshwater component; only the land snails and slugs within the class Gastropoda are terrestrial, so the phylum as a whole is not terrestrial.
- The phylum is bilaterally symmetrical in its ground plan, but gastropods undergo torsion during development, a twisting of the visceral mass that makes the adult snail asymmetrical in its coiling and internal organs.
- Most molluscs possess a radula, a rasping ribbon of chitinous teeth used in feeding, and it is absent in the bivalves, which filter their food from water drawn through the mantle cavity.
Two opposite errors are on offer: rejecting b or d because each admits an awkward case, and accepting c because it arrives third in printed order and the first two were true. Test each statement on its own before looking at any option.
- Rejecting a hedged statement such as 'generally bilaterally symmetrical' as false because an exception exists, when the hedge is what makes it true
- Accepting the absolute claim that molluscs are only terrestrial, when the phylum is predominantly marine and only some gastropods live on land
- Dropping the muscular foot because it is unrecognisable in a squid or octopus, when the arms of a cephalopod are that same foot heavily modified
- Reading the options before judging the statements, which encourages the assembly of a plausible-looking combination instead of a verified one
Animal phyla are asked in MPSC papers as statement-judgement sets like this one, as matching items joining phyla to characteristic examples, and as single-fact questions on which phylum is largest, which possesses a particular organ, or which shows a named type of symmetry. Mollusca recurs more often than most because it carries several memorable superlatives and a small number of clean defining characters. Expect at least one statement in any such set to be an absolute containing 'only', 'all' or 'never', and expect that statement to be the false one more often than not, since a phylum with tens of thousands of species rarely admits a claim that admits no exception.
No directly related past PYQ was found.
- practice — not a real PYQ
Which of the following is the largest phylum in the animal kingdom by the number of described species, ranking ahead of Mollusca ?
- (a)Chordata
- (b)Annelida
- (c)Arthropoda
- (d)Cnidaria
Answer(c) Arthropoda — the insects, crustaceans, arachnids and myriapods together account for more described species than any other phylum, which is why Mollusca is described as the second largest. Chordata, which contains all the vertebrates, is far smaller in species number despite its familiarity, and Annelida and Cnidaria are smaller still.
- practice — not a real PYQ
The three principal parts into which the body of a mollusc is divided are which of the following ?
- (a)Head, thorax and abdomen
- (b)Foot, visceral mass and mantle
- (c)Scolex, neck and proglottids
- (d)Radula, siphon and gill
Answer(b) Foot, visceral mass and mantle — the muscular foot serves locomotion, the visceral mass holds the internal organs, and the mantle encloses the mantle cavity and secretes the shell. Head, thorax and abdomen is the arthropod division, while scolex, neck and proglottids describes the body of a tapeworm.