Test: Basis of Classification - NEET MCQ

• Animals that belong to the phylum Platyhelminthes to Chordata are all triploblastic.
• But Platyhelminthes and aschelminthes belong to pseudocoelomates.
• Annelids, arthropods, echinoderms and chordates are under eucoelomates. This means not all triploblastic animals are eucoelomates. 
• Pseudocoelomates have false coelom while eucoelomate animals only have the true coelom. 

Hence, both statement 1 and statement 2 are incorrect. 

Bilateral symmetry is defined as the symmetry in which animals can be divided into two halves in one plane only, and these halves are identical to each other. The organisms have paired body organisms that occur on both sides of the body; if this will not happen then the symmetry will no longer remain. This type of symmetry is found in all vertebrates and few invertebrates. Some invertebrates also show radial symmetry, for example, animals belonging to coelenterates, ctenophores and echinoderms. These organisms can be divided into two identical haves through any plane passing through the central axis. So, the incorrect statement regarding bilateral symmetry is option C.

Line in NCERT: "Animals like annelids, arthropods, etc., where the body can be divided into identical left and right halves in only one plane, exhibit bilateral symmetry."

A - Ectoderm
B - Endoderm
C - Mesoglea
D - Mesoderm
So, the correct answer is 'A - Ectoderm, B - Endoderm, C - Mesoglea, D - Mesoderm'.

Line in NCERT: "Animals in which the cells are arranged in two embryonic layers, an external ectoderm and an internal endoderm, are called diploblastic animals, e.g., coelenterates. An undifferentiated layer, mesoglea, is present in between the ectoderm and the endoderm."

(i) Diploblastic animals have two germinal layers (ectoderm and endoderm) during embryonic development
Example: Poriferans.
(ii) Triploblastic animals have three germinal layers (ectoderm, mesoderm and endoderm) during embryonic development
Example: Platyhelminthes.

Line in NCERT: "Animals in which the cells are arranged in two embryonic layers, an external ectoderm and an internal endoderm, are called diploblastic animals." "Those animals in which the developing embryo has a third germinal layer, mesoderm, in between the ectoderm and endoderm, are called triploblastic animals."

1. Symmetry

• Asymmetrical: Animals like sponges show no definite body symmetry; any plane passing through the center does not divide them into equal halves.

• Radial symmetry: In animals such as ctenophores, coelenterates, and echinoderms, any plane passing through the central axis divides the body into identical halves.

• Bilateral symmetry: Animals like platyhelminthes, annelids, and arthropods have body plans where only one plane divides the body into equal left and right halves.

2. Germ Layer Organization

• Diploblastic animals (e.g., ctenophores, coelenterates) have two germ layers—ectoderm and endoderm—with an undifferentiated mesoglea in between.

• Triploblastic animals (e.g., platyhelminthes, annelids, echinoderms) possess a third germ layer—mesoderm—in addition to ectoderm and endoderm.

3. Coelom Type

• Acoelomates: Animals like platyhelminthes lack a body cavity.

• Pseudocoelomates: In animals like aschelminthes, the coelom is not lined by mesoderm but appears as scattered mesodermal pouches.

• Coelomates: Animals such as annelids, echinoderms, and chordates have a true coelom lined completely by mesoderm.

(A) Cellular level
(B) Radial symmetry
(C) Bilateral symmetry
(D) Pseudocoelomates
(E) Coelomates

Coelenterates have radial symmetry. Aschelminth are pseudocoelomates. Molluscs do not show metamerit Sponges are diploblastic.

Line in NCERT: "Coelenterates, ctenophores and echinoderms have this kind of body plan."

The animals in which the coelom is absent are called acoelomates, for example, flatworms (Planaria). In them, the space between ectoderm and endodorm is filled with parenchyma derived from mesoderm.

Topic in NCERT: flatworms

Line in NCERT: "Flatworms are bilaterally symmetrical, triploblastic and acoelomate animals with an organ level of organisation."

(i) Diploblastic: Coelenterates
Correct. Poriferans (sponges) do not have true tissue layers and are not considered diploblastic. Only Coelenterates (Cnidarians) are diploblastic (having ectoderm and endoderm).
(ii) Triploblastic: Platyhelminthes to Chordates
Correct. All animals from Platyhelminthes to Chordates are triploblastic (having ectoderm, mesoderm, and endoderm).
(iii) Acoelomate: Poriferans, Coelenterates, Platyhelminthes
Correct. Poriferans, Coelenterates, and Platyhelminthes are acoelomates as they lack a true coelom.
(iv) Pseudocoelomate: Aschelminthes / Roundworms
Correct. Aschelminthes (now called Nematoda) are pseudocoelomate.
(v) Eucoelomate: Annelids to Chordates
Correct. From Annelids to Chordates, animals are eucoelomate (true coelom).
Therefore, only statements (ii), (iv), and (v) are correct.

Topic in NCERT: Diploblastic and Triploblastic Organisation

Line in NCERT: "Animals in which the cells are arranged in two embryonic layers, an external ectoderm and an internal endoderm, are called diploblastic animals, e.g., coelenterates. Those animals in which the developing embryo has a third germinal layer, mesoderm, in between the ectoderm and endoderm, are called triploblastic animals (platyhelminthes to chordates)."

Option A is correct: Radial symmetry is advantageous for sedentary organisms as it allows them to interact with their environment from all directions.
Option B is correct: The notochord is mesodermal in origin and is positioned between the dorsal hollow nerve cord and the alimentary canal in some animals.
Option C is incorrect: A pseudocoel (false coelom) is derived from the blastocoel and is only partially lined by mesoderm. It is bounded by mesoderm on one side and ectoderm on the other, not fully mesodermal like a true coelom.
Option D is correct: Cephalization refers to the concentration of sensory organs, nervous tissue (including the brain), and food-capturing structures at the anterior end of an organism.

Line in NCERT: "Pseudocoel (false coelom) derived from blastocoel of the embryo and is bounded by ectoderm and mesoderm."