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."

Platyhelminthes are triploblastic, acoelomate organism showing bilateral symmetry. Ctenophores have borads symmetry, are acoelomates having diploblastic organisation. In echinoderms, larvae have bilateral symmetry whiled add have radial symmetry. They are coelomates with triplobla organisation. Annelids are triploblastic, coelomaterorganism showing bilateral symmetry.

Line in NCERT: "The basic fundamental features such as level of organisation, symmetry, cell organisation, coelom, segmentation, notochord, etc., have enabled us to broadly classify the animal kingdom."

(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.

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

(i) Diploblastic - Poriferans, Coelenterates

(ii) Triploblastic - Platyhelminthes to Chordates

(iii) Acoelomate - Poriferans, Coelenterates, Platyhelminthes

(iv) Pseudocoelomate - Aschelminthes/Roundworms

(v) Eucoelomate - Annelids to Chordates

So, the correct answer is '(i), (ii), (iii), (iv) and (v)'.

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."