Q.38 Arrange the following animals on the basis of their phyla, according to increasing level of complexity:

1. Scolopendra
2. Nereis
3. Dentalium
4. Euplectella
5. Physalia

Choose the correct answer from the options given below:

1. (D), (E), (B), (C), (A)
2. (D), (E), (C), (B), (A)
3. (E), (D), (A), (B), (C)
4. (E), (D), (B), (C), (A)

   The correct sequence is (E), (D), (B), (C), (A). Euplectella (Porifera) to Physalia (Cnidaria) to Nereis (Annelida) to Dentalium (Mollusca) to Scolopendra (Arthropoda) follows increasing organizational complexity.

   Animal and Phyla Analysis

   Scolopendra (Phylum: Arthropoda)
   Centipede with jointed appendages, segmentation, open circulation, compound eyes—highest complexity via tagmosis (fused segments), chitin exoskeleton.​

   Nereis (Phylum: Annelida)
   Marine polychaete worm showing metamerism (true segmentation), setae, closed circulation, parapodia—organ system level precedes arthropod specialization.​

   Dentalium (Phylum: Mollusca)
   Tusk shell with muscular foot, mantle, radula, trochophore larva—coelomate, advanced bilateral with shell, gills, complex nervous system.​

   Euplectella (Phylum: Porifera)
   Venus’ flower basket sponge—asymmetrical, cellular organization only, choanocytes for filter-feeding, no tissues/organs—most primitive.

   Physalia (Phylum: Cnidaria)
   Portuguese man o’ war—polymorphic colony, radial symmetry, diploblastic, cnidocytes, gastrovascular cavity—tissue level exceeds sponges.​

   Phyla Complexity Order

   Animal	Phylum	Key Complexity Traits
   Euplectella	Porifera	Cellular level, asymmetrical ​
   Physalia	Cnidaria	Tissue level, diploblastic ​
   Nereis	Annelida	True segments, closed circulation ​
   Dentalium	Mollusca	Foot/mantle, radula, coelom ​
   Scolopendra	Arthropoda	Exoskeleton, appendages, tagmosis ​

   Arranging Animals by Phyla Complexity

   Arranging animals by phyla increasing complexity—from cellular Porifera to advanced Arthropoda—tests tissue/organ progression in GATE Life Sciences. Euplectella (sponge) starts, Scolopendra (centipede) ends via symmetry, coelom, segmentation advances.

   Why This Exact Order Works

   Porifera (Euplectella): Basal cellular level, no true tissues. Cnidaria (Physalia): First tissues, nematocysts. Annelida (Nereis): Metamerism elevates. Mollusca (Dentalium): Specialized organs. Arthropoda (Scolopendra): Peak via appendages.​

   Options breakdown:

   • Correct: (E), (D), (B), (C), (A)

   • Wrong: Others disrupt Porifera→Cnidaria→Annelida sequence.

   GATE Tip: Complexity Hierarchy

   Memorize: Porifera (asymmetry) < Cnidaria (diploblast) < Annelida/Mollusca (triploblast coelomates) < Arthropoda (segmented ecdysozoan). Nails arrange animals phyla increasing complexity PYQs.​