$A$ student enters the museum and observes the animal specimens. He looks at the one marked 'whale' and calls it a type of fish. How will you explain to him that whales and fishes belong to two different classes? List any three distinguishing features to justify your statement.

Classify the following organisms based on the absence/presence of a true coelom (i.e., acoelomate, pseudocoelomate, and coelomate): $Spongilla$, $Sea \text{ } anemone$, $Planaria$, $Liver \text{ } fluke$, $Wuchereria$, $Ascaris$, $Nereis$, $Earthworm$, $Scorpion$, $Birds$, $Fishes$, $Horse$.

Name two mammals which lay eggs. Between which two phyla do they serve as connecting link? Name a mammal which gives birth to poorly developed young ones.

State reasons for the following:
$(a)$ Mammals have mammary glands.
$(b)$ Annelids have a true body cavity.
$(c)$ Fish possess gills.

Identify and write the names of the parts labeled as $A, B, C$ and $D$ in the given figure of a seahorse in your answer book.

State the type of body cavity found in Platyhelminthes. Write whether they are parasitic or free-living.