In the given figure,what do $a$ and $b$ represent respectively?

Corals belong to the phylum . . . . . . .

$A$ coral island with a central shallow lake is known as

Which one of the following animals possesses nerve cells but no nerves?

Identify the correct features for $Medusa$ and $Polyp$ forms in Phylum $Coelenterata$ (Cnidaria) from the given list:
$I$ - $Sessile$
$II$ - $Umbrella-shaped$
$III$ - $Free-swimming$
$IV$ - $Produced$ $by$ $asexual$ $reproduction$
$V$ - $Cylindrical$
$VI$ - $Produced$ $by$ $sexual$ $reproduction$
$Medusa$ $\quad\quad$ $Polyp$

Free-swimming,radially symmetrical animals with cnidocytes belong to