In the family Asteraceae the heads are of two types, depending on the type of flowers which they host.
Without tubular flowers: all flowers are irregular (bilaterally symmetrical) and strap-shaped:

With tubular flowers: in this case the flowers which are in the centre of the head are regular (radially symmetrical) and have 5 petals resembing triangular teeth, fused below into a sort of tube. Strap-shaped, irregular flowers are totally absent, or are limited to the margin of the heads, such as in daisies: