A)Those with separate sexes would need to locate one another when mating.
B)Parasitic infection is typically organ specific; parasites need to sense their location before attaching to a body structure.
C)Parasite life cycles require dispersal to new hosts, some parasites actively sense potential hosts and move toward them.
D)All of the above are reasonable hypotheses.

A)zygote to blastula to gastrula.
B)gastrula to zygote to blastula.
C)blastula to zygote to gastrula.
D)zygote to gastrula to blastula.

A)Spicules were probably absent in the earliest sponges and evolved later as predation began.
B)Spicules performed other tasks in early sponges, possibly structural support, and were later modified for defense.
C)Spicules were present in some sponge species and absent in others; predation became a selection pressure that eliminated some species entirely.
D)All of the above are valid hypotheses.