While considering phylum coelenterata..are the sexes separate or not? And is fertilisation internal or external?

Dear student, 

Cnidarians or coelentrates are hermaphrophites. Some of the species are capable of self-fertilization. Otherwise, the gametes are generally shed into the sea(external fertilization), where the eggs are fertilized. This is followed by larval stage called planula larva, which may be crawling and benthic or free-swimming and planktonic. 

Regards.