Mutations and rearrangements that occur by misalignment during DNA replication are frequent sources of genetic variation in bacteria. Dislocations between a replicating strand and its template at repetitive DNA sequences underlie the mechanism of these genetic events.
Cnidarians take on two basic forms, a medusa and a polyp. The medusa form is a free-swimming structure which consists of an umbrella-shaped body (called a bell), a fringe of tentacles that hang from the edge of the bell, a mouth opening located on the underside of the bell, and a gastrovascular cavity. The mesoglea layer of the medusa body wall is thick and jelly-like.
The cnidarian body consists of a central blind sac, the coelenteron (= gastrovascular cavity), enclosed by a body wall comprising two epithelia, the outer epidermis and the inner gastrodermis (Fig 7-1, 7-2*). A gelatinous connective tissue layer, the mesoglea, lies between the two epithelia.