Final answer:
Scyphozoans are bell-shaped jellies with a nerve net, muscle ring, and a life cycle with medusoid and polypoid stages, while Cubozoans are box jellies with pedalia and a similar life cycle. Both have a gastrovascular cavity and capture prey with nematocysts on their tentacles.
Step-by-step explanation:
Basic Anatomy of Helmet Jellyfish (Scyphozoans and Cubozoans)
Helmet jellyfish, belonging to the classes Scyphozoa and Cubozoa, exhibit distinct anatomical features. Scyphozoans are characterized by their bell-like morphology, a nerve net with sensory organs called rhopalia, and a ring of muscles that facilitate swimming and feeding. They have separate sexes with gonads formed from the gastrodermis, and the life cycle includes both medusoid and polypoid stages. Cubozoans or box jellyfish, while similar to scyphozoans, have a notable difference in the arrangement of their tentacles, with muscular pads called pedalia on their square bell canopy. Cubozoans also have a life cycle that includes polyp and medusoid forms, where the polyps have the capacity to bud and transform into medusas.
In terms of digestion, scyphozoans possess a gastrovascular cavity with interconnected sacs called diverticuli and, in some species, further branching into radial canals for increased nutrient absorption and structural support. The mouth is located on the underside, surrounded by tentacles with nematocysts for prey capture. Cnidarians like helmet jellies can exist in both the sessile polyp form and the motile medusa form, exhibiting amazing versatility in their life cycles and morphology.