Insect Animal Hydrozoans Obelia geniculata is a distinctive species, with slender zigzag stems it forms. It forms colonies of polyps and is common in shallow, rocky habitats of northwestern Europe, although it is almost worldwide in its distribution. Colonies can grow up to 1.6 inches (4 cm) in height.
Common Name: Hydrozoans, hydroids, hydrocorals, hydras, Portuguese men of war
Number of species: about 3,000
Size: Varies greatly; individual polyps mostly very small, but hydroid colonies can be up to 6 in (15 cm) tall; some deep-sea species produce solitary polyps over 6 ft (1.8 m) long; floating Portuguese man-of-war colonies can be several feet long including tentacles
Key features: very varied group; in all cases mesogela laks cells; stinging cells are on the outside of the body, never inside; gonads shed gametes externally; basic froms include solitary and colonial polyps with or without a hard skeleton and some small medusa
Habits: Live in open seas, attached directly to substratum
Breeding: asexual by duding (may lead to separate individuals or expansion into a colony); sexual reparate individuals or ecpansion into a colony); sexual reproduction occurs in medusa phase or in gonophores ( reproductive zooids) attached to polyp colony
Diet: small particles of food collected from the water; prey animals killed by stinging cnidocytes
Habitat: aquatic, mostly marine; some species (notably hydras) live in fresh water
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