Marine biology

Hydroids are a life stage for most animals of class Hydrozoa

Hydrozoa

Hydrozoa are a taxonomic class of very small, predatory animals which can be solitary or colonial and which mostly live in saltwater. A few genera within this class live in freshwater...

, small predators related to jellyfish

Jellyfish

Jellyfish are free-swimming members of the phylum Cnidaria. Medusa is another word for jellyfish, and refers to any free-swimming jellyfish stages in the phylum Cnidaria...

.

Botany

In mosses, hydroids form the innermost layer of the stem of long, colourless, thin walled cells of small diameter.The cells are dead and lack protoplasm.They function as water and nutrients conducting tissues analogous to tracheids. However, they lack any pitting or surface texture.

Hydroids have been found in some Rhynie chert

Rhynie chert

The Rhynie chert is an Early Devonian sedimentary deposit exhibiting extraordinary fossil detail or completeness . It is exposed near the village of Rhynie, Aberdeenshire, Scotland; a second unit, the Windyfield chert, is located some 700 m away...

 plants, including Aglaophyton

Aglaophyton

Aglaophyton major was the sporophyte generation of a diplohaplontic, pre-vascular, axial, free-sporing land plant of the Lower Devonian . It had anatomical features intermediate between those of the bryophytes and vascular plants or tracheophytes.A. major was first described by Kidston and Lang in...

, where they were initially mistaken for xylem vessels.

See also

• Leptoid
  Leptoid
  A leptoid are elongated food-conducting cells in some mosses. They are surrounded by strands of hydroids. They have some structural and developmental similarities to the sieve elements of seedless vascular plants. At maturity they have inclined cell walls with small pores with degenerate nuclei...
  
  , a related sucrose-transporting vessel analogous the phloem
  Phloem
  In vascular plants, phloem is the living tissue that carries organic nutrients , in particular, glucose, a sugar, to all parts of the plant where needed. In trees, the phloem is the innermost layer of the bark, hence the name, derived from the Greek word "bark"...
  
  of vascular plants