The Collaborative International Dictionary of English v.0.48:

Desmid \Des"mid\, Desmidian \Des*mid"i*an\, n. [Gr. desmo`s
   chain + e'i^dos form.] (Bot.)
   A microscopic plant of the family Desmidi[ae], a group of
   unicellular alg[ae] in which the species have a greenish
   color, and the cells generally appear as if they consisted of
   two coalescing halves.
   [1913 Webster]

WordNet (r) 3.0 (2006):

desmid
    n 1: freshwater green algae