The Collaborative International Dictionary of English v.0.48:

Mouthless \Mouth"less\, a. [AS. m[=u][eth]le['a]s.]
   Destitute of a mouth.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mouthless
    adj 1: having no mouth or mouthlike opening [syn: astomatous,
           mouthless] [ant: stomatous]