Sentence:  Segments with more than 25 pairs of veins; indusium minute, ciliate. soon disappearing; a linear-attenuate brown <WSD>aerophore</WSD> present at base of each pinna 16. 
Meaning of the word marked with <WSD> tags:  A thin-walled area of the roots of some plants growing in swampy conditions through which gasses are exchanged.