Sentence:  On one side of the body it has formed an arch forwards, its apex nearly touching the second band; and in the wider space thus left between the third band and the fourth, the <WSD>pseudostome</WSD> forms a deep involution of the sarcode, keyhole-shaped, richly ciliated, shallow anteriorly and deepening posteriorly into the short curved pseudocele, which merely dips under the fourth band, breaking through immediately behind it as a small round anal aperture. 
Meaning of the word marked with <WSD> tags:  The oral opening on the pseudembryo of an echinoderm.